Musical telephone ringer

  
The heart of the circuit is IC1, General Instrument's AY-3-1350 melody-synthesizer IC. IC2 is a TCM1512 telephone ring detector IC that is powered by the telephone line. The circuit's operation begins when IC2 senses a ring pulse on the telephone line. The detector (internally) rectifies the ring signal and then outputs a voltage to relay RY1 (an SPST reed-type relay with 5 volt contacts), causing its contacts to close. That pulls pin 12 (the ON/OFF control) of IC1 low (logic `0`), causing it to output a signal—the selected tune—to transistor amplifier Q2.
Musical telephone ringer  - schematic

The amplified signal is then fed to the speaker. The melody continues to play either until the tune is finished (at which time IC1 returns to the standby mode), or until someone takes the phone off the hook. Taking the phone off the hook discontinues the ring pulses to IC2, which opens RY1. When the relay contacts open, pin 12 of IC1 goes high, returning the circuit to the standby mode to wait for the next incoming phone call.




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