Here is the circuit diagram of an ultra sensitive intruder alarm. The shadow of an intruder who passes a few meters near the circuit is enough to trigger the alarm.
Here IC2 uA 741 is wired as a sensitive comparator, whose set point is set by R6 and R7. The voltage division by LDR and R9 is given in the non-inverting pin of IC2. In Standby mode, these two voltages are set equal by setting R9. Now the output (pin6) of the comparator will be high. Transistor Q1 will be off. The voltage on the IC1 pin will be positive and there will be no alarm. When there is an intruder near the LDR, the shadow causes its resistance to increase. Now the voltages at the comparator inputs will be different and the IC2 output will be low. This causes Q1 on. This causes a negative pulse to activate the IC1 which is wired as a monostable multivibrator. The out put of IC1 will be amplified by Q2 (SL 100) to produce alarm.
Intrusion alarm circuit diagram with parts list
Notes
• To set the alarm, turn on the circuit and set R9 so that LED D1 turns off.
• The LDR can be housed in a dark tube to increase sensitivity.
• Sensitivity is very important here. If you can not adjust the required sensitivity correctly, use a LOW resistance (~ 1K) POT in series with R9 for fine adjustment.