Abstract: A vehicle disabling system for connecting to a vehicle ignition system to disable the associated vehicle after a pre-determined length of time following an actuation of the system by a user. The device includes a disabling circuit which may be installed in series with the ignition system of the vehicle. The circuit includes a panic switch operable to actuate a timer and, after a pre-determined length of time has elapsed, subsequently actuate a relay. The relay is operable to then close a switch which results in a current surge through a fuse, thereby blowing the fuse and interrupting a supply of power to the vehicle ignition system. An alternate embodiment of the present invention includes a key-contained fuse holder for containing the fuse within the ignition key of the vehicle.

Abstract: A motorized vehicle anti-theft system which is passive by being automatically armed (i.e., does not require a vehicle occupant to set the system). Once power has been turned on to the vehicle, detection of an attempt to start the vehicle motor or the opening of a vehicle door starts a predetermined time cycle. If a first reset signal is not received before the end of the time cycle, a vehicle disabling circuit is set. After that, a detection of actuation of the vehicle brake will cause the disabling circuit to only permit the vehicle engine to operate properly near the engine idling speed, and set off an alarm. Only proper generation of the first and a second reset signal will allow the vehicle to be operated properly again.

Abstract: A cellular mobile radiotelephone (CMR 12) which detects and reports security breach conditions is disclosed. The CMR (12) includes a remotely programmable unit (RPU 48) in which alarm parameter data (1200) may be recorded from a remote programming (RP) host (22). An input (54) may be adapted to receive a vehicle alarm system (26) output. When the alarm system activates, a Mode 1 Alarm procedure (1700) is performed. If the alarm indication is deemed genuine, the CMR (12) automatically places a call to a security service (28). When a vehicle ignition signal (52) has just turned "on", a Mode 2 Alarm procedure (1800) is performed. A user is given a period of time, which is established remotely (1214), to enter a disarming code. If the disarming code is not entered in time, a genuine alarm event is assumed, and the CMR (12) places a call to the security service (28). In placing calls to the security service (28), numerous alarm parameters are established remotely.

Abstract: A motorized vehicle anti-theft system which is passive by being automatically armed (i.e., does not require a vehicle occupant to set the system). Once power has been turned on to the vehicle, detection of an attempt to start the vehicle motor or the opening of a vehicle door (514) starts a predetermined time cycle (518). If a first reset signal is not received before the end of the time cycle, a vehicle disabling circuit is set (524). After that, a detection of actuation of the vehicle brake (526) will cause the disabling circuit to disable the vehicle (530) and set off an alarm (532). Only proper generation of the first and a second reset signal will allow the vehicle to be enabled again.

Abstract: In a glass smashing alarm system for generating an alarm when a glass smashing sound is detected, the present invention is directed to reduce the possibility of occurrence of an erroneous alarm, and includes a normal alarm mode in which a preliminary alarm is raised for a short time when a glass smashing sound is detected, and a special alarm mode in which a main alarm is raised for a long time when the glass smashing sound is detected, and is constituted in such a manner that mode switching is effected so as to shift the mode to the special alarm mode at the end of the preliminary alarm, and to return it to the normal alarm mode after the passage of a predetermined time if the glass smashing sound is not detected at the end of the main alarm or during the special alarming mode. In another aspect of the invention, generation of an alarm is inhibited if a key unlock switch is turned ON before passage of a predetermined time from detection of the glass smashing sound.

Abstract: A time delay and alarm system is provided to ensure safe operation of periodically activated apparatus having potentially dangerous moving parts. The system preferably employs a 555 type integrated timer circuit, a horn, and a pulse generator which causes the horn to produce a cyclical sound.

Abstract: An emergency lighting system includes a lighting unit having a battery which is kept in a charged condition in the presence of voltage on a power line to which the unit in connected. Upon loss of voltage, the battery is caused to discharge into a pair of flood lamps to provide emergency illumination to an area surrounding the unit. To provide for periodic testing of the unit without requiring the user to actuate a switch on the unit the system includes a radio frequency receiver in the unit which responds to signals generated by a hand-held radio frequency transmitter actuated by the user. A two stage battery charging circuit and a low voltage lamp cut-off circuit are included in the lighting unit for improved battery life.

Abstract: An alarm system includes an electrical energy source; an alarm for producing an alarm signal when electrically coupled to the electrical energy source; a switch for movement from a first position to a second position upon the occurrence of the monitored event; and timing circuitry for being activated when the switch is moved from the first position to the second position, for electrically coupling the alarm to the electrical energy source, for continuing to electrically couple the alarm to the source of electrical energy for a set period of time even if the switch is subsequently moved back to the first position during the set period of time, for deactivating the alarm a certain period of time after the alarm is activated, and for reactivating the alarm a certain period of time after the alarm is deactivated if the switch remains in the second position.

Abstract: A personal security system includes at least one movable transmitter adapted to produce an emergency signal transmission when activated and a plurality of transceivers adapted to receive emergency signal transmissions from the movable transmitter. False alarms are avoided by inhibiting multiple transmissions by a movable transmitter within a predetermined time period.

Abstract: A system for measuring inter-nodal transmission delays in a communications network interconnecting a source node, a destination node and a plurality of intermediate nodes requests a response from the destination node, receives a response from the destination node and then determines both an issue time at which the request is issued by the source node and a receipt time at which the response is received by the source node. A transmission delay between the source node and the destination node is deterined by calculating a difference between the issue time and the receipt time.

Abstract: A smoke detector has an alarm which may be turned off for a short interval to prevent its sounding continuously upon actuation by nonthreatening smoke or a malfunction. A timing circuit having a switch is connected between the smoke detector battery and the smoke detector, and actuating the switch will deactivate the smoke detector for a short period. If the alarm sounds after the smoke detector is automatically reconnected to the battery, the switch may be reactuated.

Abstract: A portable security alarm system includes a rechargeable battery and an infrared sensor to detect the presence of an intruder in a protected area. Three time delays are provided to allow the user to exit without setting off the alarm; to allow a legitimate user to disarm the system upon entry; and to limit the time that the alarm will sound. The system can only be disarmed by the entry of a multi-digit code that is kept secret whereas the device can be armed by anyone. The system is specifically designed to be resistant to spurious signals associated with electrical storms and is also designed to minimize power consumption. The system further includes circuitry to shutdown the alarm system when the voltage is low enough to risk damage to the battery. There are indicators associated with system operation including low voltage warning.

Abstract: An intruder detection system includes a self-supervision feature which, on the basis of detecting authorized pedestrian traffic within a region under surveillance, alerts the system user of potentially faulty system components. Such system comprises a programmable timer which is repeatedly reset to a programmed maximum time interval by each occurence of an alarm-producing event, such as a disturbance of standing microwaves and/or a slight increase in ambient temperature in a region under surveillance. During "disarm" periods when the system alarm is intentionally deactivated to enable use of the protected region without the production of any alarm signals, each occurence of an alarm input to the system alarm (as produced by authorized traffic within the protected region) is used to repeatedly reset the timer to its preselected time interval (e.g., 1,4,8 or 16 days).

Abstract: An alarm system for generating an alarm from a remote location and which utilizes electrically generated codes to access selected functional portions of the alarm system circuitry and which further utilizes a locking transmitter which must be reset after a single use thereof before the transmitter may be re-used, the transmitter resetting being achieved by means accessible only to authorized personnel.

Abstract: A multizone intruder detection system comprises a supervisory circuit for verifying, from normal traffic or activity in each of the zones of protection while the system is disarmed, that each of the intrusion sensors is, indeed, functional. In the event one or more sensors does not trip (i.e., sense activity) during the disarm period, the supervisory circuitry inhibits re-arming of the system until the non-tripped senso(s) is "walk-tested" to verify that it is still functional. According to the invention, a programmable timer enables re=arming of the system for a predetermined (preferably brief) time interval immediately following disarmig of the system. This feature offers the edvantage of allowing the user to re-enter the protected premises "after hours" for a brief period of time without having to walk-test all zones of protection before re-arming the system.

Abstract: A multizone intruder detection system comprises a supervisory circuit for verifying, while the system is disarmed, that each of a plurality of intrusion sensors is, indeed, functional. The supervisory circuit inhibits rearming of a disarmed system until it determines that each sensor has successfully operated within a relatively brief time interval just prior to the time an attempt is made to arm the system. A timing circuit, activated by a preliminary arm signal, operates to establish a time window (e.g. 10 minutes) within which the operability of each sensor must be verified (i.e. walk-tested) as a precondition to system arming. Preferably, the timing circuit is reset by each sensor alarm output, whereby the system user is given the full time window to walk-test each sensor. By virtue of the invention, sensor sabotage in a disarmed system can be mitigated.

Abstract: An automatic temperature monitoring system comprised of a central control circuit and one or more processing circuits connected thereto. Each processing circuit(s) has one or more remote temperature sensing probes connected to remote refrigerating equipment. The processing circuit(s) has switching devices for programming a microprocessor circuit of the processing circuit and for setting high and low limit parameters as well as delay parameters for analyzing current signals generated by each of the remote temperature sensing probes. A display device displays the numerical values of the limit parameters. An access circuit provides access to the stored information signals in the microprocessor and representative of accumulated monitored signals received from the sensing probes on a predetermined periodic basis. An alarm device generates an alarm upon detection of an alarm condition by the microprocessor.

Abstract: A sensor for monitoring the status of an alarm system is disclosed. The sensor includes a base station at the location of the alarm system and arranged to respond to an alarm condition. When an alarm occurs, the base station latches the alarm condition and retains it until it is manually reset. A remote station includes a transceiver for interrogating the base station to determine whether an alarm condition has occurred. The remote unit may also be used to arm or disarm the alarm system, and to operate other controllable devices.

Abstract: A currency alarm pack especially adapted for use in the cash supply of an automatic teller machine comprises a motion detector, an exit field detector, a smoke generating device, and electronic circuitry, responsive to the motion detector and the field detector, for triggering the smoke generator if, and only if, the field detector detects the exit field and thereafter ceases to detect the exit field, and the motion detector detects motion of the alarm pack during a predetermined delay interval following the time at which the field detector ceases to detect the exit field.

Abstract: An alarm system comprises of a closed circuit includes an alarm cord connected to a DC power source, a disconnect signal detector having an inverter circuit and a thyristor and a push-button switch. An alarm generator produces an alarm sound by the detection signal from the disconnect signal detector. A first charging-discharging circuit is connected to the DC power source through a first transistor which is turned on when the push-button switch is depressed, and a first Schmitt trigger circuit transmits a connect signal to the disconnect signal detector only for a time period set by the first charging-discharging circuit. The Schmitt trigger circuit transmits a disconnect signal to the disconnect signal detector after the setting period determined by the first charging-discharging circuit.

Abstract: A touch-controlled switch and alarm system for protecting electrically conductive objects. The touch switch automatically adjusts itself within a few seconds to a wide range of ungrounded conductive objects, is not triggered by AC power line transients or failures, has a back-up battery for operation during power failures, and is automatically triggered if the power plug is disconnected. A unique underdamped automatic gain control circuit allows a simple unidirectional trigger to sense both increases and decreases of capacitance to ground. The alarm includes a lock switch for arming and disarming the alarm, and status indicators to show whether or not the system has been armed or triggered. The lock switch itself is protected by the alarm system because the lock switch has an electrically conductive body which is connected to the touch switch along with the protected objects.

Abstract: Livestock entering a border zone along a vehicle roadway are deterred by devices triggered into operation simultaneously with a remote alarm by interruption of photo-electric beams at ends of the zone from which the animals enter or exit. The deterring apparatus is reset to prevent operation in response to travel of a vehicle through the zone and is temporarily disabled by selective switch actuation.

Abstract: A security system which includes remotely controlled entrance monitors (12) to indicate when an intrusion has occurred, an arming/disarming control (18), a console (14) responsive to the entrance monitors and arming/disarming control, and an alarm (16) responsive to the console for audibly indicating intrusion. These components are completely wireless with one another. Radio coded signals are transmitted between components requiring a coded combination between the console and entrance monitors, and a coded combination between the console (14) and alarm (16) and arming/disarming control (18). The arming/disarming control (18) includes an arming button (20) for arming the system and a disarming button (22) for disarming the system with a slideable protective cover thereover to prevent accidental depression of either button.

Abstract: A system for testing a remotely located sensing unit includes a photosensor located within the sensing unit. A control beam of incident electromagnetic energy can be provided from a remotely located portable source such as a flashlight. Directing the beam of radiant energy from the flashlight against the sensor in the unit causes the unit to initiate a test sequence. The unit can be equipped with a photo-detector to terminate an alarm generated in response to a sensed condition. The unit can include a sonic detector. Control circuitry in the unit can decode a sensed encoded incident beam to minimize false tests or to provide multiple remotely initiated functions.

Abstract: An active entry delay device which uses a wired or wireless momentary switch to be placed at edge of door or car door, so that the entry delay device can be initiated by turning on the switch; if the switch is not turned on before opening of door/car door, there will not be any entry delay, and alarm system will be triggered to sound alarm immediately.

Abstract: An audio signal device for a door mounted advertising display, the device includes a signal unit, a battery connected to said signal unit, the unit including a tone generator, a pulse generator connected to control the tone generator, a timer connected to control the time of operation of the pulse generator, and a motion switch connected to energize the timer in response to the motion of the door, the timer further including a circuit for preventing reactivation of the timer for a predetermined period of time.

Abstract: A system for testing a remotely located sensing unit includes a photosensor located within the sensing unit. A control beam of incident electromagnetic energy can be provided from a remotely located portable source such as a flashlight. Directing the beam of radiant energy from the flashlight against the sensor in the unit causes the unit to initiate a test sequence. The unit can be equipped with a photo-detector to terminate an alarm generated in response to a sensed condition. The unit can include a sonic detector. Control circuitry in the unit can decode a sensed encoded incident beam to minimize false tests or to provide multiple remotely initiated functions.

Abstract: A warning light system is provided for a vehicle powered by an engine employing an engine brake. The warning system takes a signal from an electrical control line associated with the engine brake and uses the signal to initiate a time delay. After the time delay, a switch is activated, turning on a warning light to warn drivers of other vehicles of the deceleration of the vehicle equipped with the warning light system.

Abstract: An advance warning system for an automatic electric door operator in which a command signal originating from a remotely located hand held radio transmitter or a wall switch located within the garage is received and processed by a signal receiver and transmitted to a signal delay unit. Upon receipt of the command signal, the delay unit activates a warning light and warning horn to alert persons in the vicinity of the door or operator mechanism that the operator is about to be activated. After a predetermined delay period, the delay unit also generates an operating signal which activates the operator.

Abstract: Electronic disabling circuits are provided to prevent the engine of a motor vehicle to be protected from starting in the event of an attempt at starting the vehicle while the disabling circuits are switched on by a switch. An inhibitor device serves to prevent the disabling circuits from stopping the engine in the event the switch is actuated in an untimely manner while the engine is running. In addition, a detector for detecting the presence of a human being serves, via a circuit, to reliably trigger an alarm in the event of an intrusion, by applying a detection signal to an input of the circuit, assuming the circuits have been switched on by the switch.

Abstract: A system for detecting a transfer of article comprising a card attached to the article and a detector. When the article having the card is transferred to a monitoring area, electromagnetic signals are transmitted in advance from the detector to the monitoring area, and received by the card. Then, high frequency signals are transmitted from the card to the detector, thereby a transfer of the article can be detected.

Abstract: A smoke alarm having a detector circuit for detecting smoke and producing a sensible signal in response to such detected smoke is provided with a manually actuated control which cooperates with the smoke detector circuit temporarily desensitize the smoke detector circuit and later to automatically re-sensitize the smoke detector circuit after expiration of a predetermined interval. A user is thus able to desensitize the smoke detector circuit to false alarming due to lower level concentrations of smoke, such as are produced during cooking, smoking, etc., while maintaining responsiveness and protection against higher concentrations of smoke, such as would result from an actual fire condition.

Abstract: A signal to indicate deposit of mail in a box remote from a residence employs an optical reflective detector which senses presence of mail in the box. The transmission may be r. f. if the box is relatively isolated from other boxes. In apartments, condominiums, and the like where multiple boxes are centrally located, transmission may be by special wiring or by imposing a coded signal on house electrical wiring. Means is provided for the detector distinguishing between outgoing mail deposited in the box and new incoming mail.

Abstract: An alarm system for signaling and reporting the occurrence of an unauthorized entry into a defined area, such as into a closed automobile. The alarm system includes means for generating vocal speech signals that are used to: (1) draw attention to the fact that an unauthorized entry event has been sensed, and (2) provide reports to indicate the status of the system and to alert the owner of the system that a prior unauthorized event was detected. At installation of the system, several alarm options and features are displayed as easy-to-understand menus on a programming unit used by an installer. The installer can thus readily select which of several available alarm options and features are desired, thereby allowing the system to be easily suited for a particular application. Further, during use of the system, the user of the system can also select various operating modes and report options, thereby providing flexibility in how the system is used.

Abstract: A switching device is enclosed in a casing fixed to a wall below a ceiling-mounted conventional smoke detector, so as to be accessible. The switching device includes a holding relay, a dry-cell battery and a time-delay circuit. The dry-cell battery replaces a similar battery normally in the smoke detector itself. In normal position, the holding relay connects the battery of the switching device to the circuit of the smoke detector. Whenever the smoke detector sounds a false alarm, a house occupant actuates the holding relay to disconnect the smoke detector circuit and stop the alarm. The battery in the switching device thereafter is connected to the time-delay circuit which returns the holding relay to its normal position after a period of, say, twenty minutes.

Abstract: A timer controlled fire and burglar alarm system using an economical circuit design and battery for prolonged and effective functioning and includes a thermo-electric infrared sensor, a filter and an amplifier to detect signal, a plurality of time priority circuits, a time-delay circuit, a timer controlled alarm setting circuit and timers with time display for setting and disarming of an alert condition and/or driving peripheral devices so as to function as a fire alarm, a burglar alarm and a visitor detection system.

Abstract: A motion sensing alarm is disclosed. It is comprised of a time delayed switching circuit connected between a supply circuit and an electronic logic circuit. A triggering circuit is connected between the switching circuit and a false triggering circuit and the electronic logic circuit. An alarm initiating circuit is connected between the electronic logic circuit and an alarm.

Abstract: A security and control system for use in a home or building which utilizes a coded audio link between the entry detectors/transmitters and the relay modules, and a digital pulse coded power line communication ("PLC") link between the relay modules and the system controller as well as between the system controller and the various remotely located slave units which control the energization of lamps, appliances, and alarms. The relay modules and controller include constant false alarm rate ("CFAR") receivers for isolating the coded audio signal from background noise and unique exclusion circuitry for decoding the isolated signal. The PLC messages are generated by impressing a pulse code modulated high frequency carrier signal onto the AC line at selected points in the AC waveform. The location of each carrier frequency pulse relative to the 60 Hz AC line cycle determines the digital value of the pulse.

Abstract: Security tags which bear a resonant circuit made of conductors on opposite sides of a dielectric are deactivated by applying to a tag sufficiently high RF power at the resonance frequency to produce breakthrough between opposed conductors. A tag presence alert signal is intentionally extended beyond the period of active tag presence detection. During high power operation, the system inhibits other nearby RF deactivating and electronic article surveillance systems. The RF transmissions of all these systems may also be slaved. The high power RF produced by the deactivating system is principally dissipated where it causes no undesirable heating effects.

Abstract: The present invention relates to a system for the vehicular transportation of a lock-equipped cassette (1) for valuable documents between a first station, e.g. a bank or a post office, to a second station. The cassette (1) is intended to be kept in a cassette holder or rack (2) in the first station and the second station, and also in the transporting vehicle. The cassette is provided with an electronic circuit (R1,22) which is characteristic to the cassette concerned and which is arranged to co-act with a controlling activating circuit (M1). When the cassette (1) is lifted from its cassette holder or rack a clock circuit located in the cassette is activated and, unless the cassette is replaced in a cassette holder within a predetermined time, causes an alarm to be activated, this alarm being an acoustic alarm, a device for generating smoke within the cassette, releasing a dye onto the valuable documents contained in the cassette, etc.

Abstract: A burglarproof device which can facilitate the activation of an alarm system and can provide lock-out protection. It uses a set of correct-signal generating reed switches and a set of incorrect-signal generating reed switches to activate the alarm signal, and the input of a correct signal within the preset lock-out time after receipt of an incorrect signal will not be able to open the door lock or disarm the alarm system.

Abstract: An intrusion detection system of the type employing a keypad utilizing the keys to conventionally enable a user to key in a secret code capable of arming or disarming the system. The present apparatus includes a hold-down feature whereby if any one of the plurality of keys on the keypad is depressed for a predetermined interval, the apparatus detects the predetermined timing interval and transmits an audible sound to the user indicating that the requisite timing interval has transpired. Upon receipt of the audible sound, as depending upon the test to be performed, the user is thus informed that the system will now commence the test procedure as defined by the selected key. Thus the system includes apparatus for detecting the requisite timing interval. Upon detection of the requisite timing interval a storage means is activated, which storage means is associated with the selected key.

Abstract: A building has at least one security guard post to control the secured building exits, each of which has an unlocking instrumentality, without forbidding their emergency use. When activating the unlocking instrument to open a secured exit, there is a transmission of information to the security guard post and an initiation of a delay during which the security guard can forbid the opening of the exit. The unlocking instrument can also visually indicate events at the exit. The apparatus has an electro-mechanical lock, a sensor which is sensitive to motion of the unlocking instrument, a control line linking each door to be controlled to the security guard post, and a central electronic processing unit. A surveillance circuit responds to the sensor. A security circuit frees the unlocking instrument when a fault is detected.

Abstract: A multiple-sensor status monitoring system for monitoring the status of an area while avoiding false alarms and detecting and identifying faulty sensors. The system uses a timer and logic to avoid false alarms by generating an alarm signal only if two sensors give a response within a preselected interval of time. The system employs latching storage elements to keep a record of which of the sensors have made spurious responses, and a visual display to give a trouble warning respecting those sensors.

Abstract: A mountable remote cutoff timer for a smoke detector that is powered by a battery and includes a timer that is operatively connected to the battery, a light emitting device for indicating improper connection of the battery, and VELCRO.RTM. for attaching the mountable cutoff timer to the wall.

Abstract: This automatic apparatus is for use by cigarette smokers who may be in danger of going to sleep with a lighted cigarette near at hand, to possibly cause fire and both personal and property damage. This apparatus is for use with an ashtray, and there is a socket in the top wall for receiving a cigarette lighter for storage therein. The apparatus includes an electric switch associated with the socket for detecting the presence of the lighter therein. In series with this switch is an ON delay timer that is in series with an audible alarm device. Also associated with the switch is a spring-biased plunger with a feeler pin that is capable of engaging the lighter. When the lighter is removed so that the user may light his cigarette, the plunger is allowed to move to close the switch thereby energizing the ON delay timer for a given timed cycle. A cigarette crusher is mounted adjacent the front of the apparatus.

Abstract: A door ajar alarm system for a refrigerator-freezer, the system having a variable alarm delay period. The door ajar alarm system includes a control which may be digital or analog for monitoring various operating conditions of the refrigerator such as the temperatures of the refrigerator compartments and whether the refrigerator is undergoing a defrost operation. The control determines the alarm delay period in accordance with the monitored conditions such that the delay period is decreased in response to each monitored condition which reaches a predetermined status. The door ajar alarm thus alerts the user to operating conditions which are likely to produce an undesirable temperature within the refrigerator more rapidly than usual or to produce temperatures from which recovery is unusually slow.

Abstract: A box formed with an aperture in which a photosensitive cell is secured includes an electronic circuit connected to be fed by the electrical signal produced by the photosensitive cell when the latter is irradiated by ambient radiation. Said electronic circuit comprises a delay circuit adapted to receive the signal and arranged to produce a control signal when the cell has been irradiated for a predetermined time interval; an audible signal generator received in the box opposite apertures and connected to produce an audible signal in response to the control signal; and a circuit adapted to interrupt the audible signal after a predetermined time interval so that the audible signal is of predetermined duration.The box has means for securing it to an object which is required not to be lost accidentally, e.g. keys, a wallet, a diary, etc. A cover can be provided for being placed over the cell to protect it and keep away external radiations when the reminder device is not required to be operative.

Abstract: An "Automatic Exit Delay" circuit of a burglar alarm is placed on Stand-By whenever a "first" door is opened and the burglar alarm automatically starts to work right after the "last" door is closed. At that moment of arming a "Quiet Quick Test" Network automatically tests the alarm for proper functioning and generates a short "beep" to indicate such as well as the armed state of the alarm system.

Abstract: A remote alarm system provides a transmitter having an audio frequency signal generator circuit, a modulator circuit forming a current source modulated by the audio frequency signal and a radio frequency oscillator circuit modulated by the current source, and a receiver having a super-regenerative detector, an amplifier connected thereto, a notch filter for selectively filtering the audio frequency signal, an alternating current rectifier for rectifying the audio frequency signal, a delay circuit for delaying activation of an alarm signal until a signal from the rectifier circuit is present for a predetermined amount of time and for determining a period of time for activation of an alarm signal, and an alarm means for generating an alarm signal in response to the output of the delay circuit.