Skip to content

Stealth-Action Laser Tripwire System

This is a companion project to the Stealth-Action Motion Sensor. It’s Hollywood-style security systems.

With these, you can make your very own security system that gives intruders a sporting chance! It’s the laser-version of saying “But before I kill you, Mister Bond…”


This laser tripwire system will trigger an audio alarm (made from a 110dB piezo “personal alarm” from the 1$ store) when something breaks the laser beam, which is provided by a simple laser pointer.

What makes this Hollywood-style is that when the beam is broken, the unit “Beeps” three times. If the beam is STILL broken after the 3rd beep, the alarm sounds. (If the intruder “backs off” and unbreaks the beam before the 3 beeps are over, the alarm does not sound).

(If you haven’t already, please read about the Stealth-Action Motion Sensor – aka the “Hollywood-style motion-sensor”.)


These are best used when placed somewhere that is physically confining, like around tight or low spaces so an intruder cannot simply run through all the tripwires. Any area that constricts movement is ideal.

For example, the tripwires – which require the intruder to quickly break contact to avoid and alarm – can work well in conjunction with the Hollywood-style motion sensors, which require the intruder to STOP moving to avoid an alarm!


This project uses some electronics and a PIC (12F629). If you own or have access to a PIC programmer (and know how to use it!) you will be able to make and modify this project as long as you possess basic electronic skills. If things and terms like regulated voltage, HEX files, and pullup resistors don’t mean anything to you then you will have problems if you try to make this project.


The laser tripwire alarm assumes the light sensor is being lit by a laser when it is powered up. It will attempt to calibrate itself at powerup. Power up the alarm, and when the LED is blinking quickly, break the beam. When the LED lights solid, unbreak the beam. The LED will turn off and begin a slow “heartbeat” blink. The sensor is now active.

If the laser beam is broken (and the laser stops hitting the light sensor) the LED will light up and the unit will emit 3 beeps in rapid succession. If this beam is STILL broken at the end of the 3rd beep, the alarm will sound. This allows someone who touches a laser beam to “back off” to avoid an alarm.

An optional “Reset” button exists to cancel the ALARM. A “Disable” input also exists to shut down the sensor until it is reset.


The following main components are used in making the laser tripwire.

(Click for a larger version)

Obtain these, park yourself at a basic electronics workbench with the means to program the PIC, and then just follow the schematic below.


  • This design assumes you have a +5V supply to feed the circuit with. (If you do not, consult the schematic for the Hollywood Motion Sensor for a suitable +5V supply.)
  • When the ALARM is triggered, the ALARM state lasts for 10 seconds (or until Disable or Reset is hit).
  • When the (optional) Disable button is pushed, the entire device is shut down until RESET is hit.
  • When the (optional) Reset button is pushed, the device goes back to “active” mode.


(Click for larger version)

HEX file ready to be programmed into a PIC: 12f629-Laser-Alarm.hex
PICBASIC Code: 12f629-Laser-Alarm.bas


  • The audio alarm is a simple “110dB personal alarm” from the 1$ store; any audio alarm that is powered by approximately 5-6V should work.
  • The laser pointer can be almost any kind, but the more powerful it is, the better the range. A cheap pointer was reliable out to 6-8 feet.
  • The CdS cell should be shrouded (protected from ambient light). Do this by wrapping tape or some other tube around the cell.


The following is from the Stealth-Action Motion Sensor Alarm project. Use the same schematic for the “shootable” sensor.

The laser tripwire’s PIC has been programmed to use a RESET and DISABLE input, same as the Hollywood Motion Sensor. That means that with the right modification you can make the unit “shootable” (really just a fancy way of hitting the DISABLE switch).

If you own an Airsoft gun, you can make the sensor “shootable” by connecting a piezo element and a small sensor circuit to a small piece of plexiglass/lexan/polycarbonate and feeding the output into PIN 5 (DISABLE) of the PIC. The sensor will be disabled if the sheet of polycarbonate is shot. The sharp impact from the BB hit will transfer into the piezo element (superglued to the sheet of polycarbonate) which will trigger the DISABLE input of the motion sensor circuit.

(This was only made with Airsoft guns in mind. If you own a Laser Tag gun, you’ll need to roll your own sensor for detecting a “shot”. If you’re using a paintball gun, you’re probably better off actually shooting a switch with a target on it to trigger the Disable input, since paintballs carry a lot of Oomph to them and can easily trigger a beefy switch. Also paintball hits are messy, so keep that in mind. Finally, if you’re using a real gun… well, I think any way you slice it the disabling of the device will be permanent in that case.)

This modification assumes you have the electronics knowledge to follow the schematic below. The circuit shown will pull the /Disable pin on the PIC low when the piezo element detects a sharp and strong impact on the polycarbonate plate it is bonded to.

This is not a foolproof system, but it seems to work fine on my workbench with a spring-powered USP and it uses very few parts. Your mileage will probably vary.

Remember that the polycarbonate sheet is what has to be hit for the shot to be detected!

The following is from the “Hollywood-style Motion Sensor Alarm” project. Use the same schematic to make the Laser Tripwire “shootable”.

This is a brief video demonstrating the basic operation of the completed prototype. It demos the triggering of the alarm as well as “backing off” to avoid an alarm. The auto-calibration at powerup is also shown.