Simple Laser Tripwire Alarm Circuit With NE555 Timer

The Laser Tripwire Alarm Circuit is a simple circuit that is used to is designed to make noise when the laser shining on the circuit is interrupted. On a bigger scale, it can be used in home security where the alarm goes off when a person enters the house and interrupt the laser shining at the sensor. I will try to explain the steps involved in constructing the circuit and the concept behind its working.

To construct a Laser Tripwire Alarm, you will need the following equipment:

• A voltage source (4.5V- 12V)
• Laser Pointer (Light Source)
• NE555 Timer
• Buzzer
• Cds Photoresistor
• Resistors: 1k, 100

The ne555 timer has 8 pins (as shown on the picture above) and our goal is to adjust the value for the OUT pin depending on the amount of resistance from the Cds photoresistor (controlling the trigger and reset inputs). The trigger pin is connected to ground to get activated and this will change the OUT pin to a high voltage. The THRESH pin is held at a medium voltage so the OUT pin is still at high volt. Since the buzzer has one end connected to it, that end will have a high voltage. The second end of the buzzer is also connected to the positive input of the battery so it will also have a high voltage. Since there is no potential difference across it, there won't be any sound. However, when the laser (light) goes off, the voltage at the THRESH will be high while the OUT pin will have a low voltage hence one end of the buzzer will have a low voltage creating a potential difference across the two ends of the buzzer. The sound won't stop until we reset it (apply a low voltage to the TRIG pin) because the THRESH still has a high/medium voltage.

Connect the circuit according to the diagram shown.

This is what it looks like after assembly. We want the resistance from the photoresistor before plugging the battery so start by shining the laser/light at the resistor then connect the battery. Afterwards, check if the circuit is working by stopping the light from hitting the resistor; you should then hear a sound from the buzzer.