Abstract:
A keyless access sensor system for use with a keyless access control mechanism (KACM) is described for controlling the operation of a locking device. The KACM receives a signal from a sensor device for keyless access to create a first output signal before the user has begun any action on the handle in order to open the door. The first output signal is sent to a general processor, which initiates a recognition process and, after recognition of the authorized user the processor then generates an unlocking signal which unlocks the locking device before the authorized user will have fully accomplished the action of opening the door. Thus the authorized user is allowed to open the door without any specific un-ergonomic and time-consuming additional action to the simple action of actuating the handle to open the door. The second signal is generated by a device, such as a fob, card or the like, carrying a unique digital or analog identification in response to RF or IR interrogation from the general processor after it receives the output signal from the sensor device for keyless access.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International application PCT/GB01/02919, entitled “Keyless Access Sensor System” filed Jun. 29, 2001 and published as International Publication No. WO 02/02893 A1, the entire content of which is expressly incorporated herein by reference thereto. 

   BACKGROUND 
   The present invention relates to a keyless access sensor system and its associated sensor device for keyless access particularly, but not exclusively, for use in allowing access by an authorized user to a vehicle, building or the like. The invention also relates to a method of using a keyless access sensor system to control entry of authorized persons and to a circuit for processing signals in a keyless access sensor system. 
   It is important, for many reasons, to control access to premises, vehicles and personal property so that only authorized users are allowed access. Typically this is done using keys which fit a lock to allow the user of the key to open the lock and gain entry. One problem with the existing key and lock arrangements is that loss or damage to the key can render access impossible. In addition, if the key lock itself is blocked or damaged this can also prevent access. One other problem is that the use of a key requires a specific action such as unlocking a door latch with the key from the authorized person before an action of opening the door associated with the door latch. This specific action is very often not easy to accomplish, is not ergonomic and is time-consuming. 
   A number of solutions have been proposed to try to overcome these disadvantages. With security devices for cars, it is well known that a keyless fob can be used, such that actuation of a button on the fob generates an infrared (IR) or radio frequency (RF) signal which is detected by a sensor in the vehicle which unlocks the doors. A key is still required by the user in order to operate the ignition system. The fob also contains a lock button which generates a similar IR or RF signal to lock the vehicle. Such vehicle keyless access systems have been known for a number of years. Such systems operate on the basis that when the IR or RF “open” signal is generated by the fob, the signal is used to actuate a mechanism which unlocks the car door so that when the user pulls on the handle, the door is already unlocked. Similar arrangements may be used for building entry. 
   One problem with this arrangement is that the user still has to initiate a specific action such as, in the case of a fob, taking the fob in his hand and pressing on the fob button, or in the case of a magnetic card or the like, inserting the card in a slot or to present it in front of a card reader/detector or the like, in order to unlock the door and have access to the vehicle, these specific actions being time-consuming and not ergonomic. 
   One other problem with this arrangement is that if the user decides not to enter the vehicle but forgets to actuate the “lock” signal, the car and/or building remains open and is thus vulnerable. In addition, with existing keyless locking systems, particularly for vehicles, a conventional locking mechanism is used which is susceptible to interference by thieves to gain access to the car. For buildings, conventional locks are actuated in the same way and are susceptible to the same procedures by intruders to gain access to the premises. 
   It is desirable to provide a system which obviates or mitigates at least one of the above mentioned problems, and this is now provided by the present invention. 
   SUMMARY OF THE INVENTION 
   The desired features are achieved by providing a keyless access sensor system for use with a keyless access control mechanism (KACM) for controlling the operation of a locking device without any specific action from the user. The KACM receives a signal from a sensor device for keyless access to create a first output signal before the user has begun any action on the handle in order to open the door. The first output signal is sent to a general processor, which initiates a recognition process and, after recognition of the authorized user the general processor then generates an unlocking signal which unlocks the locking device before the authorized user will have fully accomplished the action of opening the door. Thus the authorized user is allowed to open the door without any specific un-ergonomic and time-consuming additional action to the simple action of actuating the handle to open the door. A signal is generated by a device, such as a fob, card or the like, carrying a unique digital or analog identification in response to RF or IR interrogation from the general processor after it receives the output signal from the sensor device for keyless access. In response to the unlocking signal, the locking device is opened for a predetermined time allowing a user entry to a car or building premises or the like. 
   The sensor device for keyless access generates a primary beam of electromagnetic radiation, particularly in the optical wavelength range and, more particularly, it is a pulsed beam, this beam being located near a door handle. In the case of a vehicle, the beam is located between the door panel and the inside of the handle. Alternatively, the beam is located between the two extremities of the handle and parallel to the door panel in order to detect and anticipate any action of opening the door made by the user. When a user inserts his hand to fully or partially interrupt or reflect the beam after the system is primed, the system detects this modification of the beam characteristics and generates the output signal which is used in anticipation with the user ID to create a control signal to unlock or open the door before any action on the door handle. The sensor device for keyless access may include a backup switch which will provide a signal to the general processor in case the modification of the primary beam characteristics due to the presence of the hand is not detected by the sensor system for whatever reason. This backup switch will be activated by the mechanical action of the user on the door handle in order to open the door. The signal issued from the backup switch will then initiate the user ID sequence and will then allow the unlocking of the door with a delay due to the lack of anticipation in the detection of the action of opening the door by the user. The backup switch may be a mechanical switch or an optical switch or the like. The sensor device for keyless access device may also include a locking switch, which purpose is to cause locking of the door when this locking switch is actuated by the user when he exits the door. In the case of a vehicle the locking switch is locatable on the handle for easy actuation by the user. 
   In the preferred arrangement, an incident beam is an infrared beam generated by a light emitting device (LED) and is detected by an optical sensing element. After the user inserts his hand to fully or partially interrupt or reflect the beam, a signal processing circuit detects when the interruption or modification of the beam of optical pulses lasts longer than a predetermined time and then generates the output signal to the general processor. 
   In the preferred arrangement, the sensor device for keyless access is a low power consumption sensor based on smart monitoring of the internal electrical function of the sensor in order to reduce to minimize the overall sensor electrical consumption. 
   In the preferred arrangement, the sensor device for keyless access is ambient light protected by measuring the level of the ambient light before producing any pulse of the optical beam, in a way which protects the sensor against any external parasitic optical light. 
   Conveniently, the access multi-sensor device includes an optical adaptive feedback arrangement which prevents the sensor from false detection which may be caused by slow variation of the optical beam characteristics due to, for example, the accumulation of dust or deterioration on the sensor external surface, the variation of electro-optical characteristic of the light emitting device or the variation of the optical sensing element during the sensor&#39;s lifetime. 
   With this arrangement a traditional key lock is not required and, consequently, it is not vulnerable to illegal entry in the same way as traditional locks. When the system is applied to vehicles, the user has no specific manual action to perform to unlock the vehicle, thus improving the ergonomics and access time to the vehicle. The main requirement is a handle or the like, a beam and an access control mechanism which generates a beam of electromagnetic radiation between the handle and the door or between the two extremities of the handle parallel to the door panel so that the beam can be fully or partially interrupted or reflected by a user, for example, when the user inserts his hand between the handle and the door. Such a beam may be modified by other means, such as a card or the like swiped through a slot to generate a-control signal for controlling a locking mechanism. 
   A particular advantage of this arrangement for use with vehicles is the low power consumption of the sensor circuit, especially in the standby mode. This low power consumption is obtained by having an ultra low consumption sensor device for keyless access and by having the general processor in a standby mode when the car is parked. When the vehicle is parked, the device is ‘woken up’ by a user interrupting or modifying the beam characteristics and only then does the general processor wake up from its standby mode and cause a RF or IR beam to be generated to verify the user ID. Thus, the RF beam is only generated in response to an access request thereby minimizing power consumption. 
   Another particular advantage of this arrangement for the use by vehicles is that it will still be fully functional even in harsh environments due to bright artificial lights in towns by night, or high temperature or presence of dust on the car, or the like. This functionality is provided by the optical adaptive feedback system and the ambient light protection function of the sensor device. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     These and other aspects of the present invention will become apparent from the following description, when taken in combination with the accompanying drawings, in which: 
       FIG. 1  is an exploded view of a car door handle assembly incorporating a sensor system in accordance with a first embodiment of the present invention; 
       FIG. 2  depicts an assembled and partly cut-away view of the car door handle assembly of  FIG. 1  incorporating the sensor system in accordance with the first embodiment of the present invention; 
       FIG. 3  is a perspective view of an assembled sensor unit as shown in the drawings of  FIGS. 1 and 2 ; 
       FIG. 4  depicts an exploded view of the sensor unit shown in  FIG. 3 ; 
       FIG. 5  is a general block diagram of the sensor device used in  FIGS. 1 to 4 ; 
       FIG. 6  is a circuit diagram of the sensor device used in  FIGS. 1 to 4 ; 
       FIGS. 7   a  to  7   j  depict timing diagrams of signals used to control the operation of the circuit of  FIG. 6  and waveform diagrams depicting signals at various parts of the circuit of  FIG. 6 ; 
       FIG. 8  depicts a handle assembly similar to that shown in  FIG. 2  but using a sensor device in accordance with an alternative embodiment of the present invention, and 
       FIGS. 9   a ,  9   b  and  10  show further embodiments of sensor devices in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   According to one aspect of the present invention, there is provided a sensor system for use with a keyless access control system, the sensor system comprising: 
   an electromagnetic radiation generating element for generating an incident beam of electromagnetic radiation in the form of a pulse train; 
   an electromagnetic sensing element for sensing the incident beam, and 
   a signal processor coupled to the sensing element for detecting an interruption to, or modification of, the incident beam, the signal processor including a timer for detecting when the duration of the interruption or modification of the incident beam is greater than a predetermined by detecting the presence of absence of a predetermined number of pulses varying from a predetermined level, the signal processor for providing an output signal to an access control mechanism when the presence of absence of a predetermined number of pulses are counted. 
   Preferably, the system includes a backup switch for sensing a mechanical opening action of the access control mechanism. 
   Preferably, the absence of a predetermined number of pulses less than a preset level results in the output signal being generated. 
   Alternatively, the presence of a predetermined number of pulses greater than a preset level results in the output signal being generated. 
   Preferably, the sensing element is disposed adjacent to the electromagnetic radiation generating element for detecting a partial or total interruption or modification of the incident beam. 
   Preferably also, the system includes an optional locking switch for manually locking the access control mechanism. 
   Conveniently, the optional backup switch is an optical switch and the optional locking switch is an optical switch. 
   Preferably, the electromagnetic radiation generating element generates an incident beam of optical radiation. Conveniently, the incident beam is an infrared beam. Conveniently, the wavelength is between 780 and 950 nanometers. 
   According to a further aspect of the present invention, there is provided a method of providing keyless access to a locked device or structure, the method comprising the steps of: 
   generating an incident beam of electromagnetic radiation, the incident beam being a pulse train, 
   sensing the incident beam of electromagnetic radiation, 
   sensing an partial or total interruption or modification to the incident beam lasting longer than a predetermined timed by detecting the presence or absence of a predetermined number of pulses varying from a predetermined level, and 
   generating an output control signal when the predetermined number of pulses are counted as the result of the partial or total interruption or modification, and processing the generated control signal to produce an actuation signal for opening the access mechanism. 
   Preferably, the method includes the step of generating a backup interruption signal as a result of a mechanical action on the handle of the access mechanism, and processing the generated interruption signal to produce an output control signal for unlocking or opening the access mechanism. 
   Preferably, the method includes the step of generating a locking signal as a result of an action on the locking switch. 
   According to a further aspect of the present invention, there is provided a circuit for use in an electromagnetic radiation sensing system, the circuit comprising: 
   a circuit power supply regulator; 
   an output stage with an optical source for emitting pulses of electromagnetic radiation of a predetermined duration; 
   a sensing and amplification stage for detecting pulses emitted by the optical source; 
   a timing circuit coupled to the power supply regulator for generating timing signals and an internal power supply, the timing signals and the internal timing power supply being fed to the amplification stage and to the output stage for synchronizing the emission and detecting of light pulses varying from a predetermined level, and a pulse counter for counting the pulses, the pulse counter generating an output signal in response to a predetermined number of pulses being counted. 
   Preferably also, the timing signals are also used to detect and remove ambient light noise. 
   Preferably, the circuitry is partially or totally realized in a monolithic ASIC (Application Specific Integrated Circuit). 
   Preferably, the ASIC includes the optical sensing element. 
   According to a further aspect of the invention there is provided a sensor device for use with a keyless access control mechanism, the sensor device comprising: 
   a post for incorporation into one end of a door handle; 
   an electromagnetic radiation emitter and receiver located in the post for generating an incident beam of electromagnetic radiation substantially parallel to the handle, and for receiving a reflected beam of electromagnetic radiation; 
   a signal processing circuit coupled to the emitter and receiver for detecting a partial or total interruption or modification of the incident beam, the signal processing unit generating an output signal when the interruption or modification to the beam is detected for transmitting to an access control mechanism. 
   Reference is first made to  FIG. 1  of the drawings which depicts a car door handle assembly, generally indicated by reference numeral  10 . The assembly consists of a door bracket assembly  12 , a door handle  14  and an access sensor device  16 . The door bracket assembly and the sensor device  16  are disposed beneath the door skin  18 . In this embodiment the door skin  18  defines an aperture  20  which receives a lens protector assembly  22  through which an infrared (IR) beam generated by the sensor device  16  passes to be reflected by a mirror  23  back to the sensor device  16 , as will be later described in detail. 
   Reference is now made to  FIG. 2  of the drawings which depicts a cross-section of the door handle assembly  10  shown in  FIG. 1 . In this diagram it will be seen that the sensor device  16  has a light emitting diode (LED)  24  which emits incident IR beam  26  which is reflected by the mirror  23  disposed in the inside  28  of the door handle  14  and the reflected beam  30  is detected by a photo-transistor  32 . As will be described, the IR beam is provided by a 1 KHz pulse frequency, to minimize power consumption. As long as the pulses are emitted and are detected by the circuit, a signal is provided from the circuit output which maintains the door in a locked position. As will also be described, if the IR beam is partially or totally interrupted or modified the beam level detected is compared with preceding pulse levels and if a reduced signal level is detected for a predetermined number of pulses, taking about 3 milliseconds, in this embodiment three pulses, the sensor interprets this as an authorized user wishing to open the door and provides an output signal which is fed to a general processor of a control module which generates a RF signal for interrogating a user&#39;s digital ID on a card. When a satisfactory response is obtained, i.e. the user&#39;s ID matches e stored digital data and a control signal is generated by the processor to unlock the locking mechanism and allows the door to be opened. 
   Reference is now made to  FIGS. 3 and 4  of the drawings, which depict the sensor device  16 . The device  16  consists of four principal parts, as best seen in  FIG. 4 : an optical enclosure  34 ; an electromagnetic shield  36 ; a printed circuit board assembly  38 , and an optical enclosure cover  40 . The optical enclosure cover  40  has a connector interface  42  which interfaces with the vehicle control conductors. The printed circuit board assembly contains a microswitch  44  which can be operated via a flexible membrane  46  disposed in the optical enclosure cover for detecting the beginning of handle motion, i.e. within a 3 mm movement. The microswitch  44  is a backup to the optical detection system to allow a user to unlock or open the door if the optical sensor fails, the signal from the backup switch replaces the signal from the sensor and is dealt with by the general processor in the same way to allow unlocking of the door. A dual lens  48  is disposed in the recess  50  in the optical enclosure for covering the LED and photo-transistor as shown in  FIG. 2 . 
     FIG. 5  shows a block diagram of the circuit used in the sensor device  16 . A control module  52  interfaces with the circuit and is coupled to current power supply  54  which supplies power to the main circuit components; pulse generator circuitry  56 ; signal processing circuit  58  for processing the output from the photo-transistor  32 , and output circuit  60  for providing an output to the control module  52 , and the microswitch  44 . 
   The pulse generator  56  generates pulses at a rate of 1 KHz and the frequency signal is fed to the LED  24  and to the signal processing circuitry  58  to synchronize detection of signals by the photo-transistor  32 . As long as both sets of pulses are received, a counter in the processing circuitry  58  is continually reset to zero and the output circuitry  60  does not generate an output signal. When the light beam is interrupted such that a predetermined number of light pulses, in this case three, are not received by the photo-transistor, the signal processing circuitry  58  detects this and actuates the output circuitry  60  to generate an output signal to the control module  52 . The control module  52 , in turn, causes a RF signal to be generated and when a suitable response is received confirming the ID of a user, the control module  52  sends a signal to unlock the door. This response time is about 3.0 to 3.5 milliseconds (MS) and by the time the user pulls the door handle  14 , the door is already unlocked. 
   Reference is now made to  FIGS. 6 and 7  of the drawings.  FIG. 6  is a circuit diagram of the circuitry used to generate the pulsed IR signal, for detecting the signals reflected from the mirror  23  and also for detecting when the reflected signal is interrupted.  FIGS. 7   a  to  7   j  depict the various signals associated with the circuit of  FIG. 5 . 
   The circuit of  FIG. 6  is designed to minimize power consumption and, consequently, in power supply  54  the supply current is limited by a 27 kΩ resistance R 29  in series with the supply which is normally between 9V and 16V. If the operating voltage is +5V, the supply current is equal to the quotient of the supply voltage tess 5V divided by the value of resistance R 29 . For example, for a 9V power supply, the supply current will be 150 A, and for a 24V power supply, the current will be 700 A. This is so that 4.7 μF capacitor C 9  can be charged sufficiently rapidly to enable the LED  24  to be driven at currents up to 100 mA in pulse mode as will be described. 
   The available supply voltage to transistor Q 9  is set by avalanche diode D 1 . Just after a measurement is taken, C 9  has been partially discharged and the voltage across C 9  is too low to maintain the operating voltage of 5V (several dozens of mV below the set voltage) and the constant supply current recharges capacitor C 9 , the voltage of which rises until the set voltage level. At this time, the transistor Q 9  conducts sufficiently to trigger the flip-flop formed by the two NOR gates  70 , 72  in IC 3 A, IC 3 B and the next measurement is initiated by synchronization signal S 1  falling to zero volts as shown in  FIG. 7   a . Capacitor C 7  filters high frequency variations in the power supply which may otherwise produce inadvertent signals. 
   Voltage level setting is principally achieved by avalanche diode D 1  which behaves like a Zener diode and is designed to operate with a weak current. The operating current is set by resistance R 30  and is about 20 A. This current value is a function of the variation in the base emitter voltage of Q 9  and temperature and the value decreases slightly at high temperatures and rises slightly at lower temperatures, varying about 1 A per 15° C. At this operating current the avalanche diode is stable at a voltage of about 4.4 V. The operating voltage (+5 V) is equal to the avalanche diode voltage (4.4 V) increased by V be  (−0.6 V) of resistor Q 9 . 
   The system is protected against excessive voltage by a shunt regulator formed by avalanche diode D 1  and the base-emitter junction of transistor Q 9 . The system is limited to supplying voltage less than 6.5 V even for an input voltage greater than 100 V. The shunt regulator allows a supply current as high as 3.5 mA resulting from 100 V continuous input supply. However, resistance R 29  is limited to the power dissipation of 0.1 W which corresponds to a permanent over-voltage of 57 V. 
   For polarity inversion, resistance R 29  limits the current without damaging the diodes in the substrates of the CMOS and HCMOS. 
   The operation of the circuit will be explained by describing how parts of the circuit are set up to generate various voltages and timing signals and then the generation and detection of pulses will be described. 
   A measurement is initiated by transistor Q 9 . The collector voltage is always around half of the supply voltage. This voltage rises when the available energy in C 9  is sufficient to perform a measurement. When the voltage reaches the threshold level of NOR gate  72 , the output changes state and the flip-flop formed by NOR gates  70 ,  72  memorizes the sequence of measurements from the start (S 1 — FIG. 7   a ). When the measurement is complete, the output of Q 9  resets the flip-flop. The R 28 , C 11  combination at the input of gate  70  and gate  74  is to provide a reset in case the system starts in a “hang-up” consumption mode with no oscillator providing a clock signal. 
   Due to the R 29 , C 9  time constant, the establishment of the 5V level is relatively slow. The flip-flop formed by NOR gates  70 ,  72  in IC 3   a  and IC 3   b  begins operating at a low voltage of 1 V to 1.5 V, before many other components on the circuit. The flip-flop can begin working with the S 1  output high or low, if the flip-flop begins working with S 1  low, i.e. 0 V, it means that the electronic circuit is powered at 1 V to 1.5 V before the 5 V level is reached. This results in a relatively high current consumption of several mA. Because the resistor R 29  limits the input current to less than 0.3 mA, the internal voltage cannot reach 5 V and the IC 3   a /IC 3   b  flip-flop cannot be reset and the circuit stays in a non-working high current consumption mode. This situation is prevented by the R 29 , C 11  combination which effectively acts as a “CPU watchdog” by resetting the IC 3   a  and IC 3   b  flip-flop after 500 s if the flip-flop remains in the state with the S 1  output in a 0 V state. This stops the power supply to the electronics and removes the electronics from the non-working high current consumption mode. The internal power supply can therefore reach +5 V required to power the circuit under normal operating conditions. Under normal operating conditions the S 1  output remains low for 45 s and the 500 s reset period does not disturb the normal functionality of the electronics. 
   The synchronization signal Si is taken from the output of gate  72 . The output of gate  70  (IC 3   a ) is fed to a sample and hold circuit  73  (IC 2 D) where it will be seen that the output at pin C, as shown in  FIG. 7   b , is the inverse of the synchronization signal S 1 . The output of sample and hold  72  is fed to pin  89  of circuit  90  to supply power to the analog circuits only during the 40 s period of the +5 V pulse. This means that all of the signal processing as shown in  FIGS. 7   c – 7   j  takes place within this 40 s period, thereby minimizing electrical power consumption. 
   NOR gates  74 ,  76  form an oscillator (see signal CLK in  FIG. 7   c ) with an oscillator period of 5 s set by the combination R 33 , C 8 . The capacitor C 8  has a thermally stable dielectric to avoid frequency variations during operation. The oscillator supplies the clock signal to the IC 1  counter which provides:
     (a) at pin D 3 , a pulse sampling the level of ambient light;   (b) at pin D 4 , a pulse indicating illumination of the LED as well as a pulse sampling the level of the signal (ambient and LED signals);   (c) at pin D 7  and pin D 8 , pulses for signals amplified by the operational amplifier  1 C 4 ;   (d) at pin D 9 , the pulse is deleted from the memory (counter IC 5 ) after the start of measurement.
 
These logic signals are depicted as signals a, b, d and e with respective pulse widths t a , t b , t d  and t e  as shown in  FIG. 7   c  of the drawings.
   

   The LED emitting stage, generally indicated by reference numeral  80 , will now be described. 
   A pulse of light is emitted by LED  24  which is connected between the supply and the collector of transistor Q 5 . The current through the LED is measured by the drop in voltage across resistances R 22 , R 23  in parallel, and is shown as signal S 3  in  FIG. 7   e . This controls the power emitted by Q 4  in the following manner. When the clock pulse rises at the output “D 4 ” of counter IC 1  at time t b , the current at the base of the transistor Q 5  rises to about 4 mA across resistor R 20 . The transistor Q 5  causes the LED shown in waveform S 3  to saturate until the current across the LED is sufficient to cause transistor Q 4  to conduct, as it receives part of the current supply from Q 5 . The combination Q 4 , Q 5  creates a feedback mechanism and the combination self-stabilizes for a LED current between 0–100 mA, the value depending on the control signal as shown in waveform S 6  in  FIG. 7   i  being supplied to transistor Q 4 . The 470 pF capacitor C 4  delays the conduction of transistor Q 5  until the switching of the general clock to avoid a current peak being produced before transistor Q 4  is enabled. The R 24 ,C 5  supply combination prevents the LED current causing a glitch in the supply voltage which could affect the operation of the photo-detector stage. The LED supply stage only operates at “high current”; the current at the base of resistor Q 5  is about 4 mA and the current at the base of resistor Q 4 , which is the current which controls supply of power to the LED, rises to 0.1 mA when the system is used in full visibility. Full visibility is the maximum level of ambient light. This is why the control current is provided only during the time the LED is illuminated. 
   The photo-detection and pre-amplification stage, generally indicated by reference numeral  82 , is provided by the photo-transistor  32  shown coupled to the emitter of transistor Q 2  which reduces the effect of high frequency signals on the capacitance of the base emitter of Q 1 . The collector voltage of Q 2  is also coupled to the collector of photo-transistor Q 1  to provide a low impedance at the stage output which is shown by pre-amplified optical signal S 2  shown in  FIG. 7   d . Resistors R 2  and R 3  form a voltage divider for transistor Q 1  and the voltage is supplied across 100 kΩ resistor R 4  to the photo-transistor Q 1 . This sets the sensitivity of the pre-amplifier to −300 mV per photo-current microamp on the base of the photo-transistor Q 1 . The pre-amplifier stage  82  thereby provides a negative voltage pulse when it receives a pulse of light. This stage consumes 600 A and has a rise time about 2 s. It is supplied throughout the cycle of the general clock which is about 40 s ( FIG. 7   c ) for a frequency of 1 KHz. 
   The operating point of the stage  82  with no photo-current is around three times V be  of Q 1 , i.e. 1.8 V at output, thereby fixing the collector current of Q 1  and Q 2  at around 100 A. The divider bridge R5R6 fixes the base potential of Q 2  at 1 V. No decoupling is present to give the pre-amplifier a very short availability time. The output signal is available after 5 to 10 s from S 2 . 
   The output of the pre-amplification stage is fed to sample and hold circuits  86 ,  88  via resistance R 7  and prevents the first stage being subjected to capacitance which can cause instability. First sample and hold circuit  86  operates during the clock cycle t a  in order to sample the level of ambient light before illumination of the LED. The second sample and hold circuit  88  operates during illumination of the LED during time t b  in order to sample the signal level. The latter sampled signal, being lower than the ambient signal, is fed to the inverting input of the differential amplifier, generally indicated by reference numeral  90 , formed by three amplifiers of  1 C 4  (IC 4 A, IC 4 B, IC 4 D). IC 4  contains four operational amplifiers, generally indicated by reference numeral  92 ,  94 ,  96 ,  98 . The differential amplifier has a gain of  10 . The operational amplifiers  92 ,  94 ,  96 ,  98  selected are classic type LM324 for low cost, low power consumption (about 600 A) and a low operating voltage of about 4 V. Its gain and slew rate are sufficient to provide stable output after 30 s. Like the photo-detection stage, the operational amplifier is only supplied for  40  s each time a measurement is taken. The amplifier output signal is shown as signal S 4  in  FIG. 7   f  of the drawings. 
   The output signal from the differential amplifier, signal S 4 , is routed through blocking diode D 2 . The output voltage is retained by capacitance C 3  and is the voltage used to control the emission of the light pulse from LED  24 . The voltage retained by C 3  can be set by adjusting the time constant set by the combination R 18 , C 3  and by the percentage of time signal S 4  is present. The discharging time constant is defined by the combination R 19 , C 3  and by the duty cycle (t b ) of closure of switch IC 2 C. Time constants can be calculated for operating at a thousand measurements per second as follows: rising time constant: R 18 =2.7 K, C 3 =4.7 pF and the signal S 4  about 20 s, giving a result of about 0.88 seconds. The discharging time constant, R 19 =1K, C 3 =4.7 pF and the switch opening time is about 5 s which gives a result of about 0.94 seconds. Signal S 6  in  FIG. 7   i  depicts the voltage for controlling the LED supply. 
   The fourth amplifier of IC 4   96  compares the voltage corresponding to the level of ambient light with a fixed threshold of 500 mV. When the pre-amplifier is illuminated by a large light signal (for example, bright sunlight), the signal is below the 500 mV threshold and the output voltage of the operational amplifier  96  rises to saturation as shown in signal S 5  in  FIG. 7   h.    
   In use, saturation is detected by the illumination of the photo-transistor, i.e. when the LED illuminates and, the signal S 5  rises to 3.8 V which is the saturation voltage of amplifier IC 4 C. The current through R 34  saturates transistor Q 6  from the time t b  until the time t e . Likewise, when the pulse from the LED  24  is correctly received, the output of differential amplifier  90  rises to around 1.4 V and the current through resistor R 14  switches on transistor Q 6 . From the time t b  until the time t e  the collector of Q 6  is pulled towards the supply potential by R 15  and R 16  during time t d  and t e . If one of the two conditions above (or if both simultaneously) are present, the transistor Q 6  will become saturated and the potential of the collector will not rise, thus transistor Q 7  will remain off. Q 7  is the transistor which blocks or allows the pulses to reset the counter  1 C 5   100 . On the other hand, if the photo-transistor  32  does not receive pulses of light, or is not saturated by ambient light, transistor Q 6  remains off and Q 7  will be saturated during time t e . 
   In addition, the counter IC 5   100  processes the output signals from amplifier  90  in accordance with the timing signals. If transistor Q 7  remains off, the counter IC 5  will be reset to zero at the end of each measurement during time t e  (signal S 7  in  FIG. 7   j ). If transistor Q 7  switches on, as indicated above, each pulse for resetting the counter to zero will not be delivered but the counter receives a clock pulse for each measurement during time t d , therefore, the counter counts as long as the signal is interrupted and the counter is reset to zero when the interruption ceases. If three successive pulses due to an interruption are counted, the counter switches off its active output until the removal of the optical barrier. The number of successive pulses measured during interruption of the signal by the system can be set between 1 and 9, although 3 has been found to be particularly convenient since at a frequency of 1 KHz this means an output is provided in 3 mS. 
   After detecting three successive pulses due to interruption of the LED signal, the output of the counter is fed to a MOS transistor  60  via the RC combination formed by R 25  and C 6  to provide a pulse of around 100 mseconds. Output as provided by the drain of Q 8  through current limiting resistor R 26 . Protection against high voltage and polarity inversion is provided by Zener diode D 4 . 
   The aforementioned circuit has the principal advantage of being low cost, uses standard components and has very low current and power consumption with an average current consumption of about 0.2 mA because self-biasing circuitry is used. Regulation of the circuit supply is used to achieve a response time which allows high frequency illumination of the LED and high frequency operation of the amplifier. The supply voltage can vary between typically 9 and 16 V and the LED needs to be energized with pulses of 5 s duration to provide satisfactory functioning. 
   In this way it will be seen that the circuitry provided minimizes power consumption because power is only supplied to the circuitry for the duration of the period of the pulses of the synchronization signal which is particularly advantageous in a vehicle or any other application where minimizing electrical power consumption is important. The use of pulses to control illumination of the LED and the detection of an absence of those pulses for a predetermined number of cycles is advantageous. 
   It will be appreciated that various modifications may be made to the apparatus described above without departing from the scope of the invention. An alternative embodiment of sensor device is shown in  FIG. 8  of the drawings which is preferred for use with vehicles. In this case the light source  110  and detector  114  are located in a post  115  disposed at one end of the handle  14 . In this case a reflector  123  (shown in broken outline) is located at the opposite end of the handle  14 . Thus, it will be seen that the incident beam  126  and reflected beam  127  are parallel to the handle  14  and to the door skin  118 . This embodiment has the advantage that an additional hole in the door skin  118 , such as that shown in  FIGS. 1 and 2 , is avoided because the post can use the same hole as the handle  14 . The reflector  123  is located to minimize the possibility of dirt being deposited, whether by a user or otherwise, on the mirror reflector  123 . Thus, a lens protector is also unnecessary in this embodiment. The user can modify the optical beam characteristic by placing his hand anywhere on the door providing an ergonomic advantage. This arrangement is simpler and is easier and less expensive to install. 
   Further, alternative embodiments are shown in  FIGS. 9   a  and  9   b  of the drawings, which depict a car door handle assembly similar to that shown in  FIGS. 1 and 2  in which LED  210  generates an incident beam  212  which is detected directly by a photo-transistor  214  without the use of a mirror. When the user inserts his hand between the LED  210  and the photo-transistor  214  it breaks or modifies the beam  212  in the same way as described above. The light emitting diodes and photo-transistors can be positioned as appropriate to facilitate interruption of a beam by a user. Thus,  FIG. 9   b  shows the beam parallel to the door skin  18  similar to that shown in  FIG. 8 . These alternative arrangements can be provided to operate with the same or similar circuit to that described above. 
   A further embodiment of the invention is shown in  FIG. 10  which is similar to the arrangement shown in  FIG. 8 . In this embodiment sensor enclosure  228  is mounted in door bracket  229 , and a post or light-pipe  230  and also may be configured to carry a light source and a detectors, which can be arranged in the same way as depicted in  FIG. 8 , that is they are disposed adjacent each other, the same distance along the post axis. The enclosure  228  also has mechanical back-up and locking switches  235 ,  237  respectively. There is no reflector in this embodiment and the sensor circuit operates by detecting light reflected from a user&#39;s hand when inserted between the handle  236  and the door skin  238 . The circuit is substantially identical to that of  FIG. 6  but as long as no reflected pulses are received, the counter in the receiving circuitry  58  is continually set to zero and the output circuitry does not generate an output pulse. The counter IC 5   100  is set up so that if three successive pulses of light are detected following reflection from a user&#39;s hand, the counter generates an output signal which is fed to the MOS transmitter as described above with reference to  FIG. 6 . Thus, the circuit only produces an output when the beam is reflected by a user, and in combination with the user&#39;s ID signal, an unlocking signal is sent to the door so that when the user pulls on the handle the door is already unlocked. The power supply to the circuit is also only supplied during the period of the synchronization circuit to minimize power consumption and, as before, all measurements and signal processing take place within this 40 s period. 
   This embodiment has the advantage of minimizing cost: a reflector is not required and the post  230  uses the same aperture  240  in the door as the handle facilitating assembly. Because a reflector is not required, problems associated with the reflector such as keeping it clean and amplifying power are avoided. 
   Reference is also made to a further embodiment of the invention which is similar to the arrangement shown in  FIG. 1  but without the reflector  23 . In this embodiment the signal is reflected back to the detector  16  by the user&#39;s hand. The sensor circuit operates in the same way as described with reference to  FIG. 10 ; counting a predetermined number of pulses present results in an output signal which is fed to a MOS transistor for generating a control signal to unlock the door as described above. 
   Various other modifications may be made to the apparatus and circuitry hereinabove described without departing from the scope of the invention. Certain applications and minimizing of power consumption may not be necessary, for example in buildings and the like where mains power supply is available and the power consumption required by the sensor system may be regarded as minimal. In such a case the IR optical signal could be provided by a continuous signal and actuation of the unlocking mechanism could be achieved by detecting the absence of the continuous signal for a predetermined period or by counting a number of pulses as described above. The LED and photo-transistor may be located separately from the handle. For example, a slot could be provided in a door or entry to a building and a plastic card, similar to a credit card of the like, could be swiped between the slot to interrupt the beam and the output of the signal processing circuitry could be used to unlock a mechanism to allow a user to open a door which is remote from a sensing mechanism. 
   The sensor device has a number of advantages which allow its use in a variety of applications, such as in vehicles, buildings and the like. The use of a partially or totally modified or interrupted beam to detect the presence and absence of an object has a variety of applications. For example, it may be used as a rain sensor and for detecting and counting the passage of objects interrupting the beam. The structure has a number of advantages which facilitate widespread use, such as low power consumption during use, the use of up to 100 mA drive current provided to the IR LED to generate a high power optical pulse to minimize the effect of dirt and the like on the lenses and reflectors, where used, fast frequency response compatible with high frequency pulses, a wide operating temperature range and good noise immunity to ambient light changes and electromagnetic interference. Synchronization of the detection of the light impulses provides good immunity against parasitic electrical signals and radio signals and the use of a counter to detect predetermined period of interruption minimizes the effect of spurious signals causing malfunctioning of the circuitry.