Phone Flasher circuit diagram
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When the phone rings, the BG2 makes IC1A output high, and in one NAND gate end of IC2B, R4, C1 forms the differential circuit to generate spike pulse and trigger IC2B NAND gate, so that the gate outputs low, D3 is conduction , the charge on the capacitor C3 rapidly discharges and remains low, then it is in high level by IC1B inverter, and the oscillator pulse can be sent to the transistor by IC2D and IC2A NAND gates, and the LED flashes with the pulse changes. When the phone is not hung up, the DC voltage on telephone lines is not enough to make the optocoupler output low level.
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