You can easily make a Burglar Alarm system using the PIR Sensor module. The PIR sensor module is readily available now and costs less than Rs.100. You can make the complete unit at the cost of less than Rs.200 but its use is worth. The PIR Sensor module has a circuit board with a timer. The timer circuit gives a time delay between 30 seconds to 3 minutes that can be adjusted using the present. It also has a sensitivity adjustment for adjusting the sensitivity from few centimeters to 10 meters. In some modules, an LDR is also provided for night time operation only.
The PIR Sensor module requires 5 volts DC for proper operation. So a voltage regulator IC 7805 is used that reduces 9 volts DC to 5 volt DC. The PIR Sensor module has three pins. One is Vcc for 5V, Middle one is output and the third one is ground. Markings are in the module for connection. Its output gives 5 volt DC when triggered. So this output can be used to drive a relay driver transistor. Load such as bulb, CFL, Electric buzzer etc can be connected through the relay contacts.
PIR (Passive Infra Red) sensor detects the human movement by comparing the surrounding temperature. Human body generates Infrared through body heat. The sensor remains stand by using the background temperature as reference level. When a human being move through its detection zone, it changes the temperature level in the area and sensor triggers and load turns on. The load turns off after the time delay as adjusted by the preset.
For best result, install the unit 3 meters above the ground at an angle of 45 degree.