The circuit diagram of super sensitive intruder alarm is given here. The main parts of this circuit are two ICs IC1 (uA 741), IC2 (NE 555) and transistors Q1 (BC 157), Q2 (SL 100). The shadow of an intruder passing few meters away the circuit is enough to trigger the alarm. In this circuit, the operational amplifier IC uA 741 acts as comparator. The set point of this comparator is determined by the resistors R6 (10K) and R7 (10K). The input to the non inverting terminal of IC2 is the divided voltage between potentiometer R9 (47K) and LDR R8. At standby mode, the voltages at both the inputs of IC2 are made equal by adjusting potentiometer R9. Then the output of the comparator becomes high and that turns off transistor Q1. That means the voltage at trigger pin of IC1 will be positive and there will be no alarm. When there is an intruder near the LDR its resistance increases and the voltages at the inputs of comparator will be different and the out put of IC2 will be low. This makes Q1 on and negative going pulse to trigger the IC1 which is made as monostable multivibrator. Transistor Q2 amplifies the output of IC1 and the alarm is produced from that output.
- Use a good quality PCB to assemble this circuit.
- Power up the circuit and adjust R9 so that LED D1 goes off while setting up the alarm.
- The sensitivity of LDR can be improved by employing a dark house.