Musical Doorbell

8 to 15 Vac is applied to terminals C and D, which produces a dc voltage across R2, and turns on Ql. This connects the bat
Musical Doorbell - schematic

teries B1 and B2 to the rest of the circuit, which activates it. Latch U3 is triggered, it remains on until Q2 turns on, charges C2, and turns off U2. When U3 is turned on, Q3 is forward-biased, which energizes Kl, powers up U4. At the time the Kl contacts close, C4 couples a positive spike to pin 4 of U4, a melody synthesizer chip. U4 generates a pre-programmed tune, at the end of which pin 1 of U4 goes positive. This activates optocoupler Ul and turns off Q2, which drops out the relay. U2 acts as an audio amplifier, which drives an 8- speaker.