Let the phone make a musical sound (2)

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When there are multiple handsets in the same office, it is difficult to tell which one is ringing when you call. Changing the adjacent phone rings to different music sounds solves this problem, and the circuit shown may change the phone rings to music sounds. IC3 is a music integrated circuit, IC1, IC2 are photocouplers. In normal hang-up state, IC1 cuts off, IC3 has no output and speaker SP is silent. When you call, the AC ring voltage makes the LED inside IC1 light-emitting, the photosensitive tube conducts, triggers IC3, and the output signal is amplified by VT1 to push SP to make music sound. After disconnecting the phone, the 6V-10V DC voltage generated in the machine makes IC2 work, the composite tube consisting of the photosensitive tube and V12 saturate, the IC3 transmission signal is short-circuited, the SP stops sounding, and the call can be made. After hanging up, the circuit returns to standby.