GB1298708A - Improvements in switching circuits - Google Patents

Abstract

1298708 Time delay switching circuits L SEYMOUR 24 May 1971 [30 May 1970] 26208/70 Heading H3T A time delayed switch or pulse stretcher comprises a solid state switch 4, a first control current path therefor including the series capacitor 12, a second solid state switch 3 having its input circuit providing a second control path and its output circuit connected across the capacitor whereby if switch 3 goes off, switch 4 remains conductive while the capacitor charges. As shown, a third control current path comprises resistor 7 and diode 9. When a positive input is provided at a-a, transistor 2 is switched on, switching 3 off and diverting the control current in resistor 7, allowing 4 to go off after the charging of capacitor 7 is complete. Diode 10 prevents capacitor 12 discharging into the supply when its upper terminal goes positive with respect to the supply line and a Zener diode 13 limits the base voltage of transistor 2. The input at a-a may be speech, in which case the circuit may be used to maintain apparatus operating during speech breaks. If an A.C. is applied to a-a the resulting pulses may cause a stepwise discharge of capacitor 12 and thus pulses of capacitor current which increase in length until the current is continuous.