This simple door-opening alarm uses a hall sensor to produce sound when someone opens the door. The circuit can be installed at a workshop, office or home for security purposes. The 3-terminal hall sensor is easily available in the market.

Fig. 1: Circuit diagram of the door-opening alarm
Fig. 1: Circuit diagram of the door-opening alarm

Circuit and working
The circuit diagram of the door-opening alarm is shown in Fig. 1. It is built around hall sensor MH183 (IC1), pnp transistor BC557 (T1), npn transistor BC547 (T2) and a few other components.

Fig. 2: Actual-size PCB of the door-opening alarm
Fig. 2: Actual-size PCB of the door-opening alarm

Hall sensor MH183 is a magnet-operated solid-state sensor. When the magnet is brought near the sensor, its output goes low. This low output is amplified by pnp transistor T1. npn transistor T2 inverts the operation. When T1 conducts, T2 does not conduct, and vice-versa.

Fig. 3: Component layout of the PCB
Fig. 3: Component layout of the PCB
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The magnet should be fitted on moving part of the door and the sensor at the door frame. When the door is closed, the magnet comes close to the sensor and the alarm is off. But when door is opened, the alarm makes a sound.

The buzzer can be replaced with a 5V relay to control a mains alarm.

When the door is closed, neither the buzzer sounds nor LED1 glows. But when the door is opened, the buzzer sounds and LED1 glows, too.

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Construction and testing
An actual-size, single-side PCB for the door-opening alarm circuit is shown in Fig. 2 and its component layout in Fig. 3. Enclose the PCB in a suitable small box and install it at a suitable place.

Download PCB and component layout PDFs: click here

Fit the magnet on a moving part of the door and sensor MH183 on the door-frame opposite to it.
Use 5V regulated power supply for the circuit.

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Door-Opening Alarm

Pradeep G. is B.Sc. (Physics) and a regular contributor to international magazines. He is also a small-business owner making school/college projects in south India

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