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Burglar Alarm to Door Buzzer Using a Monostable Circuit

Former Member

At work we are currently using what I suspect is a burglar alarm as a door buzzer. It's inappropriately loud to have buzzing the entire time the door is ajar (from barely open onward), as we're a motel and it's frankly obnoxious. However in order to hear people come in from the basement, it's what the folks in charge decided to use. I would very much like to create a monostable circuit so it buzzes for a small amount of time then turns off until the door is shut again, re-arming the buzzer. The buzzer works using a momentary switch that is pressed when the door is shut and released when the door is open. This switch is connected to what looks like a transformer core, and the device also plugs into our AC outlet.

 

I have looked at circuits online, but even using a few simulators I was unable to produce results like I wanted. Can I have some advice? I'm new to electrical engineering.

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  • This is (yet another) diagram.

     

    image

    As the buzzer is currently hooked up to a transformer of unknown ratio, it may be possible to run 12V through the circuit using that transformer and a rectifier setup and pass it (minus about 1.7V) to the buzzer. That's dependent on it working with DC of course. Since it's a magnetic bell hammer I've assumed it will for now. Thanks for the feedback on my progression. (I never mentioned it, but SW2 in this diagram and the related switch in others is an on/off switch for ease of toggling.)

  • in reply to Former Member

    Hi Simon, No, the 555 can supply at the most 200 mA which will not power the buzzer. It is better to get a low current relay as you drew before and let the relay fit into the place of your present door switch and leave the transformer and the rest of the buzzer circuit alone. You will have to get power for the 555 circuit but you can use a 12 volt DC wall adapter which will safely provide that power. All you will have to do is make sure you connect the positive lead from the wall adapter to the 12V + of the circuit board. You are on the right track.

    John

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  • in reply to Former Member

    Hi Simon, No, the 555 can supply at the most 200 mA which will not power the buzzer. It is better to get a low current relay as you drew before and let the relay fit into the place of your present door switch and leave the transformer and the rest of the buzzer circuit alone. You will have to get power for the 555 circuit but you can use a 12 volt DC wall adapter which will safely provide that power. All you will have to do is make sure you connect the positive lead from the wall adapter to the 12V + of the circuit board. You are on the right track.

    John

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