Doorbell time limiter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Doorbell time limiter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Calls and audio simulators

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Doorbells, as you know, are turned on by a button at the door. They work as long as the button is pressed. If the button is accidentally short-circuited, which happens when it is made of low-quality plastic, or it is specially closed, for example, with a match, then the bell will work continuously.

The call is not designed for this mode of operation. At best, it will burn, and at worst, a fire is possible. But even when the caller keeps the button pressed for a long time, the long ringing gets on the nerves, so it is advisable to make the call time limiter up to 5-7 s.

The scheme described below (see figure) allows you to do this.

Doorbell time limiter

The scheme works like this. When the SB1 button (at the door) is pressed, voltage is supplied through the normally closed contacts K1.1 to the bell. It starts to sound. At the same time, voltage is applied to the chain R1, VD1, K1, C1. At the initial moment, C1 represents a short circuit for the current limited by resistor R1. Capacitor C1 starts charging through R1, VD1. After a few seconds, C1 will charge up to the actuation voltage of relay K1. The relay is activated, contacts K1.1 open and the call is disconnected from the network.

When the SB1 button is released, the capacitor C1 is discharged through the coil of the relay K1. When the voltage on C1 becomes less than the release voltage of relay K1, it will return to its original state, contacts K1.1 will close and you can call again.

By selecting R1 and C1, you can adjust the ringing time.

This scheme has been working for the author of these lines for more than 25 years without a single failure.

Author: O.G.Rashitov, Kiev

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