The figures below showing two projects of simple burglar alarm circuits. There are many types of burglar alarms are available to build using different techniques to get alert of burglar. The circuits given here are using simple and effective technique for this purpose, and can be used for home security at the
doors.
The two points of wire connections shown in both the circuits to connect a very thin copper wire from one end to another at the door or at any other place that you want to secure. Now when any body will pass that door or place the wire will break and the transistor will get switch on and activate the device connected to it.
The circuit shown in figure 1 is using only two components, which are a transistor and a 100K resistor. At the output of the circuit you can connect any type of 12V and 200 to 300mA current rating buzzer, siren, horn, alarm etc
The circuit in figure 2 is using a 12V relay at the output. In this circuit you can connect any alarm device with operating current rating of 10mA to 10A.
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Figure 1.
Figure 2.