Great Light Sensitive Intruder Alarm Circuit

Intruder alarms are a device that is quite popular in areas that have a high level of security, and civil homes that can detect actions that can interfere. There are many types of Intruder alarms, from the simplest to the most sophisticated using laser motion detection.

The set of intruder alarms used here is simple but very effective in homes. The alarm intruder circuit used here is using a Light-dependent resistor (LDR) as the transducer. LDR is one type of resistor made with semiconductor materials.

LDR resistors are also called photoresistors. LDR resistance value will vary according to the intensity of light entering the LDR field. Type a lot of light on the LDR, then the resistance will decrease.

intruder alarm circuit

These intruder alarms must be placed opposite the light source because the LDR detects motion that uses the principle of changing the intensity of the light on the LDR. Principally when LDR resistance increases, the alarm will be active.


The intruder alarm circuit has a high level of sensitivity to detect any movement that passes several meters from the circuit. This is more than enough.

IC UA741 functions as a comparator circuit whose set point by two 10k resistors which function as voltage dividers. The voltage divider value of the LDR is fed to pin3 IC UA741 to be compared with the set-point voltage of pin 2 IC. The comparator circuit is designed as a non-inverting circuit.

When the comparator output is high, the transistor output value will be low so that IC2 555 has no trigger on pin2. This means that the alarm is off. When an intruder passes the LDR resistor, the shadow of the intruder causes the comparator output to be low. That way the transistor output will be active and trigger pin2 IC 555. IC 555 functions as a monostable multivibrator circuit. with a trigger on pin2, the alarm will be activated which sounds through the buzzer.

Before use, set the alarm first by setting the potentiometer before the LED turns off. To be effective, the LDR resistor is placed in a dark tube so that its sensitivity increases.