Telephone instrument flash disconnection signalling circuit - has time constant circuit charged from line to switch transistor and trigger thyristor

摘要

The circuit id designed to give a definite time interruption of a telephone line, and is primarily designed to open the loop through a telephone instrument for short time period signalling as when 'flashing' the operator. A push-button (T) actuates the circuit and, wen operated, the circuit causes an 80 ms line disconnection. The circuit has a rectifier bridge (G) input for polarity protected connection to the line supplying its power. The unit, will operate from line voltages between 24 and 60V is protected by a potential divider (R3, R4) on the transistor (T1) base. The operation of the push-button connects line (a1, a2) across a capacitor (C2) of a time constant circuit (R2, C2). When the capacitor charges beyond the base emitter circuit, of the transistor, bias-level, the transistor switches on and triggers a thyristor (T2) so ending the disconnect period.