Sound Operated Music Bell

Sometimes ringing the bell manually becomes difficult, especially when you are holding something in your hands. The sound-operated music bell takes care of the problem

In this circuit, the use of an external relay has been eliminated. The circuit comprises a trigger stage (around BC548B), a timer stage (NE555) and a melody stage (around UM66 and SL100).

Transistor BC548B is biased inn class C operation. Every second sound cause triggering at pin 2 of 555 timer IC.

When VR1 is at maximum, the hold-on time of NE555 is around 30 seconds. But this time period can be set to a lower value by VR1, as per requirement, using the relationship.

T = 1.1*VR1*C1

circuit diagram of sound operated music bell

Where VR1 is the actual resistance of the preset in circuit. UM66 IC has  ROM memory of 64 notes which are produced one by one and then over with a sound of a clap, the timer is triggered and the output at pin 3 of IC NE555 goes high which gets applied through diode 1N4001 to pin 2 of musical IC UM66. S UM66 gets the positive supply, it starts giving electrical fluctuations of the music to the base of SL100, and the charming music comes and fed to 4-ohm speaker.


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 10 KΩ

R2 = 470 KΩ

R3 = 2.2 KΩ

R4 = 150 KΩ

R5 = 100 Ω

VR1 = 1 MΩ


C1 = 22 µF/16V

C2 = 0.1 µF

C3 = 0.22 µF

C4 = 220 µF/10V

C5 = 0.01 µF


IC1 = NE555

IC2 = UM66

T1 = BC548B

T2 = SL100

D1 = 1N4001


MIC1 = Condensers microphone 34 LOD

SW1 = On/Off switch

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