Laser Tripwire Security Alarm Using NE555 Timer IC

What is a Laser Tripwire Security Alarm?

A laser tripwire security alarm is a simple security system that can detect the movement of any intruding person or object passing through the beam of a laser & triggers an alarm accordingly. The red line laser actually triggers a latching switch, that can be connected to any external load/security device such as a loudspeaker alarm, floodlight, or CCTV cameras. Laser tripwire alarms have become a staple for home security systems & mandate their presence in every security detail for any place. So, in today’s tutorial, we will go over a step-by-step procedure on how to design a simple Laser Tripwire Security Alarm using an LDR & a NE555 timer IC.

The heart of this laser security system is a light-dependent resistor or an LDR. LDRs are electronic components that are sensitive to light intensity. Its behavior is similar to a photocell that works on the principle of photoconductivity. The resistance value of an LDR decreases when the intensity of light increases. The threshold voltage of the NE555 timer IC is decided by the output voltage of the voltage divider configuration (LDR & R2), here the NE555 is running in astable (free running mode).

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Hardware Components

The following components are required to make Laser Tripwire Alarm Circuit

S.no	Component	Value	Qty
1.	Laser diode/pointer	5V, 650nm, 5mW	1
2.	IC	NE556 Timer	1
3.	Laser Security Alarm PCB	AllPCB	1
4.	LDR	5mm	1
5.	Electrolytic Capacitor	100uF, 470uF	1
6.	Ceramic Capacitor	0.1uF	2
7.	Resistor	1K, 10K, 22K	2
8.	Soldering Iron	45W – 65W	1
9.	Transistor	2n3904	1
10.	DC Battery	9V	1
11.	Speaker	8 ohms	1
12.	Jumper wires	–	As per need

NE556 Pinout

For a detailed description of pinout, dimension features, and specifications download the datasheet of NE556

Laser Tripwire Alarm Circuit

Working Explanation

This laser tripwire circuit triggers an alarm when the line of sight between the laser beam & the LDR is interrupted. The design of this circuit is based on a 555 timer IC. A photoresistor (LDR) is used to detect the laser light from the laser diode. The LDR & the 220Ohm resistor together form a voltage divider circuit.

When the laser beam is interrupted, the light intensity over the LDR decreases, increasing its resistance, this causes the voltage at pin 6 (Threshold) to rise above the reference threshold voltage. This triggers the output at pin 3 (Output) to go low & activate the Buzzer. When the line of sight between the laser pointer & the LDR is re-established its resistance decreases, causing the voltage divider output to fall well below the reference threshold voltage at pin 6 of the IC, disconnecting the output pin 3 from the latch switch which turns off the alarm.

Applications

• Laser security alarms are used in places such as residential, commercial, industrial, and military properties for protection against theft or property damage.
• Also used in high-risk/sensitive areas such as military bases, security vaults & supermax prisons.