Super light Sensitive Intruder Alarm Circuit
Intruder alarms are popular devices used in high-security areas as well as civilian houses to detect and alarm the presence of any intruders. There are different kinds of intruder detection alarms, some detect movements by using a laser, some use pressure variations etc. Our intruder alarm works using an LDR (light dependent resistor). Also known as photoresistor its resistance varies with the intensity of light falling on it. Its resistance decreases with increase in light.
Our intruder alarm needs to be placed opposite to a light source since it detects movements using the changes in the intensity of light falling on it. When a shadow falls on the LDR its resistance increases and triggers the alarm circuit.
Here is 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.
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. At standby mode, these two voltages are set equal by adjusting R9. Now the output (pin6) of the 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 the comparator will be different and the output of IC2 will be low. This makes Q1 on. This makes a negative going pulse to trigger the IC1 which is wired as a monostable multivibrator. The output of IC1 will be amplified by Q2 (SL 100) to produce an alarm.
Intruder Alarm Circuit Diagram with Parts List
- To set up the alarm, power up the circuit and adjust R9 so that LED D1 goes off.
- The LDR can be housed in a dark tube to increase sensitivity.
- 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.
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