Tiny Door Guard with Alarm

This simple Door Chime protects the door and gives a loud Alarm tone when there is an attempt of theft. The circuit is too simple and battery operated.

A Normally Closed (NC) reed switch and magnet is used to trigger the circuit. Alarm generator is the popular ROM IC UM 3561. This 8 pin IC has an inbuilt oscillator to generate 4 siren tones like, Ambulance siren, Police siren, Fire brigade siren and Gun sound. The different tones can be selected using its pin 6 connected to VCC, Ground or not connected. Frequency of oscillation is determined by the 220K resistor connected to the pins 7 and 8 of the IC.UM 3561 is the low power IC and its maximum voltage rating is 3 volts. So Zener diode ZD is used to give 3 volts supply to IC. Medium power NPN transistor T1 amplifies the output pulses from IC1 to a loud siren.

Tiny Door Guard Circuit

Magnet can be a small sized one that is to be fixed in the door using double sided adhesive tape. Fix the circuit board in the door frame. Fix Reed switch in the door frame, very close to the door. So that, when the door is closed, the magnet will pull the contacts of the reed switch to break supply to the IC. When the door opens, contacts of the reed switch make contact and IC gets power to give alarm.


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  • Christian

    I was wondering if it is possible to add a LED to to the circuit……..thanks 😀

    • D.Mohankumar

      you can add an LED with a resistor in series with the lower side of the reed switch. When the door opens, reed switch make contact and LED lights along with alarm.

  • Darcy Roberts

    Thanks for a nice easy door alarm. I was wondering how to replace the magnetic switch with a refective opto isolator devise to make this into a “interupte the light beam to alarm” divice. The 9v battery would last forever. I need some thing to keep track of my mom when she comes out of her bedroom. I am taking care of her as she has Alzheimers disease.
    They make several IR devices on the market real cheap but having spent 20 years of my life as a electronic equipment repair tech and done some of my own designs I thought it would be funner to build my own or maybe use a modified version of yours?

    Thanks , Darcy Roberts, Central Oregon, USA

  • Arjun.R.Swamy

    Respected sir,
    In your ckt BD139 is shown like a darlington transistor pair.Is it right?I am an ITI student;please.

    • D.Mohankumar

      Dear Arjun

      BD 139 is a Medium power NPN transistor. Mistake in the description is corrected. Thanks for pointing out the error

  • faruk