Doorbells, as it is known, included a button at the door. They work until the button is pressed. If the button is accidentally securetide that happens during manufacture from inferior plastic, or it is closed, for example, by means of a match, the call will run continuously.
The call is not designed for this mode of operation. At best it will burn, but in the worst possible fire. But when the caller long holds the button down, then a long ringing sound is very irritating, so it is advisable to stop time sound the call to 5-7 C.
The following diagram (see figure) allows you to do this.
The operation of the circuit is as follows. When you press the button SB1 (at the door) voltage is applied through the normally closed contacts of K1.1 on a call. He begins to sound. At the same time voltage is applied to the chain R1, VD1, K1, C1. In the initial point C1 is short circuit current limited by resistor R1. Capacitor C1 starts charging through R1, VD1. After a few seconds C1 charged up to the voltage of the relay K1. The relay is activated, the contacts K1.1 opens and the call is disconnected.
When you release the button SB1, the capacitor C1 discharges through the relay coil K1. When the voltage on C1 will be less voltage release relay K1, it will return to its original state, the contacts K1.1 will be closed and it is possible to re-call.
The selection of R1 and C1 can adjust the ring time.
The author of these lines, this scheme employs over 25 years without a single failure.
Author: O. G. Rashitov, Kiev