Foolproof Laser Security Alarm Circuit

The post discusses a a simple yet versatile foolproof laser security alarm circuit which can be used for securing any concerned premise with extreme accuracy. The idea was requested by GPS.

The Request

Dear swagatam

Your circuits were very useful to me. Thanks for all
your posts, but i wanted something different now. I made Transistor Latch Circuit and it worked fine and i am adding dark activated circuit
to it.

I used it for laser security system. So now suppose i used this
circuit to cover a room using small mirrors, once the beam is displaced
the whole system will be activated but if some one points another laser
to the ldr and replace it with the original one the system will not
response because ldr will not even know that the beam was replaced.

So i
want a circuit that should be activated if anyone points another laser
or light because adding two laser will increase little amount of output
from the ldr, so my circuit must be activated if beam is broken or if
the amount of light is increased. will you please help me out to make
our area secure from the persons that comes at night without invitation.


The Design

The proposed foolproof laser security circuit can be witnessed in the following diagram:

Foolproof Laser Security Alarm Circuit

In the earlier article we saw a rather simple Laser controlled burglar alarm circuit where the alarm is sounded whenever the laser beam incident on the LDR is interrupted.

However as requested above, this could be possibly overridden and disabled by focusing a dummy laser beam at the sensor by a smart intruder.

In order to counter this the design shown above can be effectively utilized which makes sure that only a specific amount of light controls and keeps the alarm deactivated, and varying this spec even minutely triggers the alarm.

Basically the design is implemented through a window comparator stage made by using a couple opamp comparator circuits.

Circuit Description

As can be seen in the figure, the lower opamp monitors the laser light interruption or any form of decrease in the laser intensity and triggers its output, while the upper opamp monitors the brightness level of the laser and triggers its output in case the laser intensity increases due to any reason.

This keeps the circuit at a sharp edge, wherein any alteration in the laser beam causes the alarm to get activated.

Since both the opamp can have a zero logic only in the perfectly normal laser condition, the attached optocoupler stays inactive under this condition.

However in case the light is disturbed either on the higher or on the lower side, the relevant opamp output goes high, enabling the opto to activate the relay driver stage.

This instantly actuates the relay and the connected alarm across its contacts.

How to Set up the above discussed fool proof laser security alarm circuit.

It is very easy.

Keep the correct laser intensity focused on the LDR and adjust the lower opamp preset such that the lower opamp output just becomes low or zero.

Similarly, also adjust the upper opamp preset such that the upper opamp's output just becomes low or zero.

That's all, the circuit is now set and ready for sensing any form of tampering of the laser beam and to activate the alarm.

4 thoughts on “Foolproof Laser Security Alarm Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Hi Swagatham
    Is it harmful to eyes, the laser light available with keychains….? Which cIass is such laser beams….? I am afraid, so I have never attempt to make circuits using laser light. Thanks in advance for your reply.

    • Hi Anil, yes definitely it is harmful if it is focused into the eyes in any manner.

      But there's nothing to be afraid about these gadgets because as long as we are not looking into the beam, it is quite safe….even high watt LEDs are as dangerous as this if looked directly into them.

  3. Good day to you sir swagatam, I am roldan from Philippines 35 years of age. I am so fond reading your articles about security system. I am not an electronic engineer, but I can manage to trouble shoot minor electronic problems. I also have a basic knowledge on electronic schematics… I have already scans all your schematics about cell phone projects, but still I did not found the right project I wanted to built… a favor from you, is all im asking for.. if gods will…
    My request :
    1.) A security alarm (i.e. burglar alarm) using laser, once that an intruder lurks in the perimeter it will eventually trigger/dial my cellphone, same as the idea of your CELL PHONE DOOR LOCK CIRCUIT. Instead of pushing the car central lock instrument, it will dial my number thru a GSM cell phone. so that I am aware that somebody is/were inside my house. I am always away from my house.

    2.) I am thinking about using 2 cell phone, 1 unit is for the alarm system that will dial me in case of an intruder is inside.

    Hoping for your kind consideration about my request sir, till next.


    • Hi Roldan, I'll try to design it soon, however it won't be a Hi-tech equipment but nevertheless will do the job faithfully.

      It won't be a Hi-tech since we are modifying a cell phone as the GSM transmitter and not using a professional GSM module,

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