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.

Technical Specifications

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:

Laser Security Alarm very reliable and secure

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.

Simulation and Working

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.

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