Abstract:
A self timing test circuit switch powered by the same conductor as used to carry data may be used in vehicles where it is desirable to test and confirm the continuity of a critical circuit by illuminating for a brief period of time an indicator lamp after power-up of the vehicle. The switch may be used in sensor circuits where the sensor is a normally open switch and the normally open configuration can not otherwise be distinguished from an open circuit due to a wiring fault or a burned out bulb. Typical applications include checking the continuity of leads leading to metallic chip sensors in the engine and drive train oil or hydraulic fluid of aircraft engines.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to test switches/circuits used in conjunction with sensors which monitor the condition or status of important or critical functions in powered vehicles. In particular, the invention relates to a test switch/circuit which is used to indicate the status of the electrical conductors connecting to a remote sensor and is powered by the same conductors over which the sensor indication is provided. In a further embodiment the invention relates to a test switch/circuit used to verify the continuity of the conductor connection between a remote sensor and an alert indicator.  
           [0003]    2. Background  
           [0004]    In most powered vehicles such as automobiles, boats, and planes several sensors are used to monitor the status of various operating parameters to insure the safe and continued operation of the vehicle. Fuel, engine temperature, battery voltage, and oil pressure are typical operating parameters whose value or condition is presented to the vehicle operator. Sometimes an actual variable gauge is presented, while at other times warning lights are activated when the values for that parameter differ from established norms. The consequences of the failure of the monitoring systems are not uniform. A burned out bulb or broken wire in an automobile may leave a motorist stranded but not fatally injured. However, nowhere is the accuracy and dependability of such indicators more important than in flying aircraft.  
           [0005]    In addition to sensors for powered performance, aircraft need to monitor the condition of the engine and transmission, lubrication and hydraulic systems, safety latches on hatches, etc. Typical sensors provide an open circuit between two conductors under normal conditions and close the circuit permitting current to flow when a fault condition is sensed. Typically, for a variety of reasons including cost it is desirable not to provide a separate power source and associated conductors for such sensors. Additionally, doing so introduces additional points for failure including the power source and the conductors leading to the sensors. Thus, it is most desirable to provide power to the sensors over the same electrical conductors as are used to convey the sensor signal. However, the sensors are generally located at some distance from the cockpit and are connected to the status indicators in the cockpit by conductors which may run over great lengths through several bulkheads, conduits, terminal blocks, and connectors. A connection failure anywhere along the path from sensor to cockpit indicator provides an open circuit. In such a case, the sensor may properly close the circuit upon detection of an appropriate condition but such information never reaches the cockpit through the open circuit. Failure of the warning system conductors could lead to catastrophic results since a fault condition would not be distinguishable from an open circuit condition.  
           [0006]    For instance, engines and transmissions generally begin to wear internally before failure generating chips of metal which are picked up in the lubricating oil. Similarly, debris from both metallic wear and seal wear contaminate hydraulic systems. Thus, aircraft depend on sensors which detect contaminant particle accumulation in the lubricating or hydraulic systems. Many of these sensors use a magnet to draw metal particles out of the oil stream into contact with conductive electrodes on the face of the sensor. The sensor is normally placed in electrical series connection with a power source and an indicator lamp. One of the electrode contacts within the sensor is connected to ground either through a separate wire or by means of connection to the airplane chassis. The contacts within the sensor normally provide an open circuit so that no power is drawn through the indicator lamp. However, when sufficient metal particles have accumulated from the oil, the metal particles bridge the gap between the contacts to complete the circuit thereby permitting power to flow through the lamp to indicate to the pilots that trouble has developed. This type of normally open circuit is also known as a “switch to ground” circuit and is employed in many aircraft sensors in addition to engine and transmission oil and hydraulic system sensors. Upon detection of the appropriate parameter, the sensors complete a circuit which draws power through an indicator lamp in the cockpit or otherwise activates circuits which indicate the fault.  
           [0007]    To eliminate, or at least minimize, the chances that an open circuit would render a sensor signal inoperative, aircraft employ systems which check electrical continuity between critical sensors and the cockpit indicators each time the aircraft is powered up. Thus, when power is first switched on to the aircraft instruments before engine start-up, indicator lamps in the cockpit associated with critical functions are turned on for a interval of time sufficient for the pilots to notice the failure of any lamp to light. These lamps are typically turned on by test means which connect the power and ground conductors of the lamp circuit at or very near the associated sensor; that is, the test means provides an alternate current path (short) in parallel with the sensor which completes the series circuit. After a predetermined time, the test means ceases to short the circuit and the indicator lamp will go off unless the sensor itself completes the circuit.  
           [0008]    As indicated above, generally it is not desirable to power such short circuit test means by use of additional wires (conductors) in an aircraft since adding such wiring to an aircraft is expensive and is itself subject to open circuit problems in the same manner as the sensor conductors. Thus, the short circuit test means are generally powered from the same conductor that supplies power (voltage) to the sensor. This requires that power be drawn from the sensor conductor for operation of the test means. One problem which is encountered is that drawing much power to power the test means from the sensor circuit during the test period reduces the voltage or current available to power the indicator lamp. A dim lamp is less easily detected under bright cockpit conditions. In other undisclosed embodiments of the invention, where additional functionality has been added, available power may be diminished to power the lamp.  
           [0009]    3. Description of Related Art  
           [0010]    One test means which is in common usage in aircraft is described by Berrier et al. in U.S. Pat. No. 5,045,840. Berrier teaches the use of a clock timer and sequencer in conjunction with a pulsing or intermittent switch which is placed in parallel with the sensor contacts. Closure of the switch across the leads shorts the sensor and permits current to flow through the power line turning on the lamp in the cockpit. The clock timer and sequencer control the duration of the test. Power is drawn from the sensor supply to power the clock timer and sequencer, but to avoid drawing too much power and dimming the lamp, the switch is not closed continuously but rather is opened and closed with a long duty cycle of approximately 90%. In the preferred embodiment the shorting switch is a MOS-FET transistor and is closed for 29 milliseconds each cycle and open for 1 millisecond each cycle. During the open time sufficient power is stored in a power supply capacitor to operate the circuit during the closed part of the cycle. Thus, in Berrier&#39;s circuit the current through the lamp is pulsed, but at a rate imperceptible to the human eye. In addition, there is some diminution in the brightness of the indicator lamp since power is not being continuously provided to it. In addition, the continual opening and closing of the switch and abrupt power surges has the undesirable side effect of generating broadband electro-magnetic interference (EMI), high frequency signals associated with the Fourier transform of the voltage surge delta function each time the switch is opened or closed rapidly and repeatedly.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    The invention described in this patent document provides a means for monitoring a piece of equipment such as an aircraft engine or transmission. More specifically, the present invention provides a means of testing the continuity of an electrical connection between an indicator and a remote sensor upon application of power to the circuit. The continuity test activates the indicator for a predetermined length of time if there is continuity of the conductor. After the completion of the desired test time, the test circuit no longer functions. The indicator lamp is not pulsed during the test time and full current is provided to the lamp generating its designed brightness. Further, the circuit across the sensor leads is not repetitively opened and closed and, therefore, no high frequency EMI is generated.  
           [0012]    The switch/circuit of the invention works in conjunction with monitoring circuits typically found in aircraft. Among others, the cockpit monitoring circuit may consist of an alarm lamp in series with a direct current power supply, or a logic assembly, or it may be formed from an operational amplifier configured as a voltage comparator. When used with an alarm lamp, the current drawn through the sensor directly powers and illuminates the lamp. When used with a logic circuit or voltage comparator, a low voltage on the sensor line causes the monitoring circuit to light a panel lamp. In the case of a voltage comparator, the comparator compares a reference voltage at a first input to the voltage at a second input which depends on the status of the sensor. If the voltage at the second input is lower than the reference voltage, the comparator provides an output signal which can be used to power a warning lamp illumination circuit. If the voltage at the second input is equal to or higher than the reference voltage, no output signal is provided and the lamp illumination circuit is not activated.  
           [0013]    In a circuit using an operational amplifier voltage comparator monitoring circuit, the second input is connected to one side of a normally open sensor and a series resistor to supply voltage. The voltage at the second input is determined by whether the sensor is open or closed. In the open position, the sensor does not connect the input to ground, and the voltage remains high. In the closed position, the sensor connects the second input to ground, driving down the voltage, which in turn causes the comparator to activate the lamp illumination circuit.  
           [0014]    The test switch/circuit of this invention is also connected to the second input through the same conductor as the sensor. Upon power up, the oscillator part of the test switch/circuit is activated and appears as a short circuit (current path) to ground. Since the oscillator circuit serves the function of a switch by conducting current to ground, this patent document characterizes the invention by use of the term switch/circuit. If an indicator lamp is in series with the test circuit, current flows through the lamp and the test circuit to ground thereby illuminating the lamp. If a voltage comparator is used with the test circuit, the oscillator short to ground drives down the voltage at the second input and causes the comparator to activate the lamp illumination circuit. The test switch/circuit contains a timing function which turns off the oscillator circuit after a predetermined time so that it no longer appears as a short circuit (current path) to ground, voltage at the second input rises and the comparator no longer activates the lamp illumination circuit.  
           [0015]    In a further embodiment, wherein the timing function is eliminated, the oscillator part of the circuit responsible for appearing as a short circuit (current path) to ground may also be used as an electronic switch in conjunction with a mechanical connection which moves the ferromagnetic core of a transformer in the oscillator circuit thereby decoupling the primary and secondary windings of the transformer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0016]    [0016]FIG. 1 is an electrical schematic showing the test circuit switch of the present invention used with a series indicator lamp and a first embodiment of a timing function.  
         [0017]    [0017]FIG. 2 shows the waveform generated by the oscillator subcircuit shown in FIG. 1.  
         [0018]    [0018]FIG. 3 is an electrical schematic showing the test circuit switch of the present invention used with a voltage comparator and a first embodiment of a timing function.  
         [0019]    [0019]FIG. 4 is an electrical schematic showing the test circuit switch of the present invention used with a series indicator lamp and the preferred embodiment of a timing function.  
         [0020]    [0020]FIG. 5 is an electrical schematic showing the test circuit switch of the present invention used with a voltage comparator and the preferred embodiment of a timing function.  
         [0021]    [0021]FIG. 6 is an electrical schematic showing the oscillator circuit used as a switch with a mechanical connection controlling the displacement of the transformer ferromagnetic core. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    As shown in FIG. 1, an indicator lamp B 1  in an electrical series circuit through conductor  4  to one side of chip sensor  5  located in the lubricating oil of an engine or power train (not shown) or a hydraulic system. Electrical power is supplied to the circuit when aircraft instruments are switched on at power-up. Sensor  5  contains a magnet  7  which draws metallic chips to contacts  6 . One plate of sensor  5  is connected to conductor  4  while the other plate is connected to ground. Examples of this sensor type are disclosed in U.S. Pat. Nos. 3,432,750, 3,753,442 and 4,070,660. In other undisclosed embodiments of the invention, a second sensor  8  may also be connected to conductor  4  to provide indications of other functions such as loss of oil/hydraulic fluid. A direct current voltage source V IN  is connected to one side of lamp B 1 .  
         [0023]    The test switch/circuit is electrically connected to conductor  4  at point  9  which is located in close physical proximity to sensor  5 . The test circuit has a step up transformer  10  with two primary coils P 1  and P 2 , and a secondary coil S. Each primary coil has M turns and the secondary has N turns. Primary coil P 1  is connected to the drain of MOS-FET transistor T 1  and primary coil P 2  is connected to the drain of MOS-FET transistor T 2 . The sources of transistors T 1  and T 2  are connected to ground. The two ends of the secondary coil are connected to the gates of T 1  and T 2 . Bias resistor R 2  connects conductor  4  directly to the gate of transistor T 2  and the gate of T 1  through the secondary winding S. Diodes D 1  and D 2  are connected to the gates of T 1  and T 2  and the common anodes are connected to ground. Zener diode Z 1  is connected to the gate of T 1  and zener diode Z 2  is connected to the gate of T 2 . The two Zener diode cathodes are common and connected to line  11  which is in series with resistor R 4  to the drain of MOS-FET transistor T 3 . The source of transistor T 3  is connected to ground. The gate of transistor T 3  is connected to the output of counter timer IC 1 .  
         [0024]    A current limiting resistor R 3  connects bias resistor R 2  to the clock input of counter timer IC 1 . Diode D 3  connects current limiting resistor R 3  to one input  12  of capacitor C 1 , the power input of counter timer IC 1 , and the power input of voltage monitor IC 2 . The other side of capacitor C 1  is connected to ground as is the ground input of counter timer IC 1 . Voltage monitor IC 2  is connected across capacitor C 1 . The output of voltage monitor IC 2  is connected to the reset input of counter timer IC 1 . Counter timer IC 1  has several selection leads indicated at “SELECT” which may be either open or shorted to ground.  
         [0025]    The operation of the monitoring part of the switch circuit of the invention will now be described assuming that the test switch/circuit starting at point  9  is not connected. Conductor  4  is not grounded provided that metallic chips do not bridge the contacts  6  in sensor  5 . Upon application of power by a switch means to the aircraft instruments at aircraft start-up, V IN  is applied to one side of lamp B 1 . Since there is no complete circuit to ground, no current flows and the lamp is not illuminated. If metallic chips are present across the contacts  6  of sensor  5 , conductor  4  is connected by sensor  5  to ground. In this case current flows through lamp B 1  and the lamp will stay on continuously. However, if the conductor to the sensor is open, closure across the contacts of sensor  5  will never turn on the lamp.  
         [0026]    The nominal values of the voltages and components used in the first embodiment are as follows:  
         [0027]    V IN =28 volts  
         [0028]    B 1 =standard panel bulb 327 or 387  
         [0029]    R 2 =221 K  
         [0030]    R 3 =7.5K  
         [0031]    R 4 =1 K  
         [0032]    C 1 =0.1 μF  
         [0033]    D 1 , D 2 , D 3 =silicon diodes  
         [0034]    Z 1  and Z 2 =1N965B or 1N4744 zener diode (15 volt)  
         [0035]    T 1  and T 2 =MOSFET 2N7000 or 2N7002  
         [0036]    IC 1 =IC MC14536, multi stage binary counter  
         [0037]    IC 2 =IC MAX6376, voltage monitor  
         [0038]    Transformer=small ferrite core transformer  
         [0039]    To test continuity of the conductor leading to the sensors, the operation of the test switch/circuit of the invention will now be described assuming that the switch/circuit starting at point  9  is connected. Initially, it should be recognized that if metallic chips bridge the gap between the contacts  6  in sensor  5 , the monitoring circuit will behave as previously indicated and the presence of the switch/circuit will have no effect. In this case, since conductor  4  will be at essentially zero voltage, no effective voltage will be applied to activate the switch/circuit. Thus, even in the presence of the test switch/circuit, the panel lamp will illuminate continuously to indicate the presence of metallic chips in the oil. However, if no chips are present across contacts  6 , the switch/circuit of the invention operates as follows.  
         [0040]    Transformer  10 , transistors T 1  and T 2 , diodes D 1  and D 2 , zener diodes Z 1  and Z 2 , and bias resistor R 2  comprise a self-powered astable low power oscillator. Upon the application of power to the aircraft instruments at aircraft start-up (the application of V IN  to the circuit), the voltage rises on conductor  4  and is applied to the dual primary windings P 1  and P 2  of transformer  10  and through them to the drain of MOSFET transistors T 1  and T 2 . Simultaneously voltage is applied to the gate of transistor T 2  through bias resistor R 2 . Initially the low power oscillator is off until the voltage on conductor  4  reaches several volts when the voltage provided by bias resistor R 2  becomes sufficient to turn on transistor T 2 . Transistor T 2  then draws current through primary winding P 2  directly to ground. At this point, the low power oscillator starts up and runs generating a periodic waveform of approximately 75 KHz. The voltage of the wave form is a function of the turns ratio N/M of the transformer primary and secondary coils. In the preferred embodiment M is 2 and N is 200 yielding a 100:1 voltage step up. Typically the peak to peak voltage will be several volts. The waveform output of the secondary at point  25  is shown in FIG. 2. Diodes D 1  and D 2  provide a return path for the drive current from the alternating waveform out of the transformer secondary S into transistor switches T 1  and T 2 . Depending on the resistance characteristics of lamp B 1 , a current sufficient to burn out transistors T 1  and T 2  may flow through the oscillator circuit to ground. Zener diodes Z 1  and Z 2  provide over-voltage burn out protection to switch transistors T 1  and T 2 . As will be seen, loading resistor R 4  and switch transistor T 3  are used to turn off the low power oscillator.  
         [0041]    When configured in the embodiment with the values of the components set forth above, the oscillator circuit has the unique characteristic of operating with an input voltage drop of less than 200 millivolts across the primary windings P 1  and P 2  from conductor  4  at point  9  to ground. While the oscillator runs, the lamp is turned on indicating that there is continuity in the sensor conductor circuit up to point  9 .  
         [0042]    As indicated above, it is necessary to turn the lamp in the cockpit off after a predetermined time. In a first embodiment of the circuit of the present invention, timing and turning off of the lamp is accomplished as follows. The low power oscillator alternating waveform is used as a clocking signal and time reference by applying it through current limiting resistor R 3  to the clock input of integrated circuit counter timer IC 1 . Integrated circuit counter timer IC 1  is a multi-stage binary counter timer. One of several counter output lines of IC 1  can be selected and connected to the output pin of IC 1  by the four wire jumper connections indicated at “SELECT”. The alternating waveform frequency and the value of the counter used to end the timer cycle set the duration time during which the oscillator runs and the panel lamp is illuminated.  
         [0043]    Power is supplied to IC 1  both through bias resistor R 2  and through the alternating waveform output of the oscillator. The alternating oscillator output is rectified by diode D 3  and applied to the power input of IC 1 , the power input of voltage monitor IC 2 , and capacitor C 1 . Capacitor C 1  is slowly charged up and provides even power to IC 1 . Voltage monitor IC 2  is used to generate a “clear reset” signal into counter IC 1  during power up until IC 1  is ready to count clock pulses and begin the timing cycle. When the voltage across capacitor C 1  increases above 3.5 volts, the reset signal from IC 2  ends which allows counter IC 1  to begin counting the clock pulses and begin the timing cycle.  
         [0044]    When the number of pulses determined by the selection of the jumper outputs has been reached, the output of IC 1  goes high. The high signal applied to the gate of transistor T 3  connects the gates of transistors T 1  and T 2  to ground through resistor R 4  and zener diodes Z 1  and Z 2  thereby turning off the oscillator. Once the oscillator is turned off, it no longer acts like a short to ground of conductor  4  and no current flows through lamp B 1 . The output of IC 1  stays high as long as the power V IN  is supplied to the system. Thus the switch/circuit of this invention stays deactivated until the aircraft system is powered down and up again. In this manner, power supplied in the sensor circuit powers the continuity testing cycle, the continuity of conductor  4  to a point  9  adjacent to sensor  5  is checked each time the system is powered up, and the system is returned to a state where only a short across contacts  6  in the sensor will activate the panel lamp.  
         [0045]    When used with a voltage comparator panel indicating circuit, the test circuit switch of this invention behaves as follows. FIG. 3 shows such a voltage comparator circuit. Again, the operation of the monitoring part of the switch/circuit of the invention will first be described assuming that the switch/circuit starting at point  9  is not connected. The nominal values of additional voltages and components are as follows:  
         [0046]    V REF =1.0 volts  
         [0047]    R 1 =3.7 K  
         [0048]    A 1 =IC LM393 (standard voltage comparator)  
         [0049]    By design V REF  is set very low with respect to V IN . In this configuration conductor  4  is not grounded provided that metallic chips do not bridge the contacts  6  in sensor  5 . Upon application of power at V IN , voltage V IN  is applied to input  2  of comparator A 1 , and, since it is substantially greater than V REF  no signal is produced at the output  3  of comparator A 1  to activate the lamp illumination circuit. If metallic chips are present across the contacts  6  of sensor  5 , conductor  4  is connected by sensor  5  to ground. In this case current flows through resistor R 1  causing the voltage at input  2  of comparator A 1  to drop to near zero. Comparator A 1  now sees a lower voltage on input  2  than on input  1  and provides an output signal at  3  to activate a lamp illuminating circuit and the lamp will stay on continuously.  
         [0050]    Again, it should be recognized that if chips are present across contacts  6  of sensor  5  the presence of the test circuit will have no effect and the panel lamp will stay illuminated. However, if no chips are present across contacts  6 , the switch of the invention operates as follows. The oscillator behaves upon power-up as previously described. Provided V REF  is set higher than the approximately 200 millivolt drop across the oscillator from conductor  4  to ground, the oscillator switch/circuit looks like a short circuit to comparator A 1 . The voltage at comparator input  2  is below V REF  at input  1  so that the comparator produces an output at  3  which activates the lamp illumination circuit. Thus, while the oscillator runs, the lamp is turned on, again indicating that there is continuity in the sensor circuit up to point  9 . The timing function also works as previously described. Once the oscillator is turned off, it no longer acts like a short to ground of conductor  4 . The voltage at input  2  of comparator A 1  rises above the 1.0 volt reference voltage and the output of comparator A 1  at  3  no longer activates the lamp illumination circuit. Since no appreciable amount of current is drawn through the oscillator in this configuration, there is little likelihood of transistors T 1  and T 2  burning out. Accordingly, zener diodes Z 1  and Z 2  are not needed and may be removed from the circuit. However, prudent design to protect against accidental overvoltage on conductor  4  would keep the zener diodes in the circuit.  
         [0051]    In a preferred embodiment, the internal oscillator frequency of IC 1  is used as a basis for counting to determine the duration of the test cycle instead of the frequency of the waveform output of the oscillator. FIG. 4 shows the preferred timing circuit used with a series connected panel lamp. Upon the application of V IN  the oscillator circuit acts as previously described. Current flows through lamp B 1  and the oscillator to ground. Once illuminated, the lamp will stay on until the oscillator is turned off. Power is again provided to IC 1  and voltage monitor IC 2  through resistor R 3  and diode D 3 . Additional power may be provided through resistor R 5  and diode D 4 . The input voltage to IC 1  and IC 2  is regulated by zener diode Z 3  and capacitor C 2  which are connected between the power input conductor and ground. As in the first embodiment, voltage monitor IC 2  is used to generate a “clear reset” signal into counter IC 1  during power-up until IC 1  is ready to count the internal oscillator frequency and begin the timing cycle. The internal oscillator frequency is determined by the values of C 3 , R 6 , and R 7 . The output of the internal oscillator is counted until a predetermined value set by the select lines is reached. When the timed value is reached, the output of IC 1  goes high and causes transistor T 3  to connect the gates of transistors T 1  and T 2  to ground thereby turning off the oscillator. No current can then flow through lamp B 1 , and it is no longer illuminated as the end of the timed interval has been reached. The nominal values of additional voltages and components are as follows:  
         [0052]    R 5 =20K ohms  
         [0053]    R 6 =1 Megohm  
         [0054]    R 7 =1 Megohm  
         [0055]    D 4 =silicon diode  
         [0056]    Z 3 =MMSZ4690 zener diode (5.6 volt)  
         [0057]    C 3 =200 pF capacitor  
         [0058]    [0058]FIG. 5 shows the preferred timing circuit used in conjunction with a voltage comparator illumination circuit. The comparator circuit operates as described above for the first timing circuit, the difference being the use of the internal oscillator of IC 1  to determine the time during which the oscillator will run. Generally, it is desirable for the panel lamp to stay illuminated for several seconds after aircraft instrument power-up to provide sufficient time for the aircraft pilots to note that the lamps are on to confirm continuity of the conductor to point  9 .  
         [0059]    Since the switch/circuit oscillator of this invention provides essentially a constant short circuit (current path) to ground while it is running, the lamp illumination circuit, whether consisting of just a lamp bulb, a logic circuit, a voltage comparator, or some other responsive circuit is constantly operated until the oscillator turns off after a predetermined time. This means that the lamp, when turned on, is provided with a constant voltage and shines at its designed brightness. Also, no repetitive switched voltage pulsing with its associated broadband EMI is generated by the switch/circuit of this invention. In vehicles and especially planes which are not constructed of electrically conducting materials, the ground to the sensor and test switch/circuit can be provided by a second conductor. If either the power conductor or the ground conductor is broken or interrupted, the panel light will not illuminate on power-up thereby indicating a fault which should be resolved before the vehicle is used. As mentioned earlier, different monitoring circuits are used in aircraft and may be employed with the test switch of this invention. All that is necessary is that the monitoring circuit be responsive to a voltage drop approximating a short circuit on the power conductor connected to the sensor.  
         [0060]    It should be clear from the description above, that the oscillator circuit can be used as a switch in other applications where a reliable electronic switch is required. FIG. 6 shows one such application where the timing circuit has been eliminated. As long as there is magnetic flux coupling between primary windings P 1 , P 2 , and secondary winding S, so that current flow in the primaries induces a voltage in the secondary, the oscillator circuit will turn on as described above when power is applied to conductor  4 . However, if there is no magnetic flux coupling between primary windings P 1 , P 2 , and secondary winding S, current flow in the primaries will not induce a voltage in the secondary and the oscillator will not turn on and appear as a short circuit (current path). Removal of the ferromagnetic core  12  from transformer  10  can sufficiently decouple the primary and secondary windings. In a preferred embodiment, the ferromagnetic core  12  is attached to a mechanical activator  13  in a sensor which moves in accordance with the desired status of the sensor.  
         [0061]    For instance, such a sensor may be designed to sense the presence of air pressure at a given level. In the normally open position of the sensor, the transformer core  12  attached to activator  13  is withdrawn from transformer  10 . Even with power applied to conductor  4 , the oscillator can not turn on since no magnetic flux connects the primary and secondary coils. However, if the pressure changes, the sensor responds causing the transformer coupling to increase between the primary and secondary windings of the transformer. In the embodiment shown, this is achieved by inserting core  12  into transformer  10 . The oscillator circuit turns on and appears as a short circuit (current path) permitting current to flow and turning on the panel lamp as described above or activates other appropriate circuits.  
         [0062]    Although this invention has been described and illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made which clearly fall within the scope of this invention. The present invention is intended to be protected broadly within the spirit and scope of the appended claims.