Audio Circuit Diagrams

Electronic bell-push

A solid-state bell-push is far more robust than the usual mechanical type. The touch contact may be made from an audio socket, which has a high insulation resistance and is practically indestructible.

The inverting (-) input of IC1 is at half the supply voltage via R4-R5, while the non-inverting input is at earth potential via R3. When the contact is touched (less than 10 MΩ, a light touch is fine), the output of the opamp goes high and the relay (a 9-V or 12-V type) is energized. The relay contacts then actuate the (existing) bell. Resistor R1 and capacitor C1 ensure that e bell cannot be actuated accidentally.

The circuit is powered by the rectified bell transformer voltage (or a second bell transformer) for safety’s sake, do not use a different type of transformer). A bell transformer can normally provide a current of 1 A, so the first solution is almost always possible.

The circuit draws a (quiescent) current of only 5 mA. If a 741 is used and only 0.5 mA if a TLC271 is used. When the relay is energized, this rises by 30 mA.

 

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