To build anti-theft circuit using electromagnetic relays



Author: Doan Thi Y Nhi

Instructor: Phan Cu

Unit: Tran Hung Dao High School, A Luoi District


1. The initiative:

Derived from the reality of A Luoi district in particular and many other places in the country that thieves break into houses to steal people’s property regularly. Pressing with the situation, after studying a lesson about magnet – application of electromagnets (physics – grade 9), I have an idea of doing a simple anti-theft circuit which could be put in houses to alarm at night. Once, it can be a tool against thieves, other, I can turn theory in our books into real and understandable model which can be served in teaching courses to help students enjoy the subject.

Structure description of the product:

The circuit consists of: 1 electromagnetic relay, 1 small transformers (220V in; 12V and 6V out), 2 source filter capacitor, 1 electric bell, 4 Led bulbs (2.5V), 1 electric plug and a wooden house as model.

Assembling: Connect 220V source into a primary coil of the transformer and divide voltage into 2 sources: 6V and 12V then bring them to the filter capacitors and connect with two electrical circuits.

– The first circuit: Includes a relay connected with doors.

– The second circuit: Includes bell which is parallel to the lights

3. The operation principle of the product:

– When doors close, power is supplied for the 1st circuit, the closed relay makes  the circuit close, while the others 2 opened, bell and light are in standby state.

– When doors open, the 1st circuit opens and the turn-off relay makes the 2nd circuit close, then the bell rings and the light is activated.

4 The novelty: Same studies have not been found in locality

5 Creativity:

It is in fact the alarm circuit, but I’ve combined with Led light since I know the psychology of crooks that they always afraid of the light.

Applicability of the product:

– The product has high applicability. It is suitable to the reality, easy to install and inexpensive in cost.

– The product may serve well for life, in learning and in teaching.


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