Pushbutton telephone dialling circuit with keyboard transistors - which are connected to exchange line loop during selection, but not during conversation

摘要

The circuit has a switching relay (R1s) connected at one end to the keyboard terminal and at the other end via a capacitor (C7) and a diode (D4) to the collector of the transistor (T2) controlled by the selection pulse terminal (11). The transistor emitter is connected to the first keyboard terminal (1). A flip-flop consisting of two transistors (T6, T7), is connected in parallel with the series circuit comprising the switching relay and the capacitor. The base of the flip-flop first transistor (T6) is connected via a resistor (R7) to the anode of the diode (D4).