Patent GB-1017053-A (GB)
Title: Improvements relating to circuit arrangements involving electromagnetic relays
Inventor: DRAKE BERNARD
IPC: H03K17/64
Filed: 1961-07-24
Abstract: 1,017,053. Transistor and valve relay circuits; exchange systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. June 29, 1962 [July 24, 1961], No. 26723/61. Headings H4K, H3P and H3T. A relay operating circuit, Fig. 1, comprising a pair of transistors is characterized in that the second transistor T2 is connected as an emitter follower with the relay forming part of an emitter load individual to this transistor, the first transistor T1 has a collector load R3  connected to control the base potential of the second, and a further resistance R2 is connected to both emitter circuits to provide a reference potential. In value the resistor R3  exceeds the value of the emitter load individual to R3 by an amount to produce positive feedback so that a gradual progressive change of input to T1 from a first extreme value to a second causes the relay to operate when the input passes through a value equal to the reference potential, and a gradual progressive change from the second extreme value to the first causes the relay to release when the input passes through a value also equal to the reference potential. A circuit for giving a " number of calls waiting " indication on a manual switchboard, Figs. 2A, 2B, comprises a resistor PR for each operator's position, a resistor CWR for each answering circuit and a contact cwc closed when the circuit is engaged by a call awaiting attention. The arrangement PR, CWR, cwc produces on line 4 a control voltage that varies in dependence upon the number of calls awaiting attention which is fed to a number of relayoperating circuits, each controlling individual relays G, W, R such that during a gradual progressive change of control voltage from one extreme value to another the relays are operated in sequence in the order G, W, R and during a progressive gradual change in the reverse direction the relays are released in the reverse order. Indicator lamps LPG, LPW, LPR are provided for giving a visual indication of the number of calls waiting which varies with the value of control voltage. In order to avoid a continually varying indication, the condition of the relays is sampled at intervals, and a change of indication is only effected if the conditions found are the same at two successive samplings. A change from lamp LPG, glowing to all three glowing takes place one step at a time as regards the display, the first step being a change from LPG glowing to LPG and LPW glowing, and the second step being a change to LPG, LPW and LPR glowing. For a sudden reduction in the number of calls from all three glowing to LPG alone the change takes place in one step. The cyclic timer, Fig. 2c, for producing sampling pulses for the " number of calls waiting "  circuit includes a relay SS which releases to energize slow to operate relay SR. Contact sr1 closes charging capacitor C1 until the potential at the grid of pentode VI reaches a value such that the anode current of the pentode operates relay ss. Contact SS1 then shorts resistor R 17 to increase the charge on C1, and relay SR releases opening srl. Capacitor C1 then discharges through R18 until the pentode anode current falls sufficiently to release SS, when the cycle recommences.