Patent GB-806233-A (GB)
Title: Improvements in or relating to electric selective signalling systems
IPC: H04W88/02
Filed: 1957-04-26
Abstract: 806,233. Staff locator systems. BULL, R. A., COATES, C., and LARDNER, W. E. April 26, 1957, No. 13353/57. Class 40 (1). [Also in Group XL (c)] An electric selective signalling system comprises a transmitter including means for producing a plurality of frequencies, means for selecting a number of said frequencies to be produced, time controlled means for causing said selected frequencies to be produced one at a time in a succession for a predetermined time, and means for transmitting said succession of produced frequencies, said system comprising also a plurality of receivers each including means adapted in response to an individual succession of said frequencies which is allocated to that receiver to cause an electrical circuit to produce a signal for the duration of the receipt of said succession of frequencies, which signal is used to indicate that the receiver is being called from said transmitter. A staff locator system is described for use in large buildings and the like in which a central control station by transmitting, by a radio or inductive link, an appropriate signal can alert the person carrying a selected receiver, which person can then receive a message transmitted over the same radio or indicative link. In a first embodiment a transmitter, Fig. 1, transmits two manually selected frequencies alternately for a predetermined time which causes the selected receiver, Fig. 4, to sound a buzzer, after which the control operator can transmit a message on the assumption that the person carrying the selected receiver has been alerted and is listening. Selection of the two frequencies to be transmitted is effected manually by switches Sa, Sb ... Sn, Fig. 1, one of which is switched upwards and the other downwards. A switch SW is then operated to energize a relay RS which remains energized for as long as it takes a condenser C1 to discharge and for the duration of this period the selected frequencies are transmitted alternately under the control of two relays RX and RY which act as a multivibrator alternately connecting two condensers, corresponding to the selected two frequencies, into the tank circuit of an oscillator 10 which feeds via two amplifiers 11 and 12 to a transmitter 13. When relay RS does drop out a neon N3, controlled by the relay multivibrator, stops flashing, a neon N2 glows, and a microphone 15 is connected to the amplifier 11 in place of the oscillator 10. The operator may then speak his message. At the receiver, Fig. 4, reception of the first frequency, say frequency A, causes a reed Va tuned to this frequency to beat and charge a condenser 32, and reception the second frequency, say B, causes a reed Vb to beat thereby transferring some of the charge from condenser 32 to condenser 34, which causes a transistor oscillator to produce an audible signal in an ear piece 40. The alerted person can then operate a switch 41 and await the message. In a second embodiment (transmitter, Fig. 3, not shown) which allows the frequency sequence A, B to be distinguished from B, A, a pause is introduced in the transmission between each frequency sequence. The receiver shown in Fig. 4 then responds to A, B but not to B, A, because in the sequence B, A the B is preceded by a pause during which time condenser 32 discharges.