Simple fix adds door-chime repeater

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).