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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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  • Hi Simon, Here is a simple monostable 555 timer circuit that I have used for similar applications. R-1 should be between 10K and 14 Meg and C-1 should be between 100pF and 1000uF. The formula for calculating the time that the output from pin 3 will be high is: Time( in seconds) = 1.10 * R-1 (in Ohms) * C-1 (in Farads). The power to the circuit should not exceed 15 volts DC. If you are going to power a buzzer with the unit you will have to use a relay or other means to boost the power as the output of the 555 timer is limited to about 200 mA.

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    John

  • in reply to jw0752

    I've seen that circuit, but been unable to simulate it before purchasing the needed parts.

     

    I would also need to invert the momentary switch output and add to it it to get something that will work with my input and output volatage and current needs.

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  • in reply to Former Member

    Hi Simon, From your description of the circuit and the application of the switch you should not have to invert the input. While your switch is a NC switch the fact that the door is normally closed pushing on it makes it actually a NO switch. You may have to put a capacitor in series with the switch to just give the trigger a pulse when the switch closes instead of a continuous closure, though I am not sure about that. A simulation or a rough prototype would let you experiment. The circuit itself is a standard off the shelf design and will work as I have used it successfully several times in my projects. You will probably have to rectify and regulate the voltage from the transformer to 12 volts for the timer circuit but you could also use the power from the transformer to continue to power the buzzer with a small relay, controlled by the 555,  to turn it on and off. The project you propose is very doable and I agree with you it would be much more professional than a continuous buzzer.

    John