WO1987006748A1 - Remote monitoring and alarm system - Google Patents

Abstract

A remote monitoring and alarm system has an FM radio link between a transmitter (30) carried by a person or object being monitored and a receiver (10) to which alarm signals are transmitted from the remote transmitter. Digitally-encoded FM signals are produced at the transmitter (30) at pre-set transmission intervals in the form of multiple digital words detected by the receiver (10). The receiver (10) is a constant listening device which produces an alarm immediately if at least one of the coded words is not received during any transmission interval. The multiple words serve as redundant words during each transmission interval to minimize false alarms at the receiver (10) due to interference. The multiple coded words are transmitted at minimum time intervals preset to maximize the number of coded pulses during each transmission interval while spacing the coded words to permit maximum allowable power transmission. The receiver (10) can monitor two transmitters simultaneously operating at the same carrier frequency by adjusting the coded words so they occur at different periods within the same transmission interval to prevent the possibility of total overlap. Emergency conditions monitored include out-of-range, anti-tampering, panic alert, immersion and breathing rate, as well as other selected emergency or status conditions.

Description

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REMOTE MONITORING AND ALARM SYSTEM

FIELD OF THE INVENTION

This invention relates to monitoring systems, and more particularly to a remote monitoring system using a radio frequency link between a transmitter, which is carried by a person or object being monitored, and a portable monitoring receiver. The receiver detects signals from the transmitter and produces alarms or displays information for various emergency or status conditions associated with the person or object being monitored.

BACKGROUND OF THE INVENTION

Many children are reported missing each year, either from child abduction or simply from straying away fromparents or guardians who are unaware of their being lost or in danger. A child also may wander away and be lost for several hours before being found by terrified parents, or parents may lose track of a child in crowded surroundings unfamiliar to the small child.

At the same time, caregivers for Alzheimers patients, and others suffering from dementia resulting in the wandering syndrome, experience difficulty in keeping track of those requiring special care. Families and institutions alike have expressed genuine concern for the well-being of the handicapped, elderly and special patients requiring close observation. Further, the bedridden have been reported to often suffer from sensitive incontinence problems which can aggravate existing symptoms and minimize overall comfort. Swimming pool owners and those involved in recreational activities near the water are well aware of the need for water safety precautions. For instance, small children may wander into the water without appreciating the dangers. It is not always feasible nor is it reasonable to expect that personal supervision of small children by a guardian can always prevent water-related accidents from happening.

There is a need for a safety device that helps a parent or guardian keep track of a person by constantly monitoring his or her whereabouts and triggering an alarm to alert the responsible parent or guardian whenever the subject has encountered an emergency, strayed too far, or is otherwise in danger. It has been proposed that such a safety device include a transmitter worn by the subject and a receiver carried by the parent, guardian or caregiver. In a child monitoring situation, the receiver can be set to monitor the child's whereabouts within a desired radius, and if the child moves beyond the pre-set range, a beeper and warning light on the receiver^" alert the parent or guardian. The beeper also can alert the guardian if the transmitter is momentarily removed from the child, submerged in water, or turned off. A call button on the transmitter can be pressed if the child becomes lost or perceives danger, such as if a stranger approaches. In a hospital or rest home setting, a call button can be used to transmit an alarm if a patient becomes ill or disorientation occurs.

The patient's whereabouts also can be constantly monitored.

There is a need to monitor for awidevariety of emergency situations. For example, emergencies monitored can be out-of-range, immersion, tampering with the transmitter, breathing rate, as examples. Call signaling can be provided. although this may not be considered to have the same priority as other emergencies.

There is also a need to ensure that such a monitoring device has an exceedingly high degree of reliability. For instance, an alarm must be activated at the.receiver with essentially 100% accuracy whenever a monitored emergency condition arises, regardless of interfering sources or other similar monitoring systems in close proximity. The alarm also should be produced immediately when an emergency is detected, since time is of the essence in most emergency situations.

In addition, it is necessary to reduce the probability of false alarms at the receiver. False alarms have been a major annoyance with monitoring devices previously used in experimental testing. These experimental units have covered a wide geographical range to determine how and whether local conditions will affect radio frequency transmission. It has been learned that false alarms can be produced from interference from AM sources or electrical noise or from other nearby electronic devices operating at the same radio frequency. Even though interference may be present from overlapping signals from other sources, false alarms even in these situations should be minimized. False alarms are not only annoying, but they are also a source of possible misinformation. An emergency could be detected at the receiver, only to have a later transmitted signal changed by an interfering signal that indicates the emergency has been corrected when, in fact, it has not. Alarm failures also should be prevented. Alarm failure can be caused by a second similar transmitter set at the same frequencytransmitting an identical address code signal which becomes substituted forthe signal fromthe firsttransmitter. This may cause the receiver for the first transmitter to operate without producing an alarm when an alarm may be necessary for an emergency situation. -4- It is also desirable to monitor signals from more than one transmitter with a single receiver so that one person can monitor the whereabouts of more than one subject without requiring multiple receivers. Transmitters worn by two different subjects, for example, should operate indepen¬ dently on the same carrier frequency, but without producing false alarms from interfering signals.

Remote monitoring devices also have a number of design requirements which are difficult to achieve concurrently in one small package. For example, in a radio frequency monitoring system, it is desirable to obtain maximum power transmission, within FCC limits, in order to maximize the range over which the device is sensitive. There is also a need for a high-sensitivity receiver which can be produced at a low cost and operate with low power consumption and at low voltages. In addition, there is a need for a safety device combining the ability to operate with low-voltage digital electronics in the same small package and in close proximity to high-power radio frequency energy. Both πust work together reliably, without interference or false alarms, and still be made available in a small package at a low cost so the system can be affordable to everyone.

This invention provides an extremely reliable radio frequency monitoring system which greatly reduces the probability of false alarms or alarm failure, while ensuring that necessary emergency alarms are immediately and reliably transmitted to the receiver. The system constantly monitors a variety of emergency and status conditions, including out-of-range, anti-tampering, panic-alert, immersion and wetness sensing, breathing rate, low-battery condition, and the like. The system can independently monitor these functions from more than one transmitter with a single receiver operating on the same carrier frequency substantially without interference, false alarms or alarm failure. The invention also makes it possible to combine maximum power radio frequency transmission in the same small package and in close proximity to low-voltage digital electronics at a reasonable cost. Range is increased and receiver sensitivity to signals from the transmitter also is increased when compared with radio frequency monitoring systems operating under similar regulations.

In addition, the invention includes a custom digital integrated circuit having utility for a variety of situations where remote monitoring is desirable. The system can be used in monitoring children, the elderly in rest homes, or those confined to prisons or detention facilities. It can also be used as a water safety device, or for tracking the whereabouts of a variety of moving objects.

SUMMARY OF THE INVENTION

Briefly, the invention provides a monitoring and alarm system with a radio frequency link between a receiver and a remote transmitter carried by a person or object being monitored. One embodiment of the invention includes an FM transmitter that produces a transmitted signal at an FM carrier frequency. The FM signal greatly reduces interference from AM sources or electrical noise. The transmitted signal is a digitally-coded signal produced at pre-set transmission intervals in the form of multiple digitally coded words detected by the receiver. The receiver is a constant listening device which produces an alarm immediately if none of the coded words is received during any transmission interval. The multiple-coded words sent during each transmission interval minimize false alarms at the receiver due to interference, since the receiver needs to validly receive only one of the multiple- coded words during a given transmission interval in order to not produce an alarm. If none of the coded words is received in a transmission interval, the receiver will produce an emergency alarm. In one embodiment, a factory adjustable encoder shifts the FM carrier frequency, during transmission of the multiple encoded words, for each transmission interval. This greatly reduces the probability of alarm failure occurring from the overlap in the frequency of transmitted' signals from other similar transmitters.

In another embodiment, the multiple-coded words are transmitted at minimum time intervals, which serves to maximize the number of coded words during each transmission interval, while spacing the coded words to permit maximum allowable power transmission.

In one embodiment of the invention, out-of-range information is transmitted to the receiver by adjustments at the receiver that constantly detect a pre-set signal strength value of the signal from the transmitter. If the magnitude of the signal from the transmitter falls below a pre-set sensitivity of the receiver, no signal is received and the receiver immediately produces an alarm.

A signal priority system also is used to transmit the signals which operate alarms at the receiver. Emergency conditions such as out-of-range, immersion, and tampering with the transmitter, for example, are given highest priority. If an alarm is generated because of any of these occurrences, other signal transmission from the transmitter to the receiver is overridden. This can avoid the possibility of alarm failure. Medium priority can be given to signals such as call-alarm. Lowest priority can be given to status signals such as wetness detection, low battery condition, and the like. In a further embodiment, the receiver can monitor two transmitters simultaneously operating at the same carrier frequency. The outputs from the two transmitters are adjusted so that their coded words occur at different time periods xvithin the same transmission interval to avoid the probability of overlap. If any coded words from the two transmitters should overlap, other valid words produced by the transmitters during the same transmission interval will be transmitted to the receiver, avoiding interference and false alarms. The duration of each of the ceded words from both transmitters also can be controlled to maintain signal integrity while permitting near maximum power transmission.

Other embodiments of the invention make it possible to operate at low voltages with low power consumption in combination with the high-power radio frequency link.

Both are made available in the same small package that can be produced at a reasonably low cost.

These and other aspects of the invention will be more fully understood by referring to the following detailed description and the accompanying drawings.

DRAWINGS

FIG.. 1 is a perspective view showing the exterior^' configuration of a housing for a radio frequency receiver portion of the monitoring and alarm system according to principles of this invention.

FIG. 2 is a front elevation view illustrating the exterior of a housing for a radio frequency transmitter of the monitoring and alarm system.

FIG. 3 is a side elevation view taken on line 3-3 of FIG. 2 for better illustrating a mounting clip on the transmitter housing.

FIG. 4 is a rear elevation view taken on line 4-4 of FIG. 3.

FIG. 5 is a functional block diagram illustrating the circuitry for the radio frequency transmitter portion of the monitoring and alarm system.

FIG. 6 is a functional block diagram illustrating the circuitry for the radio frequency receiver portion of the monitoring and alarm system. FIG. 7 is a fragmentary perspective view showing a safety clip attached to the transmitter.

DETAILED DESCRIPTION

The present invention is described with reference to a remote child monitoring and alarm system. In this system, a radio frequency transmitter worn by the child sends digitally-coded signals to a portable receiver carried by a guardian. When the child strays beyond a desired range, or when other monitored emergency conditions are detected, the radio frequency signal activates an alarm at the receiver. The invention is not intended to be limitedto suchchildmonitoringand alarmsystems, however, because the invention is useful for a variety of other remote monitoring applications.

Referring to FIG. 1, a portable monitoring receiver 10 contains internal circuitry for receiving digitally- coded FM signals from an FM transmitter. The receiver housing has an external mounting clip 12 so the receiver can be worn by a parent or guardian. The exterior of the receiver housing includes a flexible antenna 14 directly affixed to a printed circuit board contained in the housing. The exterior of the receiver housing also includes a number of alarm or status displays and controls. Some of these are located in a recessed region on the front face of the housing. They include an EΞD bar signal strength indicator 16 for displaying a relative approximation of the distance from the receiver to the transmitter. The desired maximum range can be set by a slidable position switch 18. If the range of the person being monitored exceeds the desired range set by the switch 18, an alarm is sounded in the receiver. If the switch is set at the low distance setting, for example, then the receiver will alarm when the person carrying the transmitter exceeds that relatively low distance from the receiver. The high switch setting allows the person to travel longer distances from the receiver before the out-of-range alarm is activated. The receiver housing also has a battery charging port 20, a slidable on/off switch 22, and an indicator lamp 24 for indicating when battery charging occurs. The on/off switch 22 also can include a check position for checking for valid signal transmission and for determining whether a constant audible update beep tone is received. The receiver housing also can include other alarms and indicator lamps. These can include LED indicator lamps 26 and 28, respectively, for visually indicating detected emergency conditions (described below) in conjunction with sounding corresponding audible alarms. The receiver alarm circuitry is set to produce the same visual alarms via flashing the LED displays 26 or 28 independently of the particular alarm being detected. Alternatively, separate LED displays can be activated to indicate the particular emergency being detected. The audible signals produced by the receiver are detected through sound holes 29. The various arrangements of the emergency and status condition warning lamps and displays and the control switches shown in FIG. 1 are illustrated as an example only, since other variations of this arrangement can be used without departing from the scope of the invention.

FIGS. 2 through 4 illustrate a portable transmitter housing 30 containing circuitry for a digitally-coded FM transmitter. The transmitter can be worn by a child whose whereabouts are being monitored. The front face of the housing includes a push button 32 for serving as a manual panic switch or call button to send an alarm to the receiver to alert the guardian when the child believes his or her safety is in danger. The receiver carried by the guardian sounds an alarm when the panic button 32 is activated. The front face of the transmitter also can include an indicator lamp 34 which blinks at a 4 hz rate to indicate transmission, alarm and status signaling.

As shown best in FIG. 3, the transmitter housing has a rear mounting clip 36 for use in attaching the transmitter to the child's clothing. The clip 36 can serve as a safety clip to activate a power switch connected to the transmitter circuitry for activating the transmitter and for sending a signal to the receiver to indicate when the transmitter has been tampered with or is removed from the child's clothing. The safety clip is mounted to the housing by a spring-biased roll pin 35. When the safety clip normally attaches the housing to the child's clothing, the power switch remains closed. The power switch opens when the safety clip is momentarily opened or removed from the clothing, thereby activating an anti-tamper alarm, described in more detail below.

The bottom face of the transmitter housing can include a pair of phone jacks 38 electrically connected to circuitry within the transmitter housing for producing signals sent to the receiver to monitor various conditions such as immersion and wetness, and breathing. If the transmitter senses wetness, or is immersed in water, if the subject stops breathing, or if any of the external sensors or probes is unplugged during use, an alarm sounds at the receiver.

The transmitter and receiver functions are understood best by referring to the functional block diagrams of FIGS. 5 and 6, Briefly, the transmitter serves as a remote transducer worn by the child being monitored. An emergency warble alarm sounds at the portable monitoring receiver if the child exceeds a pre-set distance range, falls into water, if transmitter removal is attempted, if the breathing sensor indicates that breathing has stopped, or if the transmitter's call button is pushed. A telephone ringing sound can be produced at the transmitter if the call button is actuated. Other status alarms are activated if the wetness sensor probe or pad become wet, or if either the transmitter or receiver battery power becomes low. A sound similar to dripping water can be produced if the wetness sensor is activated. The receiver is constantly listening for multiple digitally-coded FM signals from the transmitter at pre-set transmission intervals of about 10 seconds. If a required signal is not received within any transmission interval, such as if a pre-set range is exceeded, an alarm at the receiver is immediately activated. TheFMcarrierandmultiple-codedburstswitheachtransmission interval reduce the probability of false alarms. The transmitter has a removable battery in a separate adapter (not shown) for permitting continuous operation of the transmitter. The transmitter and receiver circuits are also designed to permit rechargeable operation of both the transmitter and receiver. In addition, the receiver is capable of simultaneously monitoring two transmitters transmitting at the same carrier frequency. FIG. 5 schematically illustrates a digitally-coded FM transmitter 40 which includes an FM hybrid transmitter 42, a 2700 logic gatecustomCMOSdigital integratedcircuit (I.e. ) 44, and associated analog components and a rechargeable battery. The transmitter is designed for a maximum battery life. A small 2.5 volt, 60 mah nicad button battery 46 directly powers the custom digital I.C., a 9 volt converter 48, and all analog loads. The 9 volt converter supplies the hybrid transmitter 42 and a low-voltage battery reference divider 50 with a regulated 9 volts. The transmitter is activated by clipping it to clothing which closes a safety clip switch 52 in response to contact by the external mounting clip 36 on a transmitter switch plunger 37, shown in FIG. 7. The plunger 37 can slide into a passage in the transmitter housing for contact with a switch that closes when the clip is in place. The plunger also can be seated in a recess in the clip. When the clip 36 is used to clip the transmitter onto the clothing of the person wearing the transmitter, the clothes create an interference which prevents the plunger from entering fully into the recess in the clip. This forces the plunger into the transmitter housing where it closes the switch to indicate that the transmitter has been properly clipped onto the clothing of the person wearing it. The safety clip switch powers all analog loads, activates the digital I.C. via a power-down section 54, and functions as the anti-tamper sensor. When the transmitter is not in use, a few microar.ps of current continue to flow to the power-down section 54 of the digital I.C. to maintain function of the power-down section. About 60 to 120 microamps of current continue to the 9 volt converter from the battery 46. The 9 volt converter is directly connected to the battery to enhance efficiency due to the relatively large switching currents developed during operation.

The radio frequency signal is produced by the FM hybrid transmitter 42. The FM frequency is generated by a surface acoustic wave (SAW) oscillator 56 for frequency stability. The output of the oscillator 56 is amplified by an amplifier 58 to produce an output of about 50 to 80 mw into a tuned resonant printed circuit antenna 60. The supply voltage from the converter 48 is applied to the

A oscillator and amplifies at all times, but neither the oscillator nor the amplifier is turned on until 0.1 ms before a coded signal is to be transmitted:" At that time, a pre- enable buffer switch 62 produces an output pulse for turning on the oscillator and amplifier before a coded signal is transmitted. The pre-enable function permits the oscillator to stabilize before transmitting. The hybrid transmitter also includes a varactor modulator 66 which receives output signals from a modulator buffer switch 68. A pulse code modulated (PCM) signal is sent to the modulator buffer switch from a Manchester encoder 70. When the PCM signal is sent to the modulator buffer switch, it causes the varactor modulator to shift the 318.0 mhz carrier frequency by 70-100 khz with each "logic 1" trans- mitted. That is, the carrier frequency is shifted when each "logic 1" is sent out so that the receiver can distinguish between a one and a zero.

The digital I.C. 44 provides all timing and logic functions for the transmitter. A 32.768 khz watch crystal oscillator 72 provides the time base for the digital I.C. A burst timer 74 causes transmission of four Manchester encoded digital words in rapid succession during each transmission interval of 10.7 seconds. Each coded word output from the Manchester encoder 70 contains an 11-bit factory set address code 75 from aprinted circuitprogramming pad and a 6-bit alarm status data cede 77 from a data register 78. The output of the burst timer 74 is applied to a pulse timer 80 that produces four enabling pulses at 103 ms intervals during each transmission interval. By changing one data bit, the transmitter can change from operating as the primary or standard transmitter to operating as a second similar but optional FM transmitter producing four enabling pulses at 110 ms intervals. The burst timer 74 instructs the four-pulse timer when to send the pulses during each transmission interval. The burst timer sets the transmission intervals at 10.7 seconds. The enabling pulses from the four-pulse timer 80 are applied to a 0.1 ms delay timer 82. The delay time^ then sends the time- delayed pulses from the four-pulse timer to the Manchester encoder, delayed by 0.1 ms. At the same time, the four output pulses from the four-pulse timer 80 are applied directly to the pre-enable buffer switch 62 for turning on the oscillator 56 and amplifier 58 in the FM transmitter 0.1 ms before the digitally-coded signals are received by the oscillator.

The four coded words sent from the Manchester encoder with each burst received from the burst timer minimize false alarms at the receiver due to interference, because the receiver needs to receive only one of the four coded words correctly during each transmission interval. Once a valid word is received, the receiver is reset for the next transmission interval. The coded words are also spaced apart in time to permit maximum power transmission under current FCC regulations. According to Part 15.122 of the FCC Rules, transmitted power is averaged over a 100 ms time interval. The coded words are transmitted at minimum intervals of 103 ms. Also, the beginning to end of each burst must not exceed 357 ms, under present regulations. This limits the number of words to four for each transmission interval. The eleventh bit of the factory set code is set to a "logic 1" on a secondary optional transmitter for sending digitally-coded signals at the same FM carrier frequency to the same receiver. This produces 110 ms word intervals between the coded v/ords sent by the Manchester encoder in the secondary transmitter, and thereby permits the two transmitters to operate independently on the same carrier frequency without mutual interference. With this arrangement, if the burst timing of the standard and secondary transmitters should coincide or overlap, only one of the four coded v/ords v/ould simultaneously overlap, leaving three valid transmitted v/ords per transmission interval; or if any of the coded words happens to overlap with one sent from the secondary transmitter, or from any other independent transmitter, other valid v/ords can be received during the set transmission interval. As a result, false alarms are essentially avoided because the system generates the redundant coded v/ords for each trans¬ mission interval, and only one valid word needs to be transmitted to the receiver to reset the receiver for the next transmission interval. By sending four coded words over a relatively long time interval with interdigitation due to their difference in time period, the probability is that at least one coded word will be passed to the receiver from each transmitter without interference. If at least one of the coded words is not received by the receiver during any transmission interval, then an emergency condition is immediately detected at the receiver, and an alarm at the receiver is activated. The duration of each coded word is limited to about 2 ms to maintain signal integrity while permitting boosting signal power due to FCC averaging over a 100-ms word interval.

As mentioned previously, the transmitter generates both emergency alarms and status information. All emergency alarms (except out-of-range) immediately trigger the burst timer 74 out of sequence to send a coded burst to the transmitter circuitry, followed by no bursts for 22.5 seconds, or until the alarm condition is corrected. A 22.5 second delay timer 84 controls the time delay following a triggered emergency alarm. The burst timer normally sends its timing pulses to the Manchester encoder each 10.7 seconds, but when the burst timer is triggered by an alarm signal, it immediately sends a timing pulse to the Manchester encoder to send the emergency alarm to the receiver. After the burst timer is triggered by the emergency condition, the burst timer is disabled for the next 22.5 second time interval to prevent other pulses from being sent by the burst timer during the emergency alarm phase. The purpose is to set--a signaling priority. If emergencies such as immersion, tampering sensed by the anti-tamper switch 36, or breathing failure are detected, an emergency alarm is immediately generated; and transmission of other secondary information, such as call-button signaling or wetness detection are overridden. Triggering inputs to the burst timer are controlled through an OR gate 96. The disable inputs to the burst timer are controlled through an _OR gate 97. During the "no burst" period, an LED 86 flashes at a 4 hz rate controlled by a 4 hz oscillator 88 coupled to the LED display through an OR gate 90 and buffer switch 92. The burst timer is disabled at the OR gate 97 to prevent any other coded signals from being sent out once an emergency alarm has been triggered. The purpose is to prevent sending other digitally ceded bursts which could be interfered with externally and which might erroneously inform the receiver that the emergency was later corrected and that the transmitted alarm signal had simply been a false alarm. That is, by stopping transmission of these further signals, interference from such signals is prevented, where such signals possibly could reset the alarm and make the alarm cease, while making the user think that the alarm that sounded was a false alarm, when it was not.

The anti-tamper alarm system sends an alarm signal to the receiver when tampering with the transmitter is detected. The output from the safety clip switch 52 is coupled to an input of an AND gate 94. The output of the safety clip switch also is coupled to the burst timer 74 through the OR gate 96 which controls the triggering input to the burst timer. When the safety clip switch is closed, the power-down section 54 is disabled and the digital IC is powered up, with all registers and timers being initialized by the system reset 98, which is coupled to the output of the safety clip switch. Within a few milliseconds, a coded signal is transmitted for indicating that the transmitter has been properly clipped to the person wearing it. An 8-sec./4-sec. delay timer 100 has an output coupled to the input of the AND gate 94. The output from the AND gate 94 is coupled to the 22.5 second delay timer 84. The AND gate 94 produces a "logic 1" when the inputs from the safety clip switch and the 8-sec./4-sec. delay timer are in a "logic 1" state. The 8-sec. output from the delay timer 100 serves as a disable signal applied as an input to the AND gate 94. When the 8-sec./4-sec. delay timer starts, it disables the 22.5-sec. delay timer 84. The output from the 22.5-sec. delay timer is coupled to the data register 78. The delay timer output also is connected to the OR gate 96 that triggers an input to the burst timer and to the disable input of the burst timer through the OR gate 97. Once the 22.5 second delay timer has been disabled, the safety switch clip can be clipped and undipped at will for the first 8 seconds v/ithout activating the 22.5 second anti-tamper alarm. The 4 second time delay output from the delay timer 100 is coupled to an input of the OR gate 90 and then to the LED display 86. During the first 4 seconds of the 8 second period after the safety chip is closed, theLED 86 stays onto indicatereclipping is still permissible without activating the 22.5 second anti-tamper alarm. If the safety clip switch is opened after the 8 second delay period, the delay timer 100 times out and enables the AND gate 94 and the burst timer 74 is triggered to transmit an immediate anti-tamper alarm signal to the receiver. The anti-tamper alarm can be activated even if the anti-tamper clip switch is opened for a few milliseconds. The anti- tamper alarm signal is followed by no signal transmission to the receiver for the 22.5 second delay time interval, even if the transmitter is immediately reclipped. If the transmitter is immediately reclipped, normal transmission will resume within 22.5 seconds. If the transmitter is not reclipped, then all transmissions cease, and the power-down section 54 is triggered to power down the I.e. The breathing alarm system produces an alarm signal if sensed breathing rate is too fast or too slow. The breathing alarm is controlled by a capacitive sensor (not shown) coupled to a. breathing monitor jack 102. The sensor detects abdominal motion, and thereby provides breathing information to the breathing alarm system. The breathing sensor can be formed by two small plates about one inch square attached to a semi-rigid plastic substrate and coated v/ith a thin layer of plastic. The plates are held in Contact v/ith the abdominal area by a strap around the body. The dielectric constant of v/ater is about 80 times that of air, and since the plates make imperfect contact with the body, inhaling increases capacitance, and exhaling causes a reduction in capacitance. No movement causes no change in capacitance. When the breathing monitor plug is inserted into the input jack 102, the breathing monitor is enabled. The 32.768 khz time base serves as the carrier for detecting capacitance changes. The time base signal is connected to one of the capacitor plates, and the other capacitor plate functions as an output plate. During use, the magnitude of the 32.768 khz signal at the output plate varies directly with breathing. The output from the breathing monitor jack 102 is coupled to a low-pass filter 104 which strips the 32.768 khz carrier from the signal, leaving a low-level, time-varying signal. This signal is amplified by a virtual ground input amplifier 106 v/ith level changes detected by a virtual ground input comparator 108. The output from the comparator 103 is coupled to the LED display 86 through the OR gate 90 and buffer switch 92. Because of a small amount of hysteresis built into the comparator, the transmitter LED flickers with a rate and duration proportional to the rate and duration of each inhaled breath. This confirms that breathing is occurring and that the monitor is properly attached to the person being monitored. The breathing monitor alarm is produced by coupling the output from the comparator to a 5 to 20 second time interval timer 110. The output of the timer is coupled to the trigger and disable inputs of the burst timer 74 through the OR gates 96 and 97, respectively. If, during any 20 second period, there are either no pulses or more than 40 pulses from the comparator 108, the timer 110 is not reset. This produces a breathing monitor alarmby immediately allowing the burst timer to send a breathing monitor fault code to the receiver, followed by no transmissions to the receiver and a continued alarm at the receiver until either breathing resumes or the breathing monitor sensor is unplugged from the jack 102. The LED S6 flashes at a 4 - hz rate during the alarm condition.

The immersion alarm is activated when the transmitter is immersed in v/ater. Immersion of the transmitter is sensed by the exposed cylindrical conductors of two adjacent subminiature phone jacks 102 and 112 at the bottom of the transmitter housing. One of the conductors is at ground potential. The other conductor is pulled to a high potential by a large value resistance and is common to the input of an immersion comparator 114. When v/ater bridges the conductors, the immersion comparator input drops below a comparator reference voltage for activating the immersion alarm. The output from the immersion comparator is coupled to the data register 78 and to the triggering and disabling inputs of the burst timer through the OR gates 96 and 97. The burst timer is immediately triggered when the output from the comparator is produced to send an immersion fault code to the receiver. The LED 86 also flashes at a 4 hz rate. All further transmissions thereby cease until the transmitter is properly dried and restored to operation.

The call alarm or panic alert operates as follows. The call button 32 on the transmitter is coupled to the data register 78 and to the burst timer 74 through a 22.5 second latch timer 116. When the transmitter call button is momentarily actuated, the 22.5 second timer is latched, v/hich triggers the burst timer to send an immediate call alarm fault code to the receiver, followed by no further signal transmissions for 22.5 seconds. During the 22.5 second time interval, the LED 86 flashes at the 4-hz rate. At the end of the that period, the call condition resets, and normal transmitter function resumes.

The monitoring system detects diaper wetness and bed wetness. Diaper wetness is sensed by an external two- conductor probe (not shov/n) , and bed wetness is sensed by an external two-conductor screen (not shown) . The output detected by either wetness sensor is shov/n coupled to the jack 112 on the transmitterhousing, although these detectors also could be coupled to other separate input jacks on the transmitter housing. The sensed wetness information is coupled to a wetness comparator 118. Conductive fluid or water at the sensor pulls the wetness comparator input below a comparator reference input to activate the wetness alarm. The output from the v/etness comparator is coupled to the data register 78, but not to the burst timer because the wetness alarm is not an emergency condition. The wetness fault code is instead transmitted to the receiver with each burst until the wetness condition is corrected or the sensor is removed from the jack. Other sensors may also plug into the v/etness jack, permitting sensing of light, heat, pressure, force, etc.

The voltage output from the 2.5 volt nicαd battery is constantly monitored by a low-battery voltage comparator 120. The battery voltage is compared v/ith a factory-set reference resistor divider voltage having a regulated 9 volts from the 9 volt converter 48 as its source. When the battery voltage falls below the reference divider factory-set voltage, the low-battery comparator causes a low-battery voltage fault code to be transmitted v/ith each burst until the battery is changed or recharged.

The FM receiver is understood best by referring to the functional block diagram of FIG. 6. The receiver includes a power supply 122, a radio frequency section 124, an intermediate frequency (I.F.) digital integrated circuit (I.C.) 126, a 2700 gate CMOS custom digital I.e. 128, a signal strength meter 129, a power switch 130, and miscellaneous analog components. The receiver also has a range switch 131, the signal strength bar indicator 16 (see FIG. 1) , red and green transmitter monitor LEDs 132 and 134 (shov/n at 26 and 28 in FIG. 1) , a charge status LED 136 (shov/n at 24 in FIG. 1), and an audible alarm 13S. Thepower supplyprovides 2.5 volts to operatethedigital I.C. and analog loads. It also provides 8.0 volts for operating the radio frequency section, the I.F.^' digital I.C. , and a low-battery reference divider 140. The main component of the power supply is a pair of series cylindrical nicad cells 142 providing 2.5 volts at 500 ah. A 6 vac household current battery charger 144 permits simultaneous charging of the battery while operating the receiver. An optional DC adapter can permit charging or operating the receiver from 12 vdc automotive current. An optional solar panel can permit charging or operating the receiver directly from sunlight. A single adapter can allow any of the chargers to charge up to tv/o transmitter batteries while charging the receiver battery. A resistor between the battery charger and the receiver battery pack 142 drops excess voltage and limits the charge current to the battery pack. The charge status LED 136 is illuminated while the battery pack is being charged. The transmitter battery charger adapter is designed the same way. When not in use, and with the power switch in the off position, no current flows from the battery pack to the loads. When the power control switch is moved to the check or on positions, power is provided to all loads. When in the check position, an audible beep is emitted each time a signal is received (approximately every ten seconds) , as well as emitting a flash from the appropriate transmitter monitor LED. When in the on position, only the LED flashes. The receiver system is a superheterodyne receiver in which the frequency of incoming radio signals is converted to an intermediate frequency by mixing v/ith a locally generated signal. The radio frequency section 124 of the receiver provides a down conversion function by converting the 318.0 mhz FM carrier signal to a 10.7 mhz intermediate frequency (I.F.) signal. The 318 mhz signal is received through a 5-inch long antenna 146, which is impedance- matched to 50 ohms v/ith a loading coil in an antenna matching circuit 148. The signal is then coupled to a pre-amplifier 150. The essential signal-to-noise ratio is determined by the gain and noise contribution of the RF pre-amplifier. The selected pre-amplifier transistor is based on the RF performance requirements and a demanding current consumption constraint necessary in order to have a product with substantial operational battery life. The output from the pre-amplifier is coupled to a -7 d3 mixer, which is a balanced Schottky barrier diode ring type mixer. This mixer is used because of the good performance with variable mismatches experienced in production and because of high immunity to interference. The mixer improves the product operating reliability. The Schottky diode is characterized by nanosecond switching speed, but relatively low voltage. A local oscillator 154 produces a 307.3 mhz signal of sufficient power level to bias the mixer. Oscillation frequency is determined by a surface acoustic wave (SAW) device designed into a transistor circuit where the DC to RF efficiency is of prime importance to conserve battery life. The out-put from the mixer 152 is a 10.7 mhz intermediate frequency, which is then filtered by a 400 khz ceramic filter 156 to preserve the difference signal (318.0 mhz - 307.3 mhz = 10.7 mhz). The output from the filter is then amplified by a high-gain, super low current-consumingtransistor stage of a 22 dB intermediate frequency pre-amplifier 158. The entire radio frequency section 124 is designed to operate on very low current (about 5 a) .

The I.F. digital I.C. stage 126 further processes the 10.7 mhz output from the radio frequency section 124. The I.F. stage 126 amplifies, filters, and detects the FM signal, as well as providing a signal strength output for operating 1 a signal strength display. The I.F. digital I.C. is speci¬ fically selected for its low current consumption of about 2.0 a to conserve battery life as well as its wide-band capability. The signal into the I.F. digital I.C. is amplified 5 by a 40 dB amplifier 160. The output impedance of the amplifier 160 forms a resistive divider v/ith single range switch resistors to ground for limiting range on low and medium range switch settings of the range switch 131. Range can be set in tens of feet on low range and hundreds

10 of feet on high range, which has no resistor to ground. A second 10.7 mhz, 400 khz band width ceramic filter 162 is coupled between the 40 dB amplifier 160 and a 50 d3 limiter amplifier 164 for further improving the signal-to-noise ratio. The 50 dB limiter amplifier further boosts signal

15 level up to 50 dB as required. The signal from the limiter amplifier 164 is detected by a quadrature detector 166 to remove the modulation from the 10.7 mhz FM intermediate frequency carrier signal. The Manchester data code detected by the receiver is originally in the form of square waves

20 on a DC level. A virtual ground buffer amplifier 168 amplifies the square wave component to provide a low impedance source to drive the digital I.C. This output from the amplifier 168 represents the digitally coded receiver input signal v/ith all emergency alarm status 25 condition data from the transmitter. This signal is _%/ decoded and further processed by the receiver I.e.^" <■

Signals from both the 40 dB amplifier and the 50 dB limiter amplifier are independently rectified by a full- wave rectifier 170 and summed in a voltage-to-current

30 converter 172 to produce an output current logarithmically proportional to signal strength. This output is used to operate the relative signal strength meter 129.

A high-impedance, current-to-voltage converter 174 changes the relative signal strength current of the I.F.

35 digital I.C. to a voltage output amplified and changed to a low-impedance source by a buffer amplifier 176. The output from the buffer amplifier is filtered by a low-pass filter 178 and then fed to a 5-element bar LED driver for a 5-LED signal strength indicator 182. The output of the amplifier is low-pass filtered so that each of the four 2 MS digital v/ords will cause much of the meter's bar to remain illuminated until the next 2 MS word arrives. The result is a bar which remains illuminated at a length proportional to relative signal strength with the most significant LED flickering. The 5-element bar LED driver converts the signal level into five discrete steps and drives the 5-LEDbar signal strength indicator. Approximately every ten seconds, the signal strength indicator flashes on v/ith the received signal giving a relative approximation of distance to the transmitter.

The digital I.C. performs all timing, logic, alarm generation, and signal decoding for the receiver. It uses a 32.768 khz watch crystal 184 as its time base. The output signal from the intermediate frequency I.C. (i.e. from the amplifier 168) provides the input signal 186 to the digital I.C. 128. This input signal is processed by a Manchester decoder 188 by comparing a 10-bit, factory set address code 190 with the incoming 11-bit address code. If the first ten bits of the address code correlate v/ith the receiver code, a valid output signal 189 is generated by the Manchester decoder. Valid output signals mean that coded information in the decoder matches coded information sent from the transmitter. For instance, as long as any of the four coded v/ords produced during each transmission interval is received, a valid output is generated. The eleventh bit determines whether the valid signal is from the standard transmitter or a secondary transmitter. The remainder of the signal includes alarm fault data, which is latched into a data register 192 for the standard transmitter or an optional data register 194 for the optional or secondary transmitter. The valid output signal from the Manchester decoder is coupled to a first AND gate 196 corresponding to the standard transmitter and to a second AND gate 198 corresponding to the optional transmitter. The standard output signal from the Manchester decoder is sent to an 8 second timer 200 and to a standard transmitter ENABLE latch 202, the output of which is applied to an 11.1 second standard transmitter timer 204. This output produces an enabling signal to the 11.1 second timer 204. The output from the first AND gate 196 is coupled as a RESET signal to the 11.1 second timer. The optional transmitter output from the Manchester decoder is similarly coupled to an 8 second timer 206 and an optional transmitter ENABLE latch 203 to provide an enabling input to an 11.1 second optional transmitter timer 210. The %^ralid output signal from the Manchester decoder is coupled with the optional transmitter output through the second AND gate 198 to the optional transmitter timer 210.

When a transmitter is first activated, it immediately sends a burst. If the burst is from the standard transmitter, it starts the 8 second timer and causes the red LED 132 to glow during the 8 second period to indicate that the transmitter clip may still be repositioned v/ithout activating the anti-tamper alarm. The 8-second timer is connected to the LED 132 through an OR gate 211. At the end of the 8 second period, the standard transmitter ENABLE latch is triggered, which causes the standard transmitter timer to start. If no valid signal is received before 11.1 seconds elapses (e.g., the transmitter moves beyond the preset range) , a warble oscillator 212 begins and continues until a valid reset signal is received. The optional transmitter signal is detected and monitored in the same manner as the standard transmitter. The 8-second timer is connected to the optional LED 134 through an OR gate 213. The LED signals indicate the time period during which reclipping is permissible without causing transmission of an alarm signal. The outputs fromthe standard and optional 11.l second timers are coupled to the warble oscillator 212 through an OR gate 214. The output from the optional transmitter also passes through the OR gate 215. The output from the OR gate 214 is coupled to the red LED 132 and the green LED 134 through a 4 hz oscillator 216. If one of two previously-functioning transmitters is switched off while the receiver is still operating, the warble alarm 212 will sound until the receiver pov/er switch is momentarily switched off. This causes the power-up system reset 218 to unlatch both ENABLE latches and terminate the alarm. If neither of the tv/o transmitters is functioning when the receiver pov/er switch is turned on, the receiver will remain silent until a valid signal is first received.

Each time a valid signal is received and indicated by signal 189, a beep timer 191 generates a 25 s beeping tone at the audible alarm 133.

The data registers 192 and 194 store alarm fault data received from the transmitter. For example, when a valid address signal from a standard transmitter is decoded, the first AND gate 196 strobes high to store the data bits of the Manchester decoder 188 in the -standard data register 192. Each of the six data bits conveys alarm status information. In addition, loss of signal or out-of-range alarm is given at the output of the transmitter timer. Receiver lowbattery is determinedbythe receiver low-battery comparator 280 external to the digital I.C. The alarms are separated into three categories listed in descending order of priority as follows: emergency, call, and status alarms. Emergency alarms include out-of-range, immersion, anti-tampering, and breathing monitor. There is only one call alarm. Status alarms include transmitter low battery, receiver low battery, and wetness detection. All emergency alarms are combined at the OR gate 214 which activates the warble oscillator 212 and disables the call and status alarms at AND gates 220, 222, and 224, respectively. The warble oscillator signal combines with a 3 khz oscillator signal from an oscillator '226 at an AND gate 228 to operate the audible alarm at a low level, through a low-level buffer 229. Simultaneously, the output from an OR gate 230 triggers a 4 second timer 232, which permits four seconds to elapse before switching to an AND gate 234 to pass the warble oscillator signal through an OR gate 236 to combine at an AND gate 238 to operate the audible alarm at high volume. The audible alarm on high volume is controlled through a high-volume buffer 239. During the audible alarm, the LED 132 is flashed by the 4 hz oscillator 216, which is activated by the OR gate 240. The output of the oscillator passes through an OR gate 242 and is passed through either or both_of the AND gates 244 and 246, depending upon which of the standard and/or optional transmitters is at fault as sensed at the OR gates 248 and 250, respectively. If, for example, the standard transmitter is in fault, either (SA) at the output of the OR gate 252 or (SA') at the output of the standard transmitter timer 204" is high. This causes only the red LED 132 to flash at the 4 hz rate. The green LED 134 is activated in a similar manner to produce an emergency alarm for an optional transmitter.

The receiver constantly monitors range information from the multiple coded bursts sent by the transmitter during each 10.7 second transmission interval. Range is determined as a function of the signal strength of the signal received by the receiver. Out-of-range is measured by controlling the sensitivity of the receiver. The range switch 131 is used to adjust the sensitivity of the receiver. If a low range is desired, the range switch on the low setting reduces the receiver sensitivity, i.e., its ability to receive signals from the transmitter. Therefore, the person wearing the transmitter can exceed a shorter out- of-range distance before the reduced sensitivity will cause the signal to drop out and produce an out-of-range alarm. The out-of-range condition produces a loss of signal at the output from the 50 d3 amplifier 164. It also causes the output from the current-to-voltage converter to drop out so that the signal strength meter will indicate an out-of-range condition. Loss of signal is detected at the output 186 from the amplifier 168. None of the signals from the transmitter v/ill be received during the 10.7 second transmission interval when loss of signal occurs. In this event, AND gate 196 does not reset the 11.1 second timer 204 and the timer times out and OR gate 240 then activates the warble oscillator 212 to indicate the out-of- range condition. The magnitude of the signals sent to the receiver is used to detect out-of-range. Although the signals sent to the receiver are digitally coded, the digital codes (for transmission of address and data infor- mation) are not used for transmitting range information. Out-of-range information is transmitted only through the signal strength of the digitally ceded signals, and the range switch adjusts receiver sensitivity to the incoming signals in order to control the level at which the out-of- range alarm is activated.

This technique for detecting out-of-range condition through the use of receiver sensitivity adjustments is important in reducing the overall cost of the receiver. Signal sensitivity is controlled by the voltage divider which divides the incoming signal so that the output level of the signal can be lov/er than its input level to use the lower signal level to reduce receiver sensitivity. This technique requires only a few resistors and a standard switch to accomplish its purpose at a modest cost. A call oscillator is generated frcm 20 pps of 25 ms, each being switched on and off at a 1 hz rate. The call alarm sound resembles a telephone ring. A call timer 253 is activated by call fault data, either from the standard data register 192, or from the optional data register 194. Either signal passes through an OR gate 254 having an outputwhichdisablesthebeep, v/etness, andlow-batteryalarms via an invertor 256 at the AND gates 222 and 224. The OR gate 254 also enables the 4 hz oscillator via the OR gate 240, which flashes the LED's 132 or 134 via the OR gate 242, depending upon whether the optional or standard transmitter, or both, are registering a call fault. If the standard transmitter is at fault, the output (SC) combines at the AND gate 244, via the OR gate 248, v/ith the output from the OR gate 242 to flash the red LED. Λ call from the optional transmitter flashes the green LED 134 in a similar fashion. The call timer generates the call alarm, which passes through the AND gate 220 and the OR gate 230 to combine v/ith the 3 khz signal at the AND gate 228 to operate the alarm at the low level. Simultaneously, the signal output at the OR gate 230 triggers the 4 second timer 232, which activates the high-volume alarm via the AND gate 234, the OR gate 236, and -the AND gate 233 after the 4 second delay has elapsed. The v/etness alarm is a 25 ms pulse at tv/o second intervals generated by a v/etness timer 260. This timer is activated by wetness data fault codes, either from the standard or optional transmitters via the OR gate 262. The output from the v/etness timer operates the high-volume alarm via an OR gate 264, the AND gate 224, the OR gate 236, and the AND gate 238. The red LED 132 is flashed with each pulse when an OR gate 266 is activated, which drives the OR gate 248 via an AND 268 simultaneously with the OR gate 264 driving the AND gate 224 and the OR gate 242. AND gates 222 and 224 disable the wetness flashing when a call or emergency alarm occurs. The green LED 134 is flashed in a similar way by an OR gate 270 via an AND gate 272 and the OR gate 250 in conjunction with the OR gate 264, the AND gate 224, and the OR gate 242. The transmitter low-battery alarm comprises a 25 ms pulse followed by a second 25 ms pulse within a 0.5 second interval. The pulses are generated at a transmitter low- battery timer 273 with each ten second transmission trigger received from the OR gate 274 when enabled by an OR gate 276. The LEDs are discriminately driven, and high-volume alarm is functioned using the same gates and in the same manner as in the wetness alarm.

The receiver low-battery alarm is generated within a receiver low-battery timer 278 v/ith each ten second trigger from the OR gate 274 when enabled by a receiver low-battery comparator 280. The alarm is tv/o 25 ms pulses separated by 62 ms. No LEDs are flashed, but the audible alarm is activated in the same way and v/ith the same gates as the wetness and low transmitter battery alarm. It is possible for all three status alarms to occur simultaneously.

The following summarizes certain characteristics of the receiver circuitry, the implementation of which will be apparent to one skilled in the art.

The 10 dB gain RF amplifier consumes less than one milliamp of current and has a 4 dB noise level, v/ith the following selected transistor device electrical character¬ istics: collector-emitter breakdown voltage 5 volts DC (Min.) collector-base breakdown voltage 10 volts DC (Min.) emitter-base breakdown voltage 2 volts DC (Min.) collector cutoff current (Max.) 50 nanoamps DC current gain-bandwidth product 3 GHZ typical collector-base capacitance (Max.) 0.5 picofarads noise figure 4 dB typical

The I.F. preamplifier consumes less than one milliamp of current and has a 22 dB gain, with the following selected transistor device characteristics: collector-emitter breakdown voltage 5 volts DC (Min.) collector-base breakdown voltage 10 volts DC (Min.) emitter-base breakdown voltage 2 volts DC (Min.) collector cutoff current (Max.) 50 nar.oar.ps DC current gain-bandwidth product 3 Gr.Z typical collector-base capacitance (Max.) 0.5 picofarads noise figure 4 d3^*'typical

The intermediate frequency I.C. consumes less than

2.5 milliamps of current and has a 90 dB gain, with the following selected transistor device characteristics: pov/er supply voltage 4.5 volts DC (MIN.) field strength range 90 d3 typical field strength accuracy +/- 1.5 c3 typical

I.F. input impedance 1500 OHMS (MIN.) quadrature output impedance 50,000 OHMS (MIN.)

Claims

WHAT IS CLAIMED IS:

1. A remote transmitter and alarm system fordetecting the range of a person wearing the transmitter and for sensing an emergency to which the person may be subjected, comprising: a radio frequency transmitter for producing digitally-encoded output signals, the transmitter producing multiple digitally-encoded words during each of a series of transmission intervals; and a radio frequency receiver for receiving the output signals from the transmitter to produce an alarm in response to reception of the coded v/ords transmitted by the transmitter; the receiver having means for activating an alarm representing the occurrence of a particular emergency when none of the digitally-coded words is received by the receiver during any transmission interval, the receiver having means for preventing activation of the alarm so long as any one of the multiple digitally-encoded v/ords is received by the receiver during said transmission interval, so that the multiple digital signals thereby serve as redundant signals during each transmission interval for use in preventing false alarms at the receiver due to interference; the receiver also having means for detecting the signal strength of the signals from the transmitter to produce an out-of-range alarm of the detected signal strength falls below a pre-selected level.

2. Apparatus according to claim 1 in which the transmitter includes:

(a) means for producing a clock signal to represent a series of transmission intervals for the output signals sent to the receiver;

(b) encoding means responsive to the clock signal for producing an encoded output signal representing a series of digitally-encoded pulses during each transmission interval; and ' (c) means for transmitting the encoded signal to the receiver.

3. Apparatus according to claim 1 in which the transmitter includes means for sensing an emergency condition and producing an input to the transmitter v/hen the emergency condition is sensed, means for transmitting a digitally- encoded alarm signal to the receiver to indicate the sensed emergency condition,and means for interrupting transmission of further digitally-encoded signals from the transmitter to the receiver after the emergency condition has been sensed.

4. Apparatus according to claim 1 including means for adjusting the time interval between the digitally- encoded signals produced during each transmission interval so that the time period of the digitally-encoded signals can vary relative to the time period of digitally-encoded signals from a similar transmitter operating at the same carrier frequency, to thereby avoid mutual interference between the signals produced by both transmitters.

5. Apparatus according to claim 1 in v/hich the radio frequencytransmitter and receiver are an FMtransmitter and receiver.

6. Apparatus according to claim 5 in which the receiver is a superheterodyne receiver.

7. Apparatus according to claim 1 in which signal strength level is controlled by means in the receiver for adjusting receiver sensitivity to produce the out-of-range alarm in response to a selected out-of-range distance.

8. A radio frequency transmitter output controller for a monitoring system comprising: means responsive to a first input for producing a first control output signal comprising multiple digitally- encoded signals occurring at short time intervals during each_of a series of longer transmission intervals as long as the first input is continuously sensed for representing a non-emergency condition; means responsive to a second input for producing an interrupt signal representing a sensed external emergency condition; means for selectively disabling the first input in response to the interrupt signal to prevent further transmission of the first control output; and means for producing a separate digitally-encoded signal to present the sensed external emergency condition, the first control output being interrupted to prevent interference from other external signal sources during the sensed emergency.

9. Aremotetransmitterandreceiversystemcomprising: a radio frequency transmitter having a first output represented by a first series of digitally-encoded signals and a second output represented by a second series of digitally-encoded signals; a radio frequency receiver for receiving the first and second output signals from the transmitter and for producing a selectively activated alarm signal; means for producing an input signal to the transmitter for representing the occurrence of a sensed emergency condition; and means responsive to the input signal to interrupt transmission of the second digitally-encoded signal to the receiver; the receiver having means for measuring the signal strength of the first digitally-encoded signal for activating the alarm signal v/hen measured signal strength falls below a predetermined level; the receiver having decoding means for detecting interruption of the second digitally-encoded signal to activate the alarm to indicate occurrence of the emergency condition.

10. Apparatus according to claim 9 in which the input represents immersion.

11. Apparatus according to claim 9 in which the input represents breathing rate.

12. Apparatus according to claim 9 including delay means to prevent further emergency conditions sensed by the first input from producing the second digitally-encoded output signal during a selected delay period, once the alarm is activated in response to the sensed input.