The post discusses the making of a simple yet effective smart laser alarm protection circuit using the IC 555, for securing a specified restricted zone from human interventions. The idea was requested by Mr. Collins.

Protecting Home with Laser Alarms

I just want to tell you what an amazing passion and dedication to electronics you have and thank you for helping others like me. My name is Collin from south Africa.

We have a huge problem of safety here.

There are countless number of robberies and house break inns its so unreal. I was hoping you can help me with regards to a circuit for a very cost effective and reliable product.

I have built a laser tripwire alarm using a 555 IC timer but the circuit design lacks a lot of features.

I need something that as soon as an intruder enters my property I will be alerted before they even get a chance to try and get into my home.

The circuit needs the following: Once alarm is activated it needs to sound for a few minutes and then go off

and arm again automatically. Don't want it ringing for hours in end if I am not at home to reset it.

It should not be triggered by pets or flying debris in the yard.

Easy alignment of the sensors. The laser alarm works OK but its very difficult to set the pointer on the LDR that keeps moving off. Any tips if we are going than route?

It doesn't have to be a laser system I am open to whatever you think will solve my problem.

Thanks a lot for everything and feel free to add whatever you think I missed out with regards to all the features.

Kind regards


The Design

Parts List

R1, R4 = 100K
R2 = 1M
C1 = 4.7uF/25V
C3 = 10nF
C4 = 0.33uF
LDR = any standard

IC 555Pinouts

Simulation and Working

The proposed smart yet simple laser alarm circuit using IC 555 can be witnessed in the following image, the functioning can be understood with the following points:

1) The IC 555 is configured in its basic monostable mode.

2) Pin#2, which is the trigger input of the IC can be seen connected with the emitter of a PNP BJT via a blocking capacitor C2.

3) We can also notice a couple of parallel LDRs secured within opaque pipes and their leads hooked up with the base of the PNP BJT, such that as long as the LDRs remain illuminated together through the laser light focus, the BJT remains deactivated. This happens because in the presence of laser light the LDR's combined resistance drops to around 30K, which keeps the base of the PNP more positive than the ground bias from R2.

4) The inclusion of two LDR ensures a fool proof alarm set-up such that only a human presence is able to interrupt the LDRs, while these remain unaffected by other smaller irrelevant intruders such as animals, birds etc.The two LDRs could be placed at a distance of around 2 feet so that only taller objects such as a human being gets detected.

5) Therefore whenever an interruption in the laser beam is detected, the LDRs go through a sudden rise in their resistances causing T2 to switch ON, which in turn triggers pin#2 of the IC via C1.

6) This prompts the IC 555 to activate its pin#3, which finally activates the connected alarm unit.

7) Since the IC 555 is configured in its monostable mode, the pin#3 remains activated only for a period determined by the RC network at pin#6 and pin#7 of the IC, or by R3,C2.

These timing components can be appropriately calculated using the IC 555 calculator software for accomplishing the desired length of time for which the alarm may remain switched ON.

How to Set Up the Laser Transmitter Units

The laser transmitter devices could be installed near the LDRs itself and focused back to the LDRs using mirror reflectors as shown below:

The installing of the laser devices near the LDRs allows the entire unit to be installed inside a single enclosure and also allows the lasers to be powered from the circuit itself.

This also facilitates securing the lasers and the LDR units firmly and accurately so that both the counterparts are unable to move or deviate from their positions even in an event of a mechanical shock or other vibrational interferences.

The mirrors could be positioned at some specified distance, exactly opposite to the laser installation , making sure that the laser beams intersect the restricted zone and the presence of a possible intrusion is detected without fail.

This concludes the making of the proposed IC 555 based smart laser alarm protection circuit, if you have any doubts please feel free to put them forth in the below given comments box.

Using BJTs for Reducing Current Consumption

The above design can be further enhanced to work with lower standby current, and also with a battery backup during of power failures, through the following upgraded schematic:

Laser Security using Single LDR

If you intend to simplify the implementation for a single LDR operation, in that case the following concept can be tried:

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