Door Ajar Alarm Circuit

Here is an another alarm circuit in the section of sound generator, the circuit posted here is of door ajar alarm which alerts in every ten seconds that your door is open.

Circuit Description of Door Ajar Alarm Circuit

The heart of the circuit is 14-stage ripple carry binary counter IC CD4060. The working principle of the circuit is very simple, when high input at reset pin (pin 12) is given all the output becomes low and vice-versa.  Switch SW1 is used as door open/close sensor, when the door is closed switch SW1 opens as we used push to off switch which further put reset pin (pin 12) high. When the reset pin becomes high the output from IC1 become low and piezo buzzer does not sound.



When the door is open, switch SW1 closed as per mechanical arrangement which put reset pin low. The low voltage as reset pin put high at output, as a result piezo buzzer start sound of 10-second long beep in interval of every 10 second.

The audio oscillator is build around resistor R2 and C2, where the time interval of beep sound is also controlled by these pair. Transistor T1 is used as piezo buzzer driver.


Resistor (all ¼-watt, ± 5% Carbon)
R1 – R3= 100 KΩ
C1 = 100 µF/16VC2 = 10 nF
IC1 = CD4060

T1 = SL100

D1, D2 = 1N4148

SW1 = Push-To-Off SwitchPZ1 = Piezo buzzer3V battery