Here is the description of the circuit diagram of an ultra sensitive intruder alarm. The shadow of an intruder passing few meters nearby the circuit is enough to trigger the alarm. The circuit is work based on the 741IC as the core of the system. This is the figure of the circuit.
Here IC2 uA 741 is wired as a sensitive comparator, whose set point is set by R6 &R7. The voltage divide by LDR and R9 is given at non inverting pin of IC2. The LDR can be housed in a dark tube to increase sensitivity. At standby mode these two voltages are set equal by adjusting R9.Now the out put (pin6) of comparator will be high. Transistor Q1 will be off. The voltage at trigger pin of IC1 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 inputs of comparator will be different and the out put of IC2 will be low. This makes Q1 on. This makes a negative going pulse to trigger the IC1 which is wired as a mono stable multi vibrator. The out put of IC1 will be amplified by Q2 (SL 100) to produce alarm. The sensitivity is very important here. If you cannot adjust the required sensitivity properly, use one LOW resistance (~1K ) POT in series with R9 for fine adjustment. To setup the alarm, power up the circuit and adjust R9 so that LED D1 goes off.
Here IC2 uA 741 is wired as a sensitive comparator, whose set point is set by R6 &R7. The voltage divide by LDR and R9 is given at non inverting pin of IC2. The LDR can be housed in a dark tube to increase sensitivity. At standby mode these two voltages are set equal by adjusting R9.Now the out put (pin6) of comparator will be high. Transistor Q1 will be off. The voltage at trigger pin of IC1 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 inputs of comparator will be different and the out put of IC2 will be low. This makes Q1 on. This makes a negative going pulse to trigger the IC1 which is wired as a mono stable multi vibrator. The out put of IC1 will be amplified by Q2 (SL 100) to produce alarm. The sensitivity is very important here. If you cannot adjust the required sensitivity properly, use one LOW resistance (~1K ) POT in series with R9 for fine adjustment. To setup the alarm, power up the circuit and adjust R9 so that LED D1 goes off.
