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Making a Laser Beam Security Alarm Circuit

We have often seen laser alarm system as an integral part of security solution especially for a location which deserves hi-level security.
Right from a museum preserving priceless historical possession; to a safety bank vault; and even in thriller flicks where the protagonist is seen on and often tied in the ray of red light beams, while trying to reach a safe vault; one is well acquainted with the usage of laser light beams. In fact it is also considered as a security device even in household today to ward off burglary or theft.

A laser beam is not a simple beam, but has the efficacy to respond when faced a distraction. This means that if the light ray gets interrupted with any objects, the photodiode receives resistance which in turn activates an alarm. The laser alarm system is an economical option when it comes to consumption of power, as the receiver needs power supply less than 10 mA on average. Setting up a laser alarm system is relatively easier as the laser and the receiver can be setup in one box, on a single power input.

Below is a circuit design diagram which illustrates the flow of a laser alarm system.

Referring to circuit diagram it is seen that a TL072 op-amp (IC1.A) is configured as a voltage comparator, placed in between the adjustable voltage driver P1/R4 and the light-dependent voltage which consists of photodiode D1 and R3 – a fixed resistor. As the laser beam receives a distraction from a foreign agent, the voltage on comparator pin drops to 2 below that at pin 3, thereby enabling the agent swing to positive voltage supply and enable an alarm situation.

As the laser alarm can detect any foreign agents, so is the alarm can be set in a more sophisticated fashion, where it can bypass accidental interrupts from agents, like an insect. This is accomplished as the Resistor R2 provides a level of hysteresis, thereby prevent oscillation when two comparator input voltage rests in almost equal state. In order to set a faster response method, it can be achieved by reducing the value to 1 µF.

A laser beam system is easier to setup, either as a single or a separate entity. If a single box is maintained to setup the alarm, then it is to be made sure that the photodiode can’t have direct contact with the laser beam. Assembling the components and the circuit in a breadboard, it should be mounted in a black box having a hole. A black drinking straw should pass through the hole to enable light flow from the direction of the laser beam to enter. Setting up the system properly makes the laser beam act even with direct sunlight, as it cannot affect the operation of the photodiode.

Article Written and Submitted by Mr. Dhrubajyot​i Biswas

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