Simple fix adds door-chime repeater
Dennis Eichenberg, Parma Heights, OH - May 13, 1999
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 resistor R5 and transistor Q1. D5 is a flyback diode that protects K1. You must select K1 to fit the specific chime. The coil current can be as high as 200 mA. The circuit normally uses closed contacts so that illuminated pushbutton switches operate properly. The 16V-ac door-chime power energizes the circuit via bridge BR1. IC1 provides voltage regulation, and C1 and C2 provide filtering. Diodes D1 and D2 provide power from the rear-door pushbutton. D3 isolates the filtered timer power from the relay. (DI #2349).