When the circuit in the illustration is connected to the telephone line, the lamp is given a light by any incoming call. The circuit consists of two parts and these two circuits are separated from each other by the MOC3020 optocoupler. MOC3020 The optocoupler is a circuit element that can control high voltages and currents with low voltages without an electrical connection between them. The first circuit is the lower part to be connected to the telephone line.
The voltage on the telephone line will reach a value of AC 75V in any call that will come through the telephone line. This value passes through the R1 and C1 materials and goes into the bridge diode made with 4 1N4007 and then processed and controlled by the circuit as a direct current. In any phone call, this section is fixed to 33V by the DZ1 signal. Thus, the optocoupler is protected.
The second circuit is the high section to be controlled by the city network. This part consists of R1, R2, C4, TRC1 BT137 triac and a 220V lamp. When there is a phone call, the gate end of the BT137 is triggered via the photo triac of the optocoupler and the lamp illuminates. The light-emitting process continues until the voltage is cut off from the mains.
MOC3020 Pinout BT137 Pinout