Abstract: An electrical signalling apparatus including a transmitter and receiver connected to the same electrical power distribution network wherein a signal generated by the transmitter is used to control apparatus connected to the receiver. The system uses a pair of amplitude-modulated low frequency signals as the control signals that react with the receiver to control apparatus connected thereto. The transmitter has an oscillator modulated by twin tuning forks and provides a signal for a period of, for example 10 seconds, determined by a time-delay relay. The receiver has a demodulator followed by two filters connected to integrators. As soon as the integrated signals reach a predetermined value, a control signal is transmitted to the output. This apparatus provides a communication system which is virtually insensitive to interferences on the distribution network and operates perfectly with weak signals.

Abstract: A surveillance apparatus against theft or intrusion includes a box containing a detector reacting to an abnormal presence and operable to selectively activate a camera, a flash and/or an acoustic warning device. The open rear of the box is hingedly attached to a support plate affixed to a wall while flexible wires from the box components include a connector part removably attachable to a mating connector part on the support plate, the latter being supplied with a source of power. A backup source of power may be included in the box as well as cooperating lock components on the box and support plate.