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Laser Alarm Trigger V2

2 years ago 1405
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Laser Alarm System Trigger V2

By: Tyler Le


  This is the second model of the receiving end of a DIY laser-based alarm system. The first one will not be published. Also, this circuit has been tested on a simulator [] and it seems to work great.


  This alarm will normally have a laser shining at U1 (a photoresistor). Once the laser is blocked (possibly by an intruder), power will be supplied to the alarm terminal, where an external alarm will be connected. Power will continually be supplied even if the laser shines back onto U1, and it can only be stopped by pressing SW1 (the reset button).


  • It has two terminals: one for power (preferably a 9-volt battery) and one to connect to an external alarm of some sort.

  • It has an ON/OFF switch and a RESET button

  • The alarm will stay on even if the laser shines back onto U1 (after being triggered of course)

  • Adjustable power for the alarm (1.25v -to- ~V+ Power)

  • Compact size

  • Laser and alarm are external

  • Cheap in price (the total components besides the PCB cost around 5 USD)

Future Improvements:

  I might replace the voltage regulator section with just a relay in order to allow larger alarm systems to draw more power than what's provided by the circuit (but the power will be from an external source). On the other hand, I might not replace it since I don't need a large alarm system. 


  This circuit is a cheap, maybe reliable alarm trigger (as I haven't tested or even assembled it yet). Hopefully, the circuitry isn't too complex and it should be relatively easy to understand. Finally, are there any unnecessary components? Could it be simplified further? Is it lacking anything? Please feel free to comment, provide suggestions, and improve this circuit! 

    Thank you!


Laser Alarm System V.2

Circuit Model 1

Circuit Model 2


ID Name Designator Footprint Quantity
1 1M R1,R2 AXIAL-0.3 2
2 3.3k R3 AXIAL-0.3 1
3 Photoresistor U1 SNR2-3.0 1
4 uA741 U2 DIP-8 1
5 10k R4,R6,R7 AXIAL-0.3 3
6 200 R5,R10 AXIAL-0.3 2
7 NE555P U3 DIP-8 1
8 10n C1 RAD-0.1 1
9 Button SW1 SWITCH-6X6X5_TH 1
10 LM317T U4 TO-220(TO-220-3) 1
11 39 R8 AXIAL-0.3 1
12 Alarm P1 WJ2EDGVC-5.08-2P 1
13 Circuit Power P2 WJ2EDGVC-5.08-2P 1
14 1k R9 TRIM_POT_PTH 1
15 IRF3205 M1 TO-220 1
16 On/Off SW2 CON3 1




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Comments (1)

Guest Reply

Hi Sir. Have you make this project? Can I build it? Thanks.

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