door-chime repeater

Posted on Apr 17, 2012

Electromechanical door chimes can enhance your home, but they are vulnerable to costly repair problems. A defective pushbutton switch or a careless visitor can maintain the chime in an energized state for a prolonged period, thereby damaging the chime. The circuit in Figure 1 prevents damage to the chime and improves the chime`s effectiveness by repeating the chime strike for as long as the pushbutton remains depressed. The circuit controls both front and rear chimes. The heart of the circuit is timer IC2, which you configure as an astable multivibrator. The timing components, R1, R2, R3, and C3, provide the required pulse widths.

The maximum duty cycle of a typical chime is 25%. The energized time is 0.76(R3+R4)C3, which you can adjust from 7.6 msec to 0.4 sec. The de-energized time is 0.693(R1+R2)C3, which you can adjust from 0.3 to 1 sec. IC2 drives the 12V relay, K1, via resist

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