Ringing signal detection circuit

Shown is the ringing signal detection circuit , which is composed of photoelectric coupler PT1. It is composed of bidirectional diode VA1, diodes D4, Ds, ZD1,
Rll and R13. This circuit is also connected in parallel with the telephone. When the ringing signal is transmitted, it passes through the half-wave rectification circuit composed of D4 and D5. During
the rectification cycle of D4, the PT1 light-emitting diode is turned on, and the corresponding phototransistor is also turned on. After being inverted by the driver IC3, it outputs a low level. , thereby
forming a CI (call indication) signal, indicating that there is a ringing call signal on the line.