Abstract: An alarm system, particularly for use on a train, responsive to a plurality of different alarm conditions for providing outputs for indicating which one or ones of the alarm conditions have been detected. A signal having at least as many frequency components as there are alarm conditions is generated on a transmission line which runs the length of the area over which the conditions are to be detected. Alarm sensing switches are connected to taps of the transmission line, and each switch is connected to a frequency dependent impedance means which is resonant and has a low impedance at one of the frequencies. A filter is provided for each alarm condition to be detected, each such filter having a band pass at one of said resonant frequencies. The presence of an alarm condition causes an alarm switch to change state, which causes one of the impedance means to short circuit a frequency which otherwise would have been passed by one of the filters, thus indicating the presence of an alarm condition.

Abstract: The supervised conductive loop comprises a plurality of switch elements connected in series with a DC current source. Encoder modules are located in parallel with each of the switch elements. The encoder module comprises an oscillator and an impedance element. When a switch element is opened, the oscillator associated with that switch is energized and places an AC signal on the loop. The frequency of each oscillator uniquely identifies the associated switch element. The impedance element, formed by a resistor and a capacitor, is used both as a filter for the power supply voltage to the oscillator as well as a bandpass filter to permit oscillator signals present on the loop to bypass open switch elements and be detected at any location in the loop. Frequency identifying means are coupled to the loop to permit an indication of which oscillators have been operated.

Abstract: An FSK receiver for a single channel analog telemetry system comprises an FSK demodulator, a frequency multiplier, and a frequency-to-voltage converter. An FSK transmitter comprises a voltage-to-frequency converter, a frequency divider, and an FSK tone generator. The principal advantages of this system are greater speed due to faster settling time and reduced errors in accuracy and linearity.

Abstract: An electric fence monitor and alarm apparatus and method is provided in which the charge condition of an electric security fence is continuously monitored, and an indication is provided when the charge on the electric fence falls below a pre-determined level. In a preferred embodiment, the charge on the line causes a circuit to be maintained in an energized condition which maintains a relay or other switch means in a first condition preventing the energization of indicator means or alarm means such as sirens, automatic telephone dialers, horns and bells. When the charge on the line falls below a pre-determined level, the relay or other switch means is switched to a second state, after a pre-determined adjustable time delay, to cause the operation of the indicator, thereby alerting the proper personnel to a failure of the charge on the line for any reason. The failure of the charge on the line is often caused by an unauthorized cutting of the line.

Abstract: Each alarm unit has a sensing circuit which supplies a signal in the presence of a particular problem such as smoke, heat, or equipment failure. The signal energizes an electronic switching device, to couple power to a horn which then produces an alerting tone. The output of the sensing circuit is also coupled by a resistor to electronic switching devices in the other alarm units in the system, so that the occurrence of a problem at one location activates the horn in each alarm unit.

Abstract: A time multiplexed sensor monitoring system. A plurality of remote sensor units are connected to a central control unit by a two-wire bus line. Power is supplied to the remote unit sensors by the central control unit. Synchronization of the remote units is provided through momentary interruptions in the power supplied to the remote sensor units. The use of a single two-wire bus line for both application of DC signals to the respective remote sensor units and communication of data from the remote sensor units to the central control unit is facilitated by use of double tone multi-frequency (DTMF) signalling techniques.

Abstract: A shielded two-wire cable used to transmit signals from one or more remote receivers to a central signal processing unit in an energy-responsive system is simultaneously used to transmit from the signal processing unit to the receivers (a) a signal proportional to the system noise level, (b) a pulsating signal when the system noise exceeds a level indicative of a desired margin of safety, and/or (c) a steady-state signal when an alarm signal is generated by the signal processing unit.

Abstract: A security system of the type including one or more electrically-continuous closed loops for detecting the presence of an intruder wherein a breach in security indicative of the intrusion is sensed by a change in potential at a point along the line with which the closed loop is electrically connected.