TELEMETRY AND CONTROL SYSTEM USING BURST MODE COMMUNICATION

Abstract: A telemetry and control system for use in direct current power wiring applications, such as those used in automobiles or other vehicles. The system includes a transmitter which encodes data as a train of current pulses which modulate the d.c. supply voltage. A receiver at another location, connected to the same power source through pre-existing wiring, amplifies the supply voltage modulation, recovers the data waveform, and decodes it, to effect an action or display a measurement or status indication. A second transmitter combined with the receiver may transmit data similarly for reception by a second receiver combined with the transmitter, to enable two-way communication therebetween. Applications of the system include an emergency battery booster, an anti-theft battery disconnection device, a distress alert system, activation and reception of data from outboard devices, and remote measurement of fluid levels, temperatures and pressures.

Description: DESCRIPTION TELEMETRY AND CONTROL SYSTEM USING BURST MODE COMMUNICATION BACKGROUND OF THE INVENTION This is a continuation-in-part application of application Serial No. 08/016,771, filed February 11, 1993, now abandoned. The invention relates to a method and apparatus for transmission and reception of control and telemetry signals through existing power distribution circuitry. More particularly, it relates to a method for such transmission and reception between apparatus at two or more different locations in an environment where only power distribution circuitry exists for application of direct current from a common power source to equipment at different locations therein. It is often desirable to install apparatus for transmitting and receiving control and telemetry signals as an adjunct to existing equipment connected to a common source of power. Such apparatus enables a utilization device in one location to transmit a signal to another location for purposes of controlling equipment at the first location, or to receive information from a remote location concerning the status of equipment or other conditions at the remote location. In known systems used for home security and lighting control purposes, for instance, coded signals are applied to an a.c. power mains outlet at one location, and are received at another location for controlling a device or for monitoring conditions at the first location. Typically, such signals are transmitted by means of a high-frequency carrier imposed upon the a.c. mains voltage, and modulated with a specific code designating the particular function desired. The receiver of such signals is adapted to detect the high-frequency signals and to demodulate them to recover the specific code and act on it to perform the desired function, for example to switch a light on or off, to change the speed of a ceiling fan, or to display a condition present at the transmitter's location. Although such systems have been applied extensively for home use, a similar system may be particularly desirable for automobile use, and because the power supply of an automobile or other vehicle employs direct current (d.c), the transmitter and receiver may advantageously be made simpler than is possible for the known systems mentioned above. However, a d.c. power supply such as that used in a vehicle direct current power system usually has an extremely low internal resistance, on the order of approximately 1-2 milliOhms. Thus, it is difficult to impose control signals on the lines of the system. In addition, the use of such control signals will often interfere with other components of the system, for example, a car stereo. Known remote control systems for use in automobiles or other vehicles therefore typically employ either a radio frequency (r.f.), ultrasonic, or infrared (IR) transmission system having no direct wiring between transmitter and receiver-actuator modules, or employ dedicated additional wiring between these modules. These systems all have their disadvantages. R.f. systems are limited to communication between modules in the same space in the car as there is usually a metal bulkhead between the passenger and engine compartments which prevents reliable radio transmission and reception between them. A similar limitation occurs with infrared and ultrasonic transmissions. Dedicated wiring between the two areas, while permitting communication throughout the entire vehicle, involves difficulties for user installation. In addition, the bulkhead of an automobile also forms a firewall and the seal must be effectively maintained. It would therefore be desirable to provide a system and apparatus that would provide the advantages of dedicated wiring which permits communication throughout the entire vehicle, with the convenience of installation of infrared and r.f. systems, without any of the drawbacks of either. SUMMARY OF THE INVENTION The foregoing problems are solved and a technical advance is achieved by the present invention that provides a system and method for transmitting and receiving telemetry and control signals between different locations over existing direct current power distribution circuitry. The system enables intercommunication and control between remote locations connected to, for examples the power harness of a vehicle such as an automobile. In a departure from the art, the system generates, at a first location connected to the wiring, a pulsed signal coded according to a desired telemetry or control function, and imposes the signal on the wiring. The pulsed signal is detected at a second location on the wiring and decoded to provide the desired telemetry or control function. The pulsed signal is similar in form to ambient noise normally present in the power distribution circuitry, for example noise due to engine ignition. This permits the pulsed signal to exist in the power circuit without interfering with the operation of other devices (for example, a car radio) that are connected to the power circuit, because such other devices are typically designed to be immune to ambient noise existing on the power distribution circuit. However, the regular and periodic nature of the pulsed signal permits the pulsed signal to be discerned from ambient noise. The system has particular application as a reliable means of transmission of coded signals between different locations connected to the direct current power harness of a vehicle, to measure various conditions and actuate devices in response to user commands. In one embodiment, the system includes a portable master control unit that may be plugged into the cigarette lighter of a vehicle to access the wiring system. The control unit monitors conditions of modules connected to the wiring at other locations, such as in the engine compartment or outside the vehicle, and responsive to user commands, effects actuation of the modules to perform specific functions. Both the control unit and the remote modules are capable of being equipped with transmitter and receiver circuitry, thereby enabling two-way communications. In another embodiment of the present invention, a modified relay or relays may be substituted for original equipment relays, the modified relay incorporating pulsed signal communication capability. Then, one or more relays may communicate with each other without the need to add dedicated communication wiring. Thus, for example, the original windshield wiper and headlight relays may be replaced with modified relays incorporating pulsed signal communication to automatically turn on the headlights when ever the windshield wipers are turned on. Other applications are also possible, including, for example, vehicle security systems, vehicle diagnostic systems, vehicle lighting and light monitoring systems, and the like. In a vehicle application, pulsed signal intercommunication between remote locations connected to the direct current harness is performed for purposes which may include: connection of an auxiliary battery to recharge the principal vehicle car battery when a monitoring module detects an insufficient charge on the principal battery for starting the vehicle; disconnection of the principal battery to immobilize the vehicle when an unauthorized person attempts to start the vehicle without first connecting a module which transmits a unique coded signal; measurement of various fluid levels, pressures and temperatures for display to the user where such gauges are not provided as original equipment; alerting others to a distress in such circumstances as kidnapping, carjacking or roadside emergencies; alerting the driver and other persons of objects behind or near the vehicle including distance information regarding such persons or objects; transmitting the response of an outboard radar detector to the driver; automatically activating or monitoring vehicle lights and other electrical devices including lights and devices within trailers connected to the vehicle; and automatically activating a function or feature of the automobile upon initiation of a different function or feature, for example, the automatic activation of a vehicle's headlights when the windshield wipers are turned on. A variety of similar uses are contemplated, and the listing of these several possible applications is simply exemplary and not limiting. In an illustrative embodiment, the system comprises a transmitter circuit at a first location connected to direct current power wiring for generating a pulsed signal coded according to a desired telemetry or control function. The transmitter imposes the pulsed signal on the wiring. A receiver circuit at a second location connected to the wiring determines the presence of the pulsed current through the wiring. The receiver regenerates and decodes the pulsed signal to perform the desired telemetry or control function. The transmitter circuit may include an electrical transducer for converting a desired measured quantity or user input to an electrical signal, a signal encoding circuit for converting the electrical signal into a pulsed signal, and a circuit for imposing the pulsed signal in the wiring. The receiver circuit includes amplifier circuitry for monitoring and amplifying fluctuations of the voltage on the wiring due to the presence of the pulsed current caused to flow therein, pulse recovery circuitry for determining from the fluctuations of the voltage the transition times of the pulsed signal and for reconstituting therefrom the coded pulsed signal, and decoder circuitry for determining from the pulsed signal the coded measured quantity or user input represented thereby. The pulsed signal used in the present invention is similar in form to ambient noise existing on the power distribution circuitry. The pulsed signal will thus not interfere with the operation of other devices connected to the power distribution circuit because such devices are typically designed to be immune to ambient noise. However, since the pulsed signal communication has a regular and controlled periodicity, the pulsed signal can be discerned from ambient noise. A technical advantage achieved with the present invention is that it provides a reliable and easy to install arrangement for transmission of coded signals through the existing electrical power wiring harness of a vehicle, requiring either a power or chassis ground connection, or both, for both transmission and reception of information. In particular, an exemplary embodiment of the present invention contemplates an apparatus for transmitting and receiving telemetry and control signals superposed on a direct current circuit in the presence of ambient electrical noise, the apparatus comprising, a transmitter connected to the circuit and having a signal generator for generating a signal having a frequency in excess of about 10 megaHertz, and for selectively interrupting the continuity of generation of the signal so as to provide an encoded signal, and a circuit for superposing the encoded signal on the direct current circuit; and a receiver connected to the circuit including, a differentiator for differentiating and integrating the encoded signal to remove ambient background electrical noise therefrom, a comparator for comparing the differentiated and integrated signal with a predetermined code sequence, and means for performing a function based upon the comparison. In accordance with other aspects of the present invention, the frequency of the signal may be in excess of about 100 megaHertz, may be in the VHF frequency range or may be in the UHF frequency range. The signal generator may comprise an incidence frequency generator which is at least approximately an order of magnitude lower than the frequency of the signal, and a trigger for triggering the signal generator to produce separated output waves therefrom, spaced on the incidence frequency baseline. The superimposing of the coded signal may include a tank circuit having a resonant frequency output in excess of about 10 megaHertz, the tank circuit being connected to the direct current circuit, and a device for interrupting the continuity of the resonant frequency output to carry an encoded signal. The differentiator may include a circuit for differentiating the frequencies and noise on the direct current line to select frequencies in the range of the signal frequency so as to attenuate the relative level of frequencies and the ambient noise not in that range, and an integrating circuit for integrating the output of the differentiating circuit so as to reconstitute the encoded signal sequence. The differentiating circuit may be comprised of a single stage or dual stage band pass filter. In accordance with another aspect of the invention, a succession of iterations of the encoded signal sequence may be repeated to assure affirmation of an uncontaminated sequence. These and other features and advantages of the present invention will become apparent to those of ordinary skill in the art with reference to the appended drawings and following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustrating a master unit and a slave unit of an exemplary d.c. power line communication system according to the invention; FIG. 2 is a more detailed block diagram of the system of FIG. 1; FIG. 3 is a block schematic of an embodiment of the system of FIG. 1 used for connecting a spare battery to recharge a main battery; FIG. 4 is a graph illustrating the variation in battery impedance with frequency for an automobile battery; FIG. 5 is a detailed schematic of a simple master transmitter according to an exemplary embodiment of the invention; FIG. 6A and FIG. 6B show respective front and side elevation views of the master transmitter of FIG. 5, having display means and user-operable switches; FIG. 7 is a block schematic of the system of FIG. 1 embodied as a vehicle anti-theft system; FIG. 8 is an enlarged, perspective view of the slave receiver unit of the system of FIG. 7; FIG. 9 is a block schematic of the system of FIG. 1 showing components configured for a distress alert system; FIG. 10 is a block schematic of the system of FIG. 1 embodied as a system for activating an outboard sensor and receiving data therefrom; FIG. 11 is a block schematic of the system of FIG. 1 embodied as a system for transmitting engine parameters such as a fluid temperature, pressure and/or level to a display device; FIG. 12A, FIG. 12B, FIG. 12C and FIG. 12D are a schematic of another embodiment of a transmitter and signals generated by the transmitter, in accordance with the present invention; FIG. 13A, FIG. 13B, FIG. 13C and FIG. 13D are a schematic of another embodiment of a receiver and signals present in the receiver, in accordance with the present invention; FIG. 14A, FIG. 14B and FIG. 14C are a schematic of yet another embodiment of a transmitter and signals generated by the transmitter, in accordance with the present invention; FIG. 15, FIG. 16, FIG. 17, FIG. 18 and FIG. 19 are schematics of various components of a vehicle security system incorporating the transmitter and receiver of FIG. 12 and FIG. 13; FIG. 20 is a schematic of a smart relay embodying the present invention; FIG. 21 is a schematic of another embodiment of a smart relay in accordance with the present invention. DETAILED DESCRIPTION In FIG. 1, there is shown in block schematic form a generalized illustrative embodiment of a communication system according to the invention. A master unit 1 is connected to receive power from a d.c. power supply 2, whose supply voltage is modulated by the master unit 1. The power supply 2 is also connected to supply power to a slave unit 3 and the modulation of its voltage is also detected by slave unit 3 and employed to perform any function designated by the master unit 1. While unit 1 is designated the master unit and unit 3 is the slave unit, the symmetry of the figure suggests that the slave unit 3 could just as easily impose modulation onto the power supply voltage from power supply 2, and the master unit could as easily detect such modulation and effect some function designated by the slave unit 3. Thus the communication between master unit 1 and slave unit 3 can be full duplex, or two-way communication. The more detailed block schematic of FIG. 2 shows how this can be achieved, and suggests possible applications for this communication method. In FIG. 2, the master unit 1 is seen to receive an input from a sensor device 4, and the slave unit 3 effects an output to an actuator device 5. In addition, optionally, a number of input devices 6 through 7 may be connected to the master unit 1, and a number of output devices 8 through 9 may be optionally connected to the slave unit 3. It can be seen that the master unit 1 comprises a master transmitter 10 and a master receiver 11. The d.c. power supply 2 is modeled by an ideal voltage source 12 in series with an impedance 13, which may be frequency-dependent, and in the case of an automotive battery as the voltage source, may also depend upon the temperature, state of charge, age, or electrolyte level thereof. The slave unit 3 is seen to comprise a slave receiver 14 and a slave transmitter 15. More particularly, the input devices 4, 6-7, are connected to the master transmitter 10, while the output devices 5, 8-9, are connected to the slave receiver 14. Similarly, the slave transmitter 15 may receive an input from a sensor device 16, the master receiver 11 being connected to an actuator device 17, and further input devices 18-19 may be optionally connected to slave transmitter 15, while optional output devices 20-21 may be connected to master receiver 11. A system such as this may be used for a variety of purposes, and although in many cases full duplex transmission may be desired as shown in FIG. 2, it is also envisaged that for some uses only the master transmitter 10, slave receiver 14, and associated devices may be provided, if one-way communication is sufficient for the particular application. The principle of operation of the system shown in FIG. 2 is the following. In order to impose a signal modulation onto the d.c. power supply 2, it is desirable that this power supply should have a non-zero impedance, which is represented by a resistor 13. If the current drawn by master transmitter unit 10 is not constant, the variations therein will be reflected by changes in voltage drop across the impedance 13, thereby modulating the supply voltage of d.c. power supply 2. While the slave receiver 14 is powered from this supply, it may also include means for amplifying and detecting the modulation of the supply voltage by master transmitter 10, and for decoding the information represented by the modulation. In response to an input from the sensor device 4, the master transmitter 10 encodes a particular modulation onto the d.c. power supply 2, this being transmitted through the power wiring of the vehicle to the slave receiver 14 which may be in a different location. This modulation is detected and decoded by the slave receiver 14 and is applied to control the output actuator device 5. For example, the sensor input device 4 may comprise a switch operated by the user, and the output actuator device 5 may comprise an illuminated sign which is flashed on and off by the slave receiver 14 when it receives a coded signal representing this function from the master transmitter 10. This sign may, for instance, flash the word "HELP" to other vehicles to indicate that the car driver needs assistance (FIG. 9) . Alternatively, the master transmitter 10 may send a coded signal to a remote slave receiver 14 to connect a spare battery through suitable current limiting means to recharge the main car battery to a sufficient voltage for starting the car (FIG. 3) . In the latter case, the slave transmitter 15, if installed, could receive an input from device 16 to determine whether the battery voltage is sufficient for starting the car, and could send an indication of status back to the master receiver 11, where it might be displayed on output device 17, which could be a lamp or a liquid crystal display (LCD) to show the actual battery voltage. These and other examples of the use of the system of FIG. 2 will be described in detail with reference to later drawings. In the prior art, control systems have been devised for installation in homes, for remote activation of lights, alarm devices, and heating controls, for example, from a central location, and for remote indication of status of devices attached to doors and windows, for example. Because such systems are powered by, and transmit signals through, the a.c. mains wiring, modulation has typically been performed by imposing a modulated high-frequency carrier signal onto the mains wiring. Such a system requires a certain complexity, as a high frequency carrier has to be generated and modulated, and the receiver has to be tuned to the carrier and must demodulate it. Where a d.c. supply is available, however, the modulation scheme can be simpler, avoiding the need for complex circuitry to generate and modulate r.f. signals. It is sufficient to draw a fluctuating current from the power supply 2 in order to provide modulation thereon, and the receiver merely has to amplify and decode the fluctuating voltage thereby produced. It is recognized that imposing a very high frequency (VHF, typically 30MHz - 300MHz) or ultra-high frequency (UHF, typically 300MHz - 3GHz) signal on the lines of a d.c. power system may cause malfunction of attached components (such as controller or ignition systems in a vehicle) . Accordingly, the technique of the present invention imposes a signal that is UHF only in its impulse response, and at the same time ultra low frequency (about 128 Hz) in period. This type of signal is similar to the "noise" induced by engine spark plugs in a vehicle power system, except that the imposed signal is highly periodic, i.e. "crystal-controlled. " Thus, the signature of the impulse (rise and fall times) is always substantially the same and predictable. The circuitry, described below, thereby takes advantage of these signal characteristics to produce signals which are discernable from system noise. Likewise, because the imposed signals have a resemblance to commonly present signal noise, the various devices attached to the power system are usually adequately designed to filter out the imposed signals, obviating any damage. Such a modulated current is produced herein by a switching device adapted to connect a load impedance across the power supply terminals in response to a coded pulsed signal applied to its controlling terminal, these components being contained in master transmitter 10. The slave receiver 14 amplifies and detects the pulsed modulation and regenerates and decodes the transmitted pulses, acting on the signal according to previously programmed functions-corresponding to the possible codes sent to it. In one embodiment of the present invention, it is contemplated that the modulating signal may comprise a series of pulses of varying length, frequency, duty ratio, amplitude, or other parameters, according to a predefined code system, each different modulation representing a different function to be effected or measurement to be displayed. For example, the output digitally coded temperature reading from a temperature sensor monitoring a fluid temperature in the engine compartment of a vehicle may be sent to a display unit plugged into the cigarette lighter in the passenger compartment of the vehicle, where the receiver decodes the pulses received and displays the temperature reading on an LCD display. Furthermore, a code may be sent along with the actual temperature reading to indicate the location of the sensor or its purpose, and to indicate the appropriate destination of the signal. Thus it may be seen that different signals may be present at the same time, differentiated by source and destination coding, emanating from different transmitter units, representing different functions or measurements, and being applied to different actuating or display devices. The degree to which such signals may interfere with one another depends upon the particular details of the modulating scheme and hardware, specific examples of which are provided later in this disclosure. FIG. 3 illustrates an illustrative embodiment of a master unit 1 and slave unit 3 of a system according to the present invention. The master unit 1 comprises a crystal oscillator 30 with an associated crystal 31, a frequency dividing circuit 33, a transistor or other switching device 35, a current limiting resistor 36 and inductor 37 in series with its collector. The crystal oscillator provides a square wave 32 at its output, with a frequency of 32.768kHz, for example. Other frequencies would be acceptable without departing from the scope of the invention. With the specific crystal 31 having a frequency of 32.768kHz and the division ratio of frequency divider 33 being 1/256, as shown in FIG. 3, a square wave 34 of frequency 128Hz is provided to the base of the transistor 35. The transistor 35 therefore connects the current limiting resistor 36 and the series inductor 37 from the positive battery voltage to ground at this same rate. Referring also to FIG. 4, the battery impedance increases with frequency, so the inductor 37 is included to compensate for the otherwise very "spiky" waveform that would result, and to act as a low-pass filter, preventing very high frequencies that could, for example, interfere with the vehicle's radio (not shown), from being imposed on the battery voltage when the transistor 35 switches on, but not when it switches off. The waveform imposed upon the battery comprises a low level signal 38 having substantial ringing at the transitions, such ringing being preferably limited to moderately low frequencies. In this instance, the master unit contains only a transmitter, which is therefore not separately identified. Referring also to FIG. 6a and FIG. 6b, the circuit of the master unit 1 is built into a small enclosure having a plug that may be plugged into the cigarette lighter socket (not shown) in the passenger compartment of a vehicle, for example. In accordance with this illustrative embodiment, the slave unit 3 (FIG. 3), located near the vehicle's battery 2 in the engine compartment, for example, is typically physically attached to a spare battery 40. The battery 40 is connected through a rectifier diode 41 to the main battery 2, which as shown in FIG. 2 may be represented by an ideal voltage source and a series impedance. The main battery 2 is also connected to the vehicle's alternator 42 through a regulator device 43. When the engine (not shown) is running, the alternator 42 charges the main battery, whose voltage may vary typically from 10.8V up to as much as 15.6V, although more typically it will be approximately 14V. Thus the spare battery 40 can be charged through the diode 41 to about 13.5V as long as the engine is running. As no current is drawn from this battery 40, it retains its charge indefinitely, ready to be used for emergency purposes. When the vehicle is not running, the main battery 2 is no longer being charged, and its voltage may fall to about 12 to 13V. This shuts off the diode 41 and prevents the spare battery 40 from discharging. If the main battery has discharged to a low enough voltage that the car will not start, this fact may be monitored by a device such as that labeled 16 in FIG. 2 attached to the slave unit 3, or by a device attached to the master transmitter 1. The slave unit 3, which has only a receiver, not separately identified, comprises an amplifier 44 which receives the pulsed coded signal 38 from the master unit 1 on the battery 2 through a coupling capacitor 45 and amplifies it to provide the signal 46 at its output. A pulse conditioning circuit 47 comprises a diode 48, capacitor 49 and resistor 50, together with an amplifier 51. The amplified spikes 46 received from amplifier 44 are applied to the cathode of diode 48, temporarily pulling the voltage on capacitor 49 more negative each time a pulse transition is detected. The amplifier 51 output swings positive to its maximum output, maintaining this until the voltage on capacitor 49, charged through resistor 50, goes positive, the output of amplifier 51 switching to its maximum negative output, where it remains until the next pulse transition occurs. Thus the series of spikes 46 are converted by pulse conditioning circuit 47 into a train of rectangular pulses 52 at the same frequency. The output of the pulse conditioning circuit is connected to one input of a frequency comparator 53. A crystal oscillator 54 employing a crystal 55 oscillates at 32.768kHz, and its output signal 56 is frequency divided by 256 in a frequency divider 57, thereby applying a 128Hz square wave 58 to the other input of the comparator 53. When the frequencies of the two signals applied to the frequency comparator 53 are equal, the comparator 53 produces an output which is connected to a transistor 60, which in turn operates a relay 61. The relay contact 62 connects the positive terminal of the spare battery 40 to that of the main battery 2, suitable limiting means (not, shown) being employed to limit the maximum current that may flow, to avoid damage to either battery or the relay contact 62. In its discharged condition, the internal resistance of the battery 2 may provide enough resistance to limit this current to a safe value. Although not shown, the crystal oscillator 54 may also comprise circuitry for locking the frequency of the oscillator in response to an output (not shown) of the frequency comparator 53 to accomplish synchronization of the internally generated frequency to the frequency of the train of rectangular pulses 52 whenever these frequencies are approximately equal. It should be noted that in the absence of the transmitted 128Hz signal from the master unit 1, no such connection will be made, as the slave unit 3 will not operate the relay 61 unless this signal is present. Furthermore, the receiver 3 circuitry only enables the comparator 53 when the battery voltage is initially too low to start the vehicle. The comparator 53 may remain enabled until the main battery 2 voltage has risen to a suitable value for starting the car. While it is possible for a slave transmitter like that labeled 15 in FIG. 2, if installed, to send to the master receiver 11 of FIG. 2, if present, a status signal indicating this condition, the master unit 1 can itself monitor the battery voltage and indicate when it has risen to a suitable value. The user may now start the vehicle, and the alternator will again recharge both the main and spare batteries 2 and 40 ready for any future emergency situation of low main battery voltage. An advantage of this scheme is that no jumper cables or outside help are needed in most situations where a main battery has been accidentally discharged, saving the vehicle owner time and money, and resulting in a safer, easier solution. The spare battery 40 preferably has sufficient capacity and voltage to ensure that an adequate charge can be transferred to the main battery to allow starting. A minimum suggested capacity would be 5Ah. Furthermore, in the event that the main battery is so badly discharged as to fail to operate the transmitter and receiver units, a manual switch (not shown) can be provided on the battery 40 to connect it to the main battery for starting. In some vehicles, the lighter socket may not be powered when the ignition switch is off, and this may also be a reason to provide a manual switch on the spare battery itself. In order for such a system to work, the battery 2 should preferably be of sufficiently high impedance that a modest transmitter such as that shown in FIG. 3 can in fact modulate its voltage sufficiently to be received by a remote receiver. FIG. 4, therefore, shows a graph of battery impedance against frequency for a typical car battery of 90Ah capacity. It can be seen that at d.c. or very low frequencies, this impedance is only a few milliOhms, but at frequencies in the MHz range it rises to a much higher impedance on the order of 0.1 to 300 Ohms. This may be taken as typical of car batteries in general. Therefore, when a transistor is used to switch on and off a square wave current a noticeable spike may occur at the transitions, and may be amplified and used for performing a desired function, as shown in FIG. 3. FIG. 5 shows a representative transmitter schematic for use in a master unit 1 according to that of FIG. 3. The crystal oscillator and frequency divider may be combined on a single CMOS integrated circuit, industry type CD4060, with a few additional components, represented by the numeral 33. The crystal 31 is loaded with capacitors Cl and C2, and connected to the oscillator pins 11 and 10 of integrated circuit UI, which should not be confused with elements 10 and 11 in FIG. 2. Resistors Rl and R2 apply the voltage at pin 10 to the crystal, a portion of this voltage being fed back to pin 11 to maintain oscillation. The integrated circuit Ul then divides the frequency by 256 to obtain a 128Hz square wave at pin 14 which is applied via a resistor R3 to the base of a transistor 35 labeled Ql, which may be for example an industry type 2N4124 device. The collector circuit of this transistor comprises the resistor 36 also labeled R5, the inductor 37, labeled Ll, and a capacitor C3 shunting these components. Also across the power connections of the unit, which are made to a plug 70 of a type suitable for connection to the car's lighter socket, is a red light-emitting diode 71 in series with a resistor R4, for indicating that the device is properly connected and receiving power. Other status lights, not shown, may indicate to the driver when the battery voltage is sufficient for starting the car. As illustrated in FIG. 6A and FIG. 6B, such a master unit 1 may be made in a small enclosure 72 attached to the plug 70, or possibly detachable therefrom and having an extension cord 73 and means for mounting the unit in a convenient position. The unit may have, in addition to the power indication light 71, other status lights 74-75, a liquid crystal display (LCD) 76, an annunciator 77 and pushbutton or other switches 78-79, for example. FIG. 7 illustrates application of an embodiment of the present invention to use as an anti-theft device. Although the master control unit 1 transmitter (FIG. 5) has no modulation of the pulse train, many known methods of modulation exist and may be adapted for use with this system; for example, the pulse width may be modulated yielding wide or narrow pulses lor representing digital "1" or "0" bits of a repeated digital code. A transmitter 1 adapted to send an unique digital code, representing for example a personal identification number (PIN) may be used as an electronic "key" to enable the vehicle to be started. This transmitter plugs into the lighter socket 80. A receiver 3 in the engine compartment detects the special coded signal and enables starting. In the absence of the coded signal, any attempt to start the car causes a switch to be operated, which disconnects the car battery from the starter motor and prevents starting the car. In FIG. 7, this security system is shown in block schematic form. The master unit 1 has a plug 70 which plugs into the lighter socket 80, the outer shell of which is grounded. The positive connection to the battery 2 is typically made through a fuse 81 and in some vehicles through the ignition switch 82. In the engine compartment, the slave unit 3 is connected to the battery to receive the signal from the master unit 1. The slave unit 3 operates a battery disconnect switch 83 which disconnects the battery 2 when activated. In addition to the slave receiver 14 for receiving the transmitted signal from the master unit 1, there are other sensors and actuator devices attached to the receiver 3. One such device, 84, senses whether the hood (not shown) is closed or open and can actuate an alarm device 85 if the hood is opened without the master unit 1 being connected to the lighter socket 80. In practice, the hood open sensor 84 is built into the slave unit 3, which in turn is strapped to the car battery 2 and preferably receives its power from the battery side of the switch 83. The sensor 84 may comprise an infrared emitter 86 and phototransistor 87 in conjunction with a reflective area or reflective tape 88 positioned under the hood, so that when the hood is closed, light from the infrared emitter 86 reaches the phototransistor 87, but is interrupted when the hood is opened. In addition a loud siren or other warning device 85 may be incorporated into the slave unit 3. Connectors 89 and 90 are shown for connection to the vehicle starter and battery 2, respectively. FIG. 8 shows a rendering of the slave unit 3, sensor 84 and alarm 84 embodied as a self-contained battery disconnect unit. Such a disconnect unit, may have alternate connector types for use with different types of car battery, as will be evident to those skilled in the art. Components of the slave unit 3 are referenced with the same numerals used in reference to FIG. 7. Thus the battery post connector 89 provides power to the rest of the car, while the short heavy wire and clamp 90 is connected to the positive terminal of the battery 2 itself. A strap 91 is provided for securing the slave unit 3 to the battery 2 (not shown) . FIG. 9 illustrates an embodiment of the system used for providing a vehicle warning or emergency indication. The master unit 1 comprises a concealed switch 92, mounted somewhere in reach of the driver for hand or foot operation, and a transmitter unit 97 which sends a specific coded signal on the 12V power wiring of the car to a receiver unit or units 100 when the switch is operated. The slave receiver unit 100 is powered from the nearest available 12V wiring, and connected to a special license plate cover 103 having concealed illuminated letters spelling out the message "HELP", for example. These may be provided, for example, by concealed wiring 104 to wire filaments 105 in a gas-filled or evacuated chamber between two transparent plates forming the body of the license plate cover 103, or alternatively by a transmissive/reflective liquid crystal display, an array of light-emitting diodes, or other known methods. The illumination may be made to blink in order to attract the attention of other road users. In FIG. 9, while the push-button switch, 92 may be hard-wired to the transmitter unit 97, an effective implementation of this embodiment comprises a small, inconspicuous, battery-powered r.f. transmitter 93 which may have a short wire antenna 94 and an adhesive surface 95, so that the button may be easily installed in a concealed location known only to the vehicle driver. The pulse transmitter 97 may include an r.f. receiver for receiving the signal from the r.f. transmitter 93 through a short antenna 96. The pulse transmitter may be located somewhere under the dash, for example, the positive connection being made to a suitable hot power line by means of an insulation displacement connector 98, while the ground connection can be made through a mounting tab 99 screwed to the vehicle's chassis. The slave receiver unit 100 is included as an electronics package with the license plate cover 103 and is powered by means of a wire with an insulation displacement connector 101 which may be connected to any suitable hot power line. A ground connection may be made through one of the license plate holder screw holes 102. The filament wires 105 forming the word "HELP" are interconnected through concealed wires 104 to the receiver electronics package of the unit 100. Such a system can be used in the event of an abduction or "carjacking" of the occupants by a criminal assailant. As an adjunct, additional slave units or actuator devices may, for example, activate a car phone to send an automated distress message to the operator, and to transmit a beacon signal to permit authorities to locate and track the vehicle. If a battery disconnect module of the type shown in FIG. 7 has been installed, it may be activated to immobilize the vehicle, if conditions permit this to be done safely. FIG. 10 shows another embodiment of the invention which may be used to transmit information from an outboard electronic sensor device 110 to the master unit 1 in a vehicle passenger compartment. The sensor device 110 may comprise, for example, a radar detector or an ultrasonic ranging device mounted at a convenient location under the front or rear bumper of a vehicle, for example. The device 110 also incorporates a slave unit 3 for reception of a signal to activate the device 110 and for transmission of appropriate data from the device 110 to the vehicle driver. The master unit 1 in the vehicle may be of the type shown in FIG. 6, having a master transmitter 10, master receiver 11 (see FIG. 2) , at least an activation switch 78 and one or more status indicators 74-75, and preferably an audible annunciator 77 of some kind and an LCD display 76. When the activation button 78 is pressed, the master transmitter 1 activates the outboard device 110 (or in the case of a ranging device for a backup alert application, it may be activated by connection of power to the vehicle's backup lights, for example.) The outboard device 110 includes a slave receiver 14 for receiving the activation signal, and a slave transmitter 15, which sends back to the master unit 1 a coded signal indicating its status. For example, if the outboard device 110 is a radar detector, it could send a signal indicating the detection of radar signals in any operating band, plus an indication of signal strength. The master receiver 11 would then cause appropriate status lights 74-75 to indicate the detection of radar signals, and sound a "beep" tone on the annunciator device to provide audible indication of the received signal strength as is done in conventional radar detectors. The beep tone could vary in pitch, or repetition frequency, depending on the signal strength received. Alternatively, in the case where the outboard device 110 is a ranging device used as a backup alert, a status light 74 on the master unit indicates the presence of reflecting objects behind the vehicle, while the distance reading for the nearest such object is displayed on an LCD display 76, and a warning tone whose pitch or repetition rate depended upon this reading is sounded on the annunciator device 77. Additionally, the outboard ranging device 110 may include a beeper 111 for sounding a warning to people in the vicinity that the vehicle is backing up. The outboard device 110 may also be mounted on a trailer whose electrical system is connected to the vehicle's electrical system, to provide an additional measure of security when backing up with a trailer in tow. FIG. 11 shows another embodiment of the invention, for use in monitoring various conditions in the engine compartment of a vehicle. In this application, patterned after the block schematic of FIG. 2, a number of different sensor elements may be connected to a common slave unit 3 for measuring and sending different environmental measurements to the vehicle driver. Typically, a dipstick sensor unit 120 may include transducers 121, 122 for measurement of a fluid level and temperature, these measurements being effected by known methods. A sensor 121 is attached by means of clips or clamps 123, with additional clips securing a flat cable 124, to a fluid dipstick 125 which is inserted into the appropriate part of the engine or transmission. Close to, or attached to, the dipstick is the electronics package 126 which interprets the data received from sensor 120. For example, a temperature measurement may be made by means of a thermistor 121 in the sensor 120 mounted to the dipstick, its resistance being measured by the sensor electronics package 126 and converted into a digital temperature reading. The electronics package 126 may be connected to the car's battery 2 (via the ignition switch to avoid unwanted battery drain when the engine is off) , and to a slave transmitter 15, forming part of the slave unit 3 and possibly integrated with the sensor electronics package 126, for transmitting the temperature reading, along with the location of the sensor itself, to the master unit 1. In addition, the sensor 120 may have a means for measurement of the fluid level. For example, a second thermistor 122 could be provided at a position on the dipstick 125 that would correspond to a low level mark. If both thermistors 121, 122 were operated at a sufficient current to provide some self-heating, when the fluid level was high enough to submerge this thermistor 122, both thermistors 121 and 122 would read the same temperature; however, if the thermistor 122 was exposed and not immersed in the fluid, its temperature could rise, and the difference in temperature could be used by the sensor electronics package 126 to signal a low fluid level condition to the driver. It is important to note that many fluids, such as the engine oil, can only be measured reliably under certain conditions, such as with the engine warmed up and stopped, as the level of the fluid varies depending upon whether the engine is running or stopped, the engine rpm. , the fluid temperature, and other factors. However, a warning of abnormally low engine oil could be provided if the factory-installed instrumentation does not include such a warning light. Similarly, a dipstick type sensor may be provided for measurement of other fluids, such as the transmission fluid, radiator fluid, windshield washer fluid, and if needed, battery acid level (most modern automobile batteries are sealed and need no such indication) . Alternatively or additionally, an oil pressure sensor 127 may be included, with a connecting wire 128, and the measurement electronics required for it may also be incorporated into the electronics package 126. The electronics package 126 may also include circuitry for determination of when the quantity being measured falls beyond preset minimum and maximum limits,, and cause an audible warning to be sounded on the master unit's annunciator 77 when this happens, automatically displaying which fluid sensor 120 or 127 is responsible for the alert on the LCD display 76. The measurements may be activated or deactivated by the use of switches 78-79 on the master unit 1, if the slave unit 3 also contains a slave receiver (not shown) and means (also not shown) for activating or deactivating the sensor electronics package 126. Referring back to FIG. 4, it can be seen that when a signal with a frequency on the order of ten megaHertz (MHz) is superposed on the output of a D.C. battery in, effect, it encounters an impedance of approximately 1.0 Ohm. Thus, a peak current of approximately 10 Amps is required to drive a nominal ± 1.0 Volt signal. The battery impedance vs. frequency relationship of FIG. 4 illustrates that a lower frequency signal of equal amplitude will demand a higher current, and a higher frequency signal will demand a lesser current. Furthermore, as long as they are characterized by the same period and rate-of-change, a signal comprising only a single cycle wave, or a continuously cycling signal, will have the same effective frequency and encounter the same impedance. The physical size of components for any electronic system is largely determined by the power capacity required. The cost of components is a function of both their power capacity and the level of precision required in their operation. As shown above, higher frequencies can be used to reduce current demands to the milliAmpere range so as to vastly reduce the power requirement for a given signal amplitude. Parts that are made or selected to have operating characteristics within ± 2% of specification are much more expensive than parts that are only required to perform within + 20%. Background electrical noise is an automotive environment is present throughout the frequency spectrum. Alternator and electric motor noise contribute engine speed related noise as do the spark plugs. Spark plug firing events are high frequency rate-of-change, short period spikes of perhaps a microsecond in duration, although they occur at rates ranging up to only about 600 Hertz Hz) . Broadcast and other frequencies abound, especially in urban areas. Thus, the coded signal of the present invention should be capable of isolation and decoding in the presence of this cluttered background. Therefore, an exemplary embodiment of the present invention uses high frequencies for the signalling function, in a manner which avoids a need for precise modulation or discrimination and thus, allows its implementation with common, inexpensive components. The use of frequencies above 10 megaHertz not only serves to reduce power requirements but also helps distinguish the signal from background noise, so that a coded signal can be recognized and isolated without requiring carefully tuned, precision circuitry. In practice, frequencies in the VHF (typically, 30MHz - 300MHz) or UHF (typically 300MHz - 3GHz) range are preferred in order to minimize power requirements, component size and cost, however frequencies above or below these ranges would also be acceptable as long as the frequencies remain high enough to minimize power requirements. In FIG. 12A, FIG. 12B, FIG. 12C, FIG. 12D, FIG. 13A, FIG. 13B, FIG. 13C and FIG. 13D are shown additional exemplary embodiments 140 and 170 of the present invention for transmitting and receiving telemetry and control signals on a D.C. circuit as powered by battery 142. Referring first to FIG. 12A, the telemetry and control signal is first created as an incidence frequency by oscillator 144 as the 8 megaHertz square wave 145 of FIG. 12B. An encoded binary signal is set into digital encoder 146 by code programming device 147 so as to mix with square wave 145 at AND gate 148 and mix the incidence frequency with timed interruptions and produce the encoded sequence signal 150 of FIG. 12C. Tank circuit 160, which has a resonant frequency of approximately 500 megaHertz, is comprised of inductor 158 and capacitor 159 and also includes current limiting resistor 156. Transistor 154 receives encoded signal 150 through base resistor 152 and the rising voltage of each square wave 145 connects tank circuit 160 to ground 157, causing the generation of one full wave at the 500 megahertz frequency for each 8 megaHertz square wave. This modified, encoded signal 150A is superposed on D.C. line 162 and, through capacitive coupling in the wiring harness bundles, upon every associated wire including the ground wire at least to some extent. Values selected for the effective frequency and incidence frequency are restricted only in that the incidence frequency should be the lower of the two by at least approximately an order of magnitude, and that the higher the effective frequency, the less the power requirement of the circuit. Thus, while the present invention may be expressed using an effective frequency of 10 megaHertz and an incidence frequency of 1 megaHertz, even high effective frequencies may be advantageous for purposes of cost and electronic package size. In FIG. 12D, encoded signal 150A is shown to comprise an effective frequency component of separated 500 megaHertz frequency waves, triggered on the 8 megaHertz incidence frequency baseline so as to pulse in encoded bursts. The duration of each 8 megaHertz burst is 625 microseconds, making the number of separated full- wave cycles in each burst to be about 5,000 rather than the 6 shown schematically. Other Burst widths would of course also be acceptable. In this exemplary embodiment, a burst interval of 2.2 milliseconds indicates a "1" and one of 3.1 milliseconds indicates a "0". Thus, 24 bits of data can be transmitted in 74 milliseconds or less. Other digital coding systems such as PWM (Pulse Width Modulation) , NRZ (Non Return to Zero) and Manchester Code are well known in the art and the particular code used is a matter of choice. An example of an alternative embodiment of a receiving system in accordance with the present invention is disclosed with reference to FIG. 13A, FIG. 13B, FIG. 13C and FIG. 13D. As described above, encoded signal 150A, along with background electrical noise, is present on input 172 of the receiving system circuit 170 shown in FIG. 13A. Diodes 173 and 174 provide a clamping circuit to protect circuit 170 from excess voltage spikes or surges, while encoded signal 150A passes through capacitor 175 to the 500 megaHertz differentiating circuit 176 which operates as a band pass filter to selectively amplify frequencies in the vicinity of 500 megaHertz and attenuate all higher and lower frequencies. Differentiating circuit stage 176 serves, in effect, as a band pass filter for frequencies from approximately 300- 700 megaHertz. This circuit removes frequencies below 200 megahertz and thereby eliminates most engine speed related, but not all background electrical noise 177. The encoded signal 150B as shown in FIG. 13B, is next passed into a 8 megaHertz differentiating circuit 178 which serves as a band pass filter for frequencies of approximately 5-12 megaHertz. The output of differentiating circuit stage 178 is modified to emerge as encoded signal 150C of FIG. 13C. Background electrical noise in the higher frequency ranges is rejected so that encoded signal 150C should be virtually free of noise as it passes through resistor 179 to the next stage. Workable results can be achieved with only a single-stage of differentiation, albeit at the expense of more precise components and tuning. Integration circuit 180 comprises diode 181, resistor 182 and capacitor 183. Here, encoded signal 150C is reconstituted as the half-amplitude envelope of encoded signal 150A, shown as encoded signal 150D in FIG. 13D. Any residual short duration spike which somehow persists through differentiation should not be removed but, because there always remains a possibility for the contamination of any given signal sequence, encoded signal 150 is repeated in a predetermined string of iterations. Encoded signal 150D is input to code recognition logic 186 for comparison to the programmed function code sequence. An affirmative comparison initiates the closing of function switch 187 so as to operate selected function 188. In FIG. 14A is shown an alternative encoded signal transmitting circuit 190 wherein an encoded signal is created without the use of an incidence frequency. Tank circuit 192, comprising coil 194 and capacitor 196 and having a resonant frequency in excess of 10 megaHertz is excited through resistor 193 to create a continuous output at the resonant frequency. Encoding of this continuous frequency output is much the same as in the example of preferred embodiment 140 of FIG. 12A. Encoded binary signal sequence 205, shown in FIG. 14B, is set into digital encoder 191 by code programming devices 195 so as to mix the continuous frequency with timed interruptions at "AND" gate 197 and produce the encoded sequence signal 205A of FIG. 14C. Signal 205A is then amplified by amplifying stage 198 and passes through coupling capacitor 201 to be superposed on line 202 in the direct current circuit of battery 189. An exemplary embodiment of a security system based on the encoded signal transmitting and receiving system of FIG. 12A and FIG. 13A is shown in FIG. 15, FIG. 16, FIG. 17, FIG. 18 and FIG. 19. Master control unit 210 as shown in FIG. 15 is a hand held RF remote, powered by battery 211. Oscillator 218 provides a signal frequency which is encoded in a 24 bit sequence for RF transmission. The first 20 bits comprise a dedicated identity code input to logic device 212 at the factory. The remaining 4 bits provide as many as sixteen encoded commands, according to input combinations of normally open switches 213, 214, 215 & 216. The 24 bit sequence is repeated in a consecutive string to provide more positive assurance of an uncorrupted signal, and sent to transmitter 220 for output at antenna 222. In FIG. 16, RF receiver 232 is shown to be a component part of smart relay control unit 230 which may be installed in replacement of an original equipment horn or light relay, or other acceptable relay. Base connector 249 is the same configuration as the original equipment relay replaced by smart relay control unit 230. Smart relay control unit 230 includes logic device 234, which may be a microprocessor or may be hard wired, with non-volatile memory 236, which may be ROM, PROM, EPROM, EEPROM, so called "Shadow RAM", or dip-switch input. Also included is encoded signal transmitter 238 which operates according to the prior description of FIG. 12A. Electronic components are almost exclusively designed for operation in the 2-6 VDC range. Thus, the 12V supply is filtered and regulated by reduced voltage power supply 240 so as to provide a substantially spike free internal 5V source. Electrical activity sensor 242 is included as also is a low voltage protection circuit 244. Electrical activity sensor 242 picks up and amplifies a transient voltage drop as from a sudden current drain by any electrical activity such as an interior light, engine starting, or the like. Low voltage protection circuit 244 sets logic device 234 to an initialization state, after any supply voltage failure. Relay driver 246 controls the function of the original equipment relay replaced by smart relay control unit 230 and is actuated according to identity and command codes received by receiver 232 and processed by logic device 234. Prong 247 connects with the vehicle 12VDC circuit, prong 243 is ground and prong 248 makes normally open connection to the controlled function. When the base of NPN transistor 241 receives a actuation signal from logic device 234, prong 247 at its collector has electrical continuity to prong 248 at its emitter and current is conducted to the controlled function. Prong 245 provides for manually operated function control. FIG. 17 shows smart relay assembly 250 such as is controlled by encoded signals sent along the DC system wiring from smart relay control unit 230. Such relays are used to replace starter, fuel pump, ignition, headlight and other function relays, as desired. Smart relay assembly 250 comprises encoded signal receiver 252 which receives the encoded signal on the 12VDC wiring and operates according to the description of FIG. 13A, the code recognition logic control 186 of which further comprises logic device 254 and memory 256. Also included in the assembly are reduced voltage power supply 240 for a spike free internal 5V source and low voltage protection circuit 244, as are shown for FIG. 16. Base connector 259 is the same base pattern as the horn or light original equipment relay which is replaced. Prong 257 connects to the vehicle 12VDC circuit, prong 258 makes normally open connection to the controlled function, prong 253 is ground and prong 255 provides connection to the manually operated function control. As in FIG. 16, transistor 261 has its emitter and collector connected across function prong 258 and 12VDC prong 257 so as to conduct current to the controlled function upon a signal from logic device 254. The dedicated identity code sequence of master control unit 210 is stored in memory 236 of smart relay control unit 230 and memory 256 of each smart relay assembly 250 when the system is initialized. Subsequently, when an encoded RF signal is received by receiver 232, the 20 bit identity code sequence and 4 bit command code are transmitted by encoded signal transmitter 238 along the D.C. wiring in encoded bursts as described with reference to FIG. 12A, FIG. 12B, FIG. 12C and FIG. 12D. The code sequence is compared by logic devices 234 and 254 with the known identity code stored in memories 236 and 256 to confirm a valid transmission from master control unit 210. Affirmation that the 20 bit identity code sequence received agrees with the stored sequence in memory 236 authorizes logic devices 234 and 254 to initiate the indicated command. The 4 bit command code is compared to the known function codes which are preprogrammed in the logic device and the matching response is implemented. According to the programming for the command response, a command can call for one or several relay controlled functions. The security system "Arm-" command, for example, may be programmed to transmit appropriate signals to the other installed smart relays to disconnect the starter, ignition and fuel pump while tapping the horn and flashing the lights briefly twice to annunciate the "armed" mode. Relay assembly 270 is a physical arrangement, generally typical for either smart relay control unit 230 or smart relay 250. Base 272 includes connecting prongs 274, arranged in a pattern according to automotive industry standards for relay bases. Although only one prong pattern is depicted it will be understood that other industry standard prong patterns may be used to permit the smart relay of the present invention to replace any relay of any vehicle. Protective cover 277 encloses and seals circuit assembly 276 so the two parts are a subassembly unit. Gibs 273, engage with ways 275 to guide circuit assembly into engagement with base 272 so that contacts 271 make proper contact and locking lugs 278 engage with receiving slots 279 to maintain this contact. In FIG. 19 electrical activity sensor 242 is shown to be AC coupled to the vehicle 12V circuit by l.Oμf capacitor 280 and protected from spike overloads by diodes 281 and 282 ahead of input resistor 283. AND gate 285 and transistor 287 cooperate to maintain the gate threshold voltage of approximately 1/2 V cc at point A, where V cc is the 5V output of reduced voltage power supply 240. With V cc at B and just slightly more than threshold voltage at A, AND gate 285 passes V cc to the base of triode 287 through resistor 286. This takes the emitter of transistor 287 to ground so as to send an inverted, negative signal back to A through feedback resistor 284 and thereby put AND gate 285 in the passive state. This cycle instantly repeats, in oscillator-like fashion, sustaining a voltage level of approximately one-half V cc at point A. This voltage level includes a high frequency component of minor amplitude which is largely filtered out by O.lμf capacitor 288. Amplification of signal input perturbations is a characteristic of this feedback circuit. Gain, as in conventional operational amplifiers, varies directly with the value of feedback resistor 284 and inversely with that of input resistor 283. A slight voltage drop in the 12V battery circuit, such as resulting from the inrush current of an interior light, results in a small voltage perturbation passing through capacitor 280 and input resistor 283 to AND gate 285. This small perturbation is significantly amplified by the return through feedback resistor 284, in the same manner as an op-amp circuit . While the foregoing circuit is employed in this exemplary embodiment of an electrical activity circuit other circuits may also be used for this purpose without departing from the scope of the invention for economy and convenience, and such small voltage perturbations may be amplified by other amplification circuitry well known in the art. The amplified signal passes through coupling capacitor 290 to point D which is biased to maintain a voltage level slightly below the gate threshold by biasing resistors 291 and 292. Thus, even a small disturbance signal will exceed gate threshold voltage so as to trigger AND gate 295 and send the "electrical activity" message. FIG. 20 illustrates a smart relay assembly 300, an alternate embodiment of the present invention, where the mission may also include direct actuation of a secondary function rather than, or in addition to, control from a central source such as smart relay control unit 230 of FIG. 16. An example of a secondary function actuation is the operation of automobile headlights anytime the windshield wipers are in use. Not only is such headlight operation considered to be good practice for safe driving, but it is a legal requirement in some states. Smart relay assembly 300 replaces a conventional windshield wiper control relay and is normally operated under manual control input to prong 303. In a secured system, of the type previously discussed, smart relay assembly 300 includes encoded signal receiver 302 which receives the identity code signal on the 12VDC wiring 301 upon initial start-up. The identity code is transmitted to logic device 304 where it is stored and updated in memory 306. Also included in the assembly are reduced voltage power supply 240 for a spike free internal 5V source and low voltage protection circuit 244, as are shown for FIG. 16. Base connector 309 is the same base pattern as the windshield wiper relay which is replaced. Prong 307 connects to the vehicle 12VDC circuit, prong 308 makes normally open connection to the wiper motor, prong 305 is ground and prong 303 is connected to the manually operated controller. Transistor 311 has its emitter and collector connected across function prong 308 and 12VDC prong 307 so as to conduct current to the wiper motor upon an input signal from either the manually operated controller at prong 303 or from microprocessor 304 in response to a system generated command. A controller input signal is also received at logic device 304 wherein it triggers transmission by encoded signal transmitter 310 of an appropriately encoded function signal along the 12VDC circuit. The proper identity code has been stored and updated as required in memory 306 so that this signal is accepted at a headlight relay, such as smart relay 250 of FIG. 17, and acted upon in the same manner as a similarly coded signal from smart relay control unit 230 of FIG. 16. Smart relay assembly 300 is also suitable to work in applications where the identity discipline of a security system is not needed, by substituting a standardized protocol or, by simply deleting the identity code sequence. FIG. 21 illustrates yet another alternate embodiment as smart relay controller 320, where the mission is direct actuation of a secondary function in response to sensor input rather than through the programming of a central control such as smart relay control unit 230 of FIG. 16. An example of the use of direct sensor actuation is the operation of automobile headlights by a photo-electric cell sensor any time the level of ambient light falls below a set threshold. Such headlight operation is desirable in twilight driving or upon entering a tunnel, as a safety measure, where the light serves more to be seen than to illuminate the road. The use of other types of sensors, such as optical, capacitive or infra-red, as appropriate to the function, may also provide condition driven control of windshield wipers, fog or brake lights. Smart relay controller base 322 may plug into a cigarette lighter socket for connection of contact 323 to the 12VDC circuit 321 and of contact 325 to ground or an accessory socket may be added to the DC circuit as needed. Smart relay controller 320 includes encoded signal receiver 326 which receives the identity code signal on the 12VDC wiring 321 upon initial start-up. The identity code is transmitted to logic device 328 where it is stored and updated in memory 330. Also included in the assembly are reduced voltage power supply 240 for a substantially spike free internal 5V source and low voltage protection circuit 244, as is shown for example in FIG. 16. Sensor 332, which in this case is a photosensitive device, sends an input signal to logic device 328 when ambient light falls below a selected level. Such a signal triggers transmission by encoded signal transmitter 334 of an appropriately encoded function signal along the 12VDC circuit. The proper identity code has been stored and updated as required in memory 330 so that this signal is accepted at a headlight relay, such as smart relay 250 of FIG. 17, and acted upon in the same manner as a similarly coded signal from smart relay control unit 230 of FIG. 16. These and other applications of the basic master and slave unit intercommunications technology which is the subject matter of the present invention will be apparent to those skilled in the art, and the description of the exemplary embodiments disclosed above is intended to illustrate, and not to limit, the many possible embodiments of this invention which may be employed in an automobile, truck, tractor trailer rig, boat, recreational vehicle, trailer, private aircraft, or any other environment where direct current power is available and pre-wired harnesses may be used for distribution of intercommunication signals for control and telemetry, and for other uses. Many other modifications and embodiments may be implemented without departing from the spirit of the invention.

Claims: CLAIMS 1. Apparatus for transmitting and receiving telemetry and control signals superposed on a direct current circuit in the presence of ambient electrical noise, comprising: a transmitter including: means for generating a signal having a frequency in excess of about 10 megaHertz, means for selectively interrupting the continuity of generation of said signal so as to provide an encoded signal, and means for superposing said encoded signal on said direct current circuit; and a receiver including: means for differentiating and integrating said encoded signal to remove ambient background electrical noise therefrom, means for comparing said differentiated and integrated signal with a predetermined code sequence, and means for performing a function based upon said comparison. 2. Apparatus according to claim 1, said frequency being in excess of 100 megaHertz. 3. Apparatus according to claim 1, said frequency being in the VHF frequency range. 4. Apparatus according to claim 1, said frequency being in the UHF frequency range. 5. Apparatus according to claim 1, said means for generating said signal further comprising incidence frequency means for triggering said signal . 6. Apparatus according to claim 1, said means for generating said signal comprising: means for generating an incidence frequency which is at least approximately an order of magnitude lower than the frequency of said signal; and means for triggering said signal generating means to produce separated output waves therefrom, spaced on the incidence frequency baseline. 7. Apparatus according to claim 1, said means for generating said signal comprising: a tank circuit having a resonant frequency in excess of 10 megaHertz; an oscillator having a square wave output voltage, the frequency thereof being at least approximately one order of magnitude lower than said resonant frequency; and a switch having first and second switch connections and a switch control input, said square wave output frequency being connected to said switch control input, said tank circuit being i connected to said first switch connection its emitter and said second switch connection being grounded, whereby each square wave triggers a full wave output from said tank circuit at said resonant frequency. 8. Apparatus according to claim 7, said switch comprising a transistor. 9. Apparatus according to claim 1, said means for superposing said encoded signal further comprising: a tank circuit having a resonant frequency output in excess of 10 megaHertz, said tank circuit being connected to said direct current circuit; and means for interrupting the continuity of said resonant frequency output to carry an encoded signal. 10. Apparatus according to claim 1, said means for selectively interrupting the continuity of the generation of said signal comprising: incidence frequency means for triggering said waveform; and means for mixing an encoded sequence with said incidence frequency. 11. Apparatus according to claim 1 wherein said encoded signal is encoded in a binary code. 12. Apparatus according to claim 44, said differentiating and integrating means further comprising: means for differentiating the frequencies and noise on said direct current line to select frequencies in the range of said signal frequency so as to attenuate the relative level of frequencies and the ambient noise not in said range; and means for integrating the output of said differentiating means so as to reconstitute said encoded signal sequence. 13. Apparatus according to claim 1, said differentiating and integrating means further comprising: first means for differentiating the frequencies and noise on said direct current circuit to select frequencies in the range of said signal frequency so as to attenuate the relative level of frequencies and ambient noise not in said range; second means for differentiating the frequencies and noise in the output of said first differentiating means to select frequencies in the range of said incidence frequency so as to attenuate the relative level of frequencies and ambient noise not in said range; and means for integrating the output of said second differentiating means so as to reconstitute said encoded signal sequence. 14. Apparatus according to claim 1, further comprising means for repeating a succession of iterations of said encoded signal sequence to assure affirmation of an uncontaminated sequence. 15. Apparatus for encoding an encoded signal sequence to be superposed on a direct current circuit in the presence of ambient electrical noise comprising: means for generating a signal waveform, the effective frequency thereof being in excess of about 10 megaHertz; and means for selectively interrupting the continuity of said waveform so as to provide an encoded signal. 16. Apparatus for encoding an encoded signal sequence in accordance with claim 15, said means for selectively interrupting comprising: incidence frequency means for triggering said waveform at a frequency at least approximately an order of magnitude lower than said effective frequency; and means for mixing an encoded sequence with said incidence frequency so as to express said decoded sequence in bursts of waves of said effective frequency. 17. Apparatus for transmitting an encoded signal sequence in accordance with claim 15, said means for generating comprising a tank circuit having a resonant frequency output in excess of about 10 megaHertz, an I input of said tank circuit being connected to said incidence frequency means and an output of said tank circuit being connected to said direct current circuit. 18. Apparatus according to claim 15, said encoded signal is being encoded in a binary code. 19. Apparatus for transmitting an encoded signal sequence in accordance with claim 16, said means for mixing comprising is an and logic gate. 20. Apparatus for receiving an encoded signal sequence transmitted in bursts of waves of a predetermined frequency and superposed on a direct current circuit in the presence of ambient electrical noise comprising: means for coupling to said direct current circuit to block direct current voltage levels while passing alternating current signals; means for differentiating the frequencies and noise passing said coupling means to select frequencies in the range of said predetermined frequency so as to attenuate the relative level of background noise not in said range; and means for integrating the output of said differentiating means so as to recover said encoded signal sequence. 21. Apparatus according to claim 20, further comprising means for clamping an output of said coupling means to remove excess voltage spikes . 22. Apparatus according to claim 20, further comprising means for repeating a succession of iterations of said encoded signal sequence to assure reception of an uncontaminated sequence. 23. Apparatus according to claim 20, said differentiating means comprising a single stage band pass filter. 24. Apparatus according to claim 20, said differentiating means comprising two stages of band pass filters . 25. A function control relay for an electrical circuit comprising: an encoded signal receiver; a comparator, connected to the output of the receiver, for comparing an encoded signal received to a predetermined function code so as to enable the relay to execute a command; a relay for controlling an electrical function; and means for actuating said relay in response to a command. 26. A function control relay according to claim 25, said receiver comprising an radio frequency receiver. 27. A function control relay according to claim 25, said receiver comprising a DC code receiver. 28. A function control relay according to claim 25, said comparator comprising a logic device. 29. A function control relay according to claim 25, said comparator comprising a logic device with memory capability. 30. Apparatus according to claim 25, said encoded signal being encoded in a binary code. 31. A function control relay according to claim 25, further comprising: a relay base having a plurality of connectors arranged in a pattern conforming to a standardized automotive relay base pattern; and means for selectively making and breaking electrical continuity across a pair of said connectors as a function of an output of said comparator. 32. A function control relay according to claim 31, said means for selectively making and breaking comprising a transistor having an emitter and collector thereof connected across a pair of said connecting prongs. 33. A function control relay according to claim 25, further comprising, means for transmitting an encoded signal including: means for generating a voltage waveform having an effective frequency in excess of about 10 megaHertz; means for selectively interrupting the continuity of generation of said waveform so as to provide an encoded signal; and means for superposing said encoded signal on said direct current circuit. 34. A function control relay according to claim 33, said means for transmitting the encoded signal further comprising incidence frequency means for triggering said voltage waveform. 35. A function control relay according to claim 34, said means for transmitting the encoded signal further comprising: means for generating an incidence frequency which is at least approximately an order of magnitude lower than the effective frequency of said waveform; and means for triggering said effective frequency generating means to produce separated output waves therefrom, spaced on the incidence frequency baseline. 36. A function control relay according to claim 35, said means for transmitting the encoded signal further comprising: a tank circuit having a resonant frequency in excess of about 10 megaHertz; an oscillator having a square wave output voltage, the frequency thereof being at least approximately one order of magnitude lower than said resonant frequency; and a transistor receiving said square wave output frequency at its base, having said tank circuit connected to its emitter and having its collector grounded so that each square wave triggers a single full wave output from said tank circuit at said resonant frequency. 37. A function control relay according to claim 25, said amplifier further comprising: an amplifier circuit having input and output terminals; means for AC coupling said input terminal to the electrical circuit to pass any voltage perturbation therein; and a feedback circuit connecting the output of said amplifier circuit to the input thereof so as to return a signal thereto and amplify any voltage perturbation in the electrical circuit.

Inventors: MODGIL O S, MODGIL, O., S.
Applicants: NAT DIGITAL ELECTRONICS INC, NATIONAL DIGITAL ELECTRONICS, INC.
Application year: 1996
Main classification: H02J13
IPC codes: H02J13