GB2112600A

GB2112600A - Out of proximity alarm system

Info

Publication number: GB2112600A
Application number: GB08224874A
Authority: GB
Inventors: Kunwar Chander Jeet Singh
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-09-02
Filing date: 1982-09-01
Publication date: 1983-07-20
Also published as: AU8794782A; EP0073681A3; JPS5875295A; EP0073681A2

Abstract

An out of proximity alarm system useful for indicating when the distance between two bodies has increased beyond a predetermined distance is disclosed. In the preferred embodiment, the system comprises a radio transmitter 500 attachable to one of the bodies, such as a personal article such as a briefcase and having a pulsed R.F. output. A radio receiver 600 carried by a second body, such as a person, receives the transmitted signal when the two bodies are within range of each other. The receiver incorporates a time delay stage 660 which is kept disabled by the received pulsed signal. Should the distance between the two bodies increase beyond a predetermined amount the pulsed signal will have insufficient strength to keep the time delay stage disabled, thus activating an alarm 690 and alerting the carrier of the receiver that he has left the personal article behind. Other embodiments utilise transponders, or ultrasonic or infra-red radiation, or the variation of capacitance with separation distance. <IMAGE>

Description

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SPECIFICATION

Out of proximity alarm system

5 BACKGROUND OF THE INVENTION. The present invention relates to an out of proximity alarm system for the purpose of indicating a separation, greater than a predetermined distance, between a first and a sec-10 ond body, e.g., a person and an article, an object being to warn a person that he has forgotten to pick up the article, e.g., a briefcase, after putting it down.

1 5 SUMMARY OF THE INVENTION

According to a first aspect of the invention, in an alarm system for the purpose hereinbefore specified, a change in the distance between a first and at least one second body, 20 one of the bodies, for example, being a person and the other body, for example, being an article, is accompanied by a change in a variable detectable by at least one detector of the system such that when a predetermined 25 and preferably adjustable, distance between the bodies is exceeded and the variable has changed by a predetermined amount, an audible, visible or tactile alarm means or combination of these coupled to the detector is acti-30 vated.

In one embodiment, the alarm system comprises an interrogator having a first part which is a transmitter of radiation and a second part which is a detector of radiation. The transmit-35 ter and the detector may both be on or in the first body, the radiation transmitted by the transmitter being transmitted back to the first body by a reflector, or retransmitted to the first body in the same or a modified form by a 40 transponder on or in the second body and detected by the detector, the time between transmitting and detecting the radiation being the variable and being directly related to the distance between the bodies. Alternatively the 45 system may include a separate transmitter and receiver, the transmitter being located on or in one of the bodies and the receiver on or in the other body, the variable being the strength of a signal produced by the radiation and re-50 ceived by the receiver, the signal decreasing with increasing distance of separation between the bodies.

The radiation may take any desired and convenient form, and may, for example, be 55 ultrasonic energy or be electromagnetic energy in the microwave, radio or infrared wavebands.

In another form of alarm system embodying the invention, the variable may be the capaci-60 tance of a body, the detector system comprising a capacitance detector, e.g., a bridge network, for detecting any change in the capacitance as a result of any separation between the bodies and adapted to activate an 65 alarm means when the value of the capacitance changes beyond a predetermined threshold value thereof.

According to a further aspect, the invention provides an out of proximity alalrm system 70 comprising a master unit which is designed to produce a continuous series of timed ultrasonic pulses, a slave unit designed to pick up and retransmit such pulses, and a receiver in the master unit which is designed to calculate 75 the time interval between transmission of a pulse and subsequent receipt of the retransmitted pulse and to trigger an alarm when the time interval exceeds a preset period.

80 Thus this system is capable of identifying and measuring the position of the slave unit relative to the master unit such that if the slave unit is moved more than a preset distance from the master unit (for example, in 85 the range 60 to 90 cm) an alarm or indicator is operated. Such a system is capable of mass production to provide a very small slave unit and a master unit no greater in size than a 10 cm cube. Such devices will ideally be battery 90 operated using alkaline cells of minimal size, and are best constructed using integrated circuits and low current techniques to produce minimum current drain.

According to a preferred embodiment of the 95 invention, the alarm system comprises a radio frequency transmitter and receiver. Preferably, the transmitter is attached to the personal article to be protected, such as a briefcase, and the receiver is carried by the user. When 100 the protected article is separated from the receiver by more than a preset, preferably adjustable distance, an alarm is activated, reminding the user that he has left the personal article behind.

105 Generally the alarm means, e.g., an electronic oscillator operating at an audible frequency, a visible light emitting means such as, for example, a light emitting diode, or a tactile alarm means, or a combination of 1 10 these, will be carried by, or secured or attached to the person or his clothing or headgear or other article carried by him, or possibly at least partially incorporated in such clothing or headgear or other article. Addition-1 1 5 ally, or alternatively, however, such alarm means may be secured or attached to, or at least partially incorporated in, the article to be protected.

The invention also extends to an article of 120 the kind normally carried by a person, e.g., a briefcase, other kind of case or wallet, which has one of the components of the alarm system according to the invention secured or attached thereto or at least partially incorpo-125 rated therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways and various embodiments thereof are 1 30 illustrated by way of example in the accom-

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panying drawings, in which:

Figure 7 is a block diagram of the circuit of a first embodiment of an out-of-proximity ultrasonic detector and alarm system according 5 to the invention;

Figure 2 is a block diagram of a second embodiment of an ultrasonic detector and alarm system according to the invention;

Figure 3 is a simplified circuit diagram of 1 0 still a further embodiment of an ultrasonic detector and alarm system according to the invention;

Figure 4 is a block diagram of a preferred embodiment of the invention illustrating a 1 5 modified form of the circuit of Fig. 3 and utilizing a radio frequency transmitter and receiver;

Figure 5 is a circuit diagram of the transmitter portion of Fig. 4;

20 Figure 6 is a circuit diagram of the receiver portion of Fig. 4;

Figure 7 illustrates an alternative embodiment of the transmitter output of Fig. 5; and Figure 8 illustrates an alternative embodi-25 ment of the front end stage of the receiver of Fig. 6.

DETAILED DESCRIPTION Referring now to the drawings, Fig. 1 illus-30 trates a first embodiment of an out of proximity alarm system having ultrasonic energy radiating and receiving means. Although ultrasonic energy radiating means are shown,

other energy radiating techniques could be 35 employed, such as radio frequency techniques. The system comprises an interrogator module 10 and transponder module 100. The interrogator module comprises a sawtooth waveform generator 12 which frequency modu-40 lates an ultrasonic frequency carrier in F.M. modulator 14. A carrier frequency in the ultrasonic range is used, preferably a frequency in the 30 to 50 KHz range. A center frequency of 40 KHz is shown in the embodi-45 ment of Fig. 1. The frequency modulated signal is then coupled to an ultrasonic transducer 16 such as a lead zirconate piezo ceramic element. The ultrasonic signal having a center frequency of 40 KHz is then 50 transmitted into the environment and is received by receiving transducer 102, which is a part of the transponder 100 and which converts the 40 KHz ultrasonic signal into an electrical signal. The received signal is ampli-55 fied in amplifier 104. The output of amplifier 104 is fed into mixer 106, wherein the received signal is mixed with a 10 KHz signal from fixed frequency oscillator 109. The mixer 106 produces sum and difference signals hav-60 ing center frequencies at 30 and 50 KHz.

High pass filter 109 allows only the 50 KHz signal to pass through to transducer driver 110, which is coupled to ultrasonic transducer 112. Thus, a signal having a 50 KHz 65 center frequency is retransmitted back to the interrogator module 10. A conversion of the received signal frequency by transponder 100 is required in order that the interrogator can distinguish between the signal transmitted by 70 the interrogator and the signal transmitted by the transponder.

Transducer 18 receives the signal transmitted from transducer 112. This signal is coupled to amplifier 20 and then mixed in 75 mixer 22. The sum and difference output signal frequencies from mixer 22 are proportional to the phase shift between the transmitted and received signals, the phase shift being proportional to the distance be-80 tween the two bodies. A bandpass filter 24 having a 10 KHz bandwidth selects the difference signal, which is then coupled to a frequency discriminator 26, which demodulates the F.M. signal, resulting in a time varying 85 signal whose amplitude is inversely proportional to distance. Beeper alarm 28 is activated when the amplitude of the demodulated signal falls below a preset limit. Should the distance between interrogator 10 and tran-90 sponder 100 increase beyond prescribed limits, the decreased amplitude of the demodulated signal will activate alarm 28.

Fig. 2 illustrates an alternative embodiment of an out of proximity alarm system according 95 to the invention.

In the apparatus of Fig. 2, unit A incorporates an ultrasonic oscillator 200 operating at a frequency in the range 30 KHz to 50 KHz whose output is gated by gate 202 to pro-100 duce a burst output to the transmitter ultrasonic transducer 204 for a period of 1 ms in every 100 ms. This ultrasonic signal is received and retransmitted by unit B to be received and amplified by an ultrasonic receiv-105 ing transducer 206 and band-pass amlifier 208 in unit A.

The output from amplifier 208 is squared in a pulse shaper 210 and applied to a timer 212. The timer 212 is gated open by the 110 timing pulse generator 214 and gated off by the received signal from pulse shaper 210. During this time interval, the signal from the ultrasonic oscillator coupled via line 216 charges a timing capacitor. If the timer 212 is 11 5 not stopped before the capacitor has reached a predetermined voltage, voltage dependent switch 218 triggers, thus operating the alarm or indicator 220.

Unit B is essentially an ultrasonic receiving 1 20 transducer 250 and transmitting transducer 252 operating at the selected frequency and may additionally comprise an amplifier stage 254. It may be possible for unit B to be a specially designed ceramic resonator which, 125 being a passive device, would not require a battery supply.

The speed of sound waves in air at 1 5°C is approximately 332 m/s. The nominal distance between the units A and B is 0.60 m. 1 30 Hence the transmitter to receiver path length

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via unit B is 1.2 m. The time taken for an ultrasonic signal to travel the path length is 3.6 msec. The timing resolution is expected to be better than ± 10%, i.e., ± 0.36 5 msec. Hence there is an expected position resolution for B of better than ± 0.06 m.

Fig. 3 illustrates a further embodiment of the alarm device. The device comprises transmitter 300 mounted to one body, such 10 as a briefcase, and receiver 400 carried by the other body, such as a person. The transmitter is constructed, for example, from CMOS integrated circuit NOR logic gates to consume low power. Furthermore, the 1 5 transmitter is of the burst amplifier type, so that only small bursts of energy are transmitted and consequently average current draw is low. The transmitter 300 comprises a first square wave oscillator or multivibrator 20 310 constructed in conventional fashion which gates a second higher frequency oscillator or multivibrator 320. Oscillator 320 oscillates in the 30 to 50 KHz ultrasonic range and preferably at 40 KHz. The output of 25 oscillator 320 is coupled in push-pull fashion to transducer 330. Thus, small bursts of ultrasonic energy are transmitted into the environment to be received by receiver 400.

Receiver 400 comprises receiving trans-30 ducer 410 followed by passband amplifier 420 shown in Fig. 4 as a type SN76660 high gain IF amplifier having a 40 KHz center frequency. The output of IF amplifier 420 is coupled to a tone decoder 430 which func-35 tions as a detector stage sensitive to the transmitted 40 KHz tone. The output of the tone decoder is then coupled to low pass filter 440 which acts as a time delay stage. When a tone signal is present, the output of tone 40 decoder 430 goes to ground, biasing the transistor 450 off and holding the alarm 460 deactivated. The time constant of the low pass filter insures that the base of transistor 450 stays near ground so long as tone bursts are 45 received i.e. the capacitor stays discharged. When the distance between the transmitter and the receiver increases beyond a predetermined amount, and the received bursts signal strength falls off, the output of the tone 5© decoder goes high, the capacitor charges up, transistor 450 is biased on after a small time delay and the alarm is activated. The alarm shown here comprises a low frequency oscillator 460 coupled to a transducer 462 such as 55 a loudspeaker or piezoelectric crystal.

The tone decoder 430 allows the receiver to discriminate between varying transmitter frequencies, thus providing about 200 or so different channels. A steep bandpass charac-60 teristic for the receiver is thus required. An alternative solution might utilize a numerical decode technicque, thus allowing all the transmitters and receivers to be the same with the exception of the code links set prior to 65 sale. For example, the transmitter could send a coded signal at a low duty cycle, e.g. 5 msec, long every 1 sec. This would allow, for example, 4096 (212) different code combinations. The precise number of code combina-70 tions necessary to avoid significant probability of overlap would depend on the estimates of the number of units sold. Additionally, to reduce spurious alarm triggers, a delay should be incorporated such that the receiver would 75 need to fail to identify its code for the delay time (say 1 5 sees) before the alarm is triggered.

Additional delays activated by a push button could also be provided which would allow 80 the alarm to be suppressed if the two bodies should be intentionally temporarily separated. For example, the delay mentioned above could increment in two minute steps instead of the much shorter times i.e., approximately 85 1 5 sees., contemplated in normal use.

Fig. 4 illustrates the block diagram of the preferred embodiment, a modified form of the out of proximity alarm system of Fig. 3. The system comprises radio transmitter 500 pre-90 ferably attached to the article to be protected and radio receiver 600 carried by the user. As shown in Fig. 4, transmitter 500 comprises a pulse generator 510 which produces 20 msec, pulses at a 100 msec, repetition rate. 95 In the embodiment shown in Fig. 4, pulse generator 510 gates a radio frequency crystal oscillator 520 on and off. Oscillator 520 may operate in any convenient R.F. band as allowed by the Federal Communications Com-100 mission in the United States or other governing authority elsewhere. The pulse burst output of R.F. oscillator 520 is then fed to an antenna and transmitted into the environment for reception by receiver 600. In order to 105 allow many of the alarm systems to be used in close proximity to each other, an alternative form of the invention includes digital encoder 530. The digital encoder, may, for instance, comprise a 1 6 bit shift register or counter 1 10 preset with a code prior to sale and having a serial output which pulse modulates R.F. oscillator 520, thus allowing up to 65,536 different code combinations. Transmitter 500 is powered by two miniature watch batteries 115 having an approximately 3V output for maximum compactness, as shown by block 540.

In one embodiment, receiver 600 carried by the user comprises a radio receiver having a single conversion superheterodyne front end 120 625 comprising local oscillator 620 and mixer 610. The pulse modulated signal from transmitter 500 is received by the receiver antenna and coupled in conventional fashion to mixer 610 where it beats with the local 125 oscillator frequency from local oscillator 620. The intermediate frequency signal produced is fed to I.F. amplifier 630. The output of I.F. amplifier 630 is coupled to envelope detector 640. The detected signal is then fed into a 130 low frequency amplifier 650 incorporating a

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range or sensitivity control to set the maximum reception distance. The output of amplifier 650 is coupled to a time delay circuit 660, the purpose for which will be described 5 hereinafter. The output of time delay 660 is then coupled to a 1 Hz multivibrator 670 which gates an audio frequency oscillator 680 on and off to produce a pulsating audio frequency signal via transducer 690. The pur-1 0 pose of time delay 660 is to prevent activation of the alarm for a small period of time if the signal strength from transmitter 500 should temporarily fade, i.e., when the user moves within a room, although within range 1 5 of the protected article. Again, receiver 600 may incorporate a digital decoder 700 which generates an output pulse each time a proper transmitted code is received. These pulses maintain the time delay in an off state, thus 20 preventing activation of the alarm. Receiver 600 is powered by, for example, two small batteries 710, having an output voltage of 3 volts. Two size AA batteries may be used. By using circuit technology having low power 25 consumption such as CMOS where possible, battery life should exceed one month of continuous use.

A circuit diagram for the alarm device illustrated in Fig. 4 is shown in Figs. 5 and 6. The 30 portions of the circuit corresponding to the blocks of Fig. 4 are indicated by phantom lines.

Pulse generator 510 comprises, in Fig. 5, an astable muiltivibrator 515 producing 80 35 msec, pulses at a 100 m sec. repetition rate. The multivibrator may be constructed in conventional form using a single quad NOR CMOS integrated circuit such as a type CD4001 B for low current draw. Two of the 40 NOR gates 511 and 51 2 on the single integrated circuit serve as the multivibrator. The remaining two NOR gates 513 and 514 are connected in parallel with one input tied low and serve as an inverting buffer stage coupled 45 to oscillator 520, thus allowing sufficient current sourcing to oscillator 520. The output of gates 513 and 514 turn crystal oscillator 520 on and off, thus producing a burst of R.F. energy which is coupled to antenna 530 by 50 transformer 525 and inductor 526. The output signal to antenna 530 is the inverted form of the signal from multivibrator 515 due to inverting gates 513 and 514, i.e., a 20 msec, pulse every 100 msecs. Antenna 530 might 55 comprise either a small whip or loop antenna. Oscillator 520 is of conventional design, having a crystal 522 oscillating at 49.890 MHz coupled to the base of transistor 524 and a tuned collector circuit comprising transformer 60 525 and capacitor 527.

The receiver is illustrated in Fig. 6. The receiver comprises super-heterodyne front end 625 including tuned circuit 626 tuned to the transmitter frequency of 49.890 MHz and a 65 frequency converter comprising fixed frequency crystal local oscillator/mixer 620 operating at a frequency of 49.435 MHz. Tuned circuit 626 includes a readily available antenna coil suitable for frequencies around 49 70 MHz. The mixer output difference frequency of 455 KHz is the I.F. frequency. The output of the mixer/local oscillator is coupled to 4 stage I.F. amplifier 630 constructed in conventional fashion. I.F. transformers 631 may, 75 for example, be readily available A.M. broadcast band components. The amplified signal is then fed into circuit 640 which functions as a simple envelope detector and voltage doubter. The detector output is then A.C. coupled to 80 low frequency high gain inverting limiting amplifier 650. The amplifier is realized in a convenient form by using one linearly biased gate 652 of a CMOS buffered Quad NOR gate, the remaining three gates of which are 85 also used in the following stages of the circuit. Amplifier 650 incorporates a sensitivity control 651 for varying the gain of the amplifier and consequently the receiver range. The gain of amplifier 650 is adjustable between 90 approximately 10 and 5000 due to resistors 651, 653 and 654. One input of NOR gate 652 is tied to ground while the other input receives the demodulated pulsed signal.

When the demodulated signal is of sufficient 95 amplitude to saturate amplifier 650, corresponding to the receiver being within range of the transmitter, the output of NOR gate 652 will be a pulsating signal having a phase opposite the signal at the input. Due to the 100 4th I.F. transformer take-off (point C), the input to amplifier 650 is a pulsating signal having an on period of 80 msec and a total period of 100 msec.

The output of amplifier 650 is then coupled 105 to time delay circuit 660 via diode 661.

When a demodulated pulsed signal is present at the input of amplifier 650, an inverted pulsed signal will be presented at the output of amplifier 650. Due to the low duty cycle of 110 the pulsed signal at the output of amplifier 650 (20 msec on, 80 msec off), capacitor 662 will be kept relatively discharged through diode 661. The capacitor is coupled to the input of NOR gate 664 of the time delay 115 stage, whose other input is tied to ground. When capacitor 662 is discharged, the output of NOR gate 664 will be high. The high level of the signal keeps 1 Hz gated multivibrator 670 in an off condition, which in turn di-120 sables 2 KHz audio oscillator 680, thus keeping the alarm deactivated.

When the receiver falls out of range of the transmitter or the range control setting 651 is reduced, the output of amplifier 650 is lin-1 25 early biased to its quiescent output voltage of approximately 1/2 the supply voltage. Capacitor 662 then charges up slowly through resistor 663 towards the supply voltage and is kept from discharging by the reverse bias on 130 diode 661. The output of time delay stage

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664 goes low once the capacitor 662 has charged up to the threshold voltage of the gate. The low condition at the output of time delay stage 660 then gates multivibrator 670 5 on, which in turn gates audio oscillator 680 on and off, resulting in a pulsing audio tone from piezoelectric transducer 690. The carrier of the receiver is then warned that he has moved too far away from the personal article. 10 Additionally, the alarm device according to the invention also has obvious use as a theft alarm.

Fig. 7 illustrates an alternative form of the transmitter output section which can re-15 place the crystal oscillator 520 of Fig. 5 by coupling to point A of Fig. 5. Fig. 7 shows a tunable oscillator having a ferrite rod antenna coil and variable capacitor in the collector circuit which can be tuned to the 455 KHz 20 intermediate frequency of the receiver, thus allowing elimination of the superheterodyne front' end 625 of the receiver. In place of front end 625 the simple antenna coil LC circuit of Fig. 8 tuned to the 455 KHz I.F. is 25 used and coupled directly to I.F. amplifier 630 at point B in Fig. 6.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, how-30 ever, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, 35 accordingly, to be regarded in an illustrative rather than in a restrictive sense.

Claims

1. An out of proximity alarm system com-40 prising:

a transmitter attachable to a first body, including means for generating a radiated signal; and a receiver attachable to a second body, 45 comprising:

means for receiving said radiated signal; means for demodulating said radiated signal and producing a demodulated signal;

alarm means; and 50 means responsive to said demodulated signal for activating said alarm means when the level of said demodulated signal falls below a threshold level;

whereby when the distance between said 55 first and second bodies increases beyond a predetermined amount, said alarm means is activated.

2. The out of proximity alarm system recited in claim 1 wherein said means for gener-

60 ating a radiated signal includes ultrasonic frequency oscillator means, means for pulse modulating said ultrasonic frequency oscillator means and ultrasonic transducer transmitting means coupled to said ultrasonic frequency 65 oscillator means and said means for receiving includes ultrasonic transducer receiving means.

3. The out of proximity alarm system recited in claim 2 wherein said means respon-

70 sive to said demodulated signal comprises time delay means.

4. The out of proximity alarm system recited in claim 3 wherein said means for receiving further comprises passband amplifier

75 means coupled to said ultrasonic transducer receiving means.

5. The out of proximity alarm system recited in claim 4 wherein said means for demodulating comprises tone decoder means.

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6. The out of proximity alarm system recited in claim 5 wherein said time-delay means comprises resistor-capacitor charge storage means.

7. The out of proximity alarm system re-

85 cited in claim 6 wherein said first body is a person and said second body is a personal article.

8. The out of proximity alarm system recited in claim 6 wherein said second body is a

90 person and said first body is a personal article.

9. An out of proximity alarm system comprising:

a transmitter attachable to a first body,

95 including means for generating a radio frequency signal; and a receiver attachable to a second body, comprising:

means for receiving said radio frequency 1 00 signal;

means for demodulating said radio frequency signal and producing a demodulated signal; and alarm means responsive to said demodu-105 lated signal;

one of said bodies being a person and the other of said bodies being a personal article;

whereby when the distance between said first and second bodies increases beyond a 1 1 0 predetermined amount, said alarm means is activated.

10. The out of proximity alarm system recited in claim 9 wherein said means for generating a radio frequency signal includes

115 radio frequency oscillator means and means for pulse modulating said radio frequency oscillator means.

11. The out of proximity alarm system recited in claim 10 wherein said receiver

1 20 further comprises means for amplifying said demodulated signal.

12. The out of proximity alarm system recited in claim wherein said means for amplifying includes gain adjusting means for

1 25 varying the reception range of said receiver.

1 3. The out of proximity alarm system recited in claim 12 wherein said receiver further comprises time-delay means responsive to said demodulated signal for activating 1 30 said alarm means when the level of said

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demodulated signal falls below a threshold level.

14. The out of proximity alarm system recited in claim 1 3 wherein said means for 5 receiving comprises:

means for converting said radio frequency signal to an intermediate frequency signal;

and intermediate frequency amplifier means 1 0 coupled to said means for converting.

1 5. The out of proximity alarm system recited in claim 1 3 wherein:

said means for receiving includes a radio frequency amplifier;

1 5 said radio frequency oscillator means includes oscillator means tuned to the center frequency of said radio frequency amplifier; and said means for receiving further includes 20 inductor capacitor tuned circuit means coupled to the input of said radio frequency amplifier and tuned to the center frequency of said radio frequency amplifier.

1 6. The out of proximity alarm sytem re-25 cited in claim 14 or 15 wherein said means for demodulating comprises envelope detection means.

1 7. The out of proximity alarm system recited in claim 1 6 wherein said alarm means 30 comprises:

sub-audio frequency oscillator means; and audio frequency oscillator means coupled to said sub-audio frequency oscillator means for producing a pulsating audio alarm signal. 35

1 8. The out of proximity alarm system recited in claim 1 7 wherein said transmitter further includes means for digitally encoding said radio frequency signal and said receiver further includes means for digitally decoding 40 said radio frequency signal.

1 9. The out of proximity alarm system recited in claim 1 3 wherein said time delay means comprises resistor-capacitor charge storage means.

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20. An out of proximity alarm system for determining when a first body has separated from a second body by a predetermined distance comprising:

transmitting means mounted on one of said 50 bodies comprising radio frequency oscillation means and means for pulse modulating said radio frequency oscillation means;

receiving means mounted on the other of said bodies comprising radio frequency recep-55 tion means tuned to the frequency of said radip frequency oscillation means, radio frequency amplifier means, demodulation means coupled to said radio frequency amplifier means, low frequency amplifier means coup-60 led to said demodulation means, alarm means and time delay means responsive to the demodulated signal from said demodulation means for activating said alarm means when said demodulated signal falls below a thresh-65 old level;

wherein one of said bodies is a person and the other of said bodies is a personal article.

21. The system recited in claim 22 wherein said radio frequency amplifier means

70 comprises intermediate frequency amplifier means and said radio frequency reception means further comprises frequency conversion means.

22. An out of proximity alarm system sub-75 stantially as herein before described with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.—1983.

Published at The Patent Office, 25 Southampton Buildings

London, WC2A 1AY. from which copies may be obtained.