Laser Trip Wire

Building a DIY Alarm System: The Concept

So, you want to protect your property, or maybe a room, car, or even your backyard. The concept of an alarm system is to notify the "alarm administrator" that there is an intruder entering the previously designated boundaries. The triggers for this boundary could be as simple as a pressure sensor, or as complicated as a laser network. They all do the same thing, but some work better than others.
Generally, in an alarm system, there is a central brain that deals with sensor input, user input, and notification output. For example, in a simple alarm system protecting a door, when the door was opened, a sensor would go "High" (or Low, depending on the configuration), and the brain would detect this abnormality, and sound an alarm.
In this article, we'll be making a laser trip wire alarm.

You Will Need

  • A laser pointer, ideally one that can run from the mains
  • A few mirrors, but these are optional (depending on how many beams you'd like)
  • A nightlight; one with a light sensor! See Fig. 1.1
  • A relay (depending on which circuit you chose to build, either 120 volt or 12 volt. These can be found at most electronics stores, or purchased online here and here)
  • Miscellaneous wire
  • A hobbybox or similar enclosure
  • Glue (hot glue works fine)
  • A 12 volt siren (again, this can be found at most electronics stores, or purchased online here)
  • Pushbutton temporary switch (this needs to have a continuous on/off option, and commonly has three prongs. It can be found at most electronics stores, or online here)
  • AC-to-DC adapter (only necessary if you chose circuit two. 120 volt to 9/12 volt, can be found at most electronics stores, or online here)

 How to Build a Laser Trip Wire Alarm System
                                                                    (Fig. 1.1)


  • Soldering iron
  • Solder
  • Hot glue gun
  • Wire cutters
  • Miscellaneous screwdrivers
  • Electrical tape

The Procedure

Before I get into the actual creating of the alarm circuit, I'll explain the theory behind it. Inside the nightlight is a small circuit with a transistor, resistor, and photo resistor (a resistor controlled by light). This is the key component of this alarm, because it senses when the laser is on and off. The second component is the relay, which is triggered by the nightlight circuit. The third component is the adapter and siren, which are in turn, triggered by the relay.
The reason I have included a relay in the circuit, rather than wiring the siren directly to the nightlight, is because the siren needs to remain on even after the trespasser moves out of the laser's path. Here are two simple schematics of the full circuit (Fig. 1.2), both should work fine depending on whether you prefer AC or DC input. Also, the first one only requires the photoresistor from the nightlight. The first circuit should function from between 9-12 volts, and the second at mains power (120 volt).
How to Build a Laser Trip Wire Alarm System
                                                              (Fig. 1.2)
1.  First, build the circuit standalone or on a protoboard.
   A. For circuit one, use the adapter to power the whole circuit. Extract the photoresistor from the    nightlight by opening it up and removing the component (Fig. 1.3) from the circuit inside.

How to Build a Laser Trip Wire Alarm System
                                                                   (Fig. 1.3)
    B. For circuit two, solder the adapter directly into the protobox and circuit. Since it's AC,
    polarity isn't a concern. However, when attaching the adapter to the siren, ensure that                positive goes to positive negative to negative. For the nightlight, remove the circuit from inside,     and place it in series as shown in the schematic.
    C. For both circuits, the relay functions as follows.
How to Build a Laser Trip Wire Alarm System
    First, the "loopy lines" on the left of the relay are the coil, which generates a magnetic                 field and moves the switches. Inside this relay, there are two switches. Shown in the                     picture, there are four lines parallel to each other, located vertically. These are two                     switches; the black dots represent ground, and the line is the second terminal. 
    Below is a better representation of a relay. A and B one are one switch, and A/B are on               another.
How to Build a Laser Trip Wire Alarm System
    Once your circuit is completed, move on to the next steps.
2.  Mount the mirrors where you'd like the laser to protect (or just the laser itself, with no mirrors).
3.  Make sure the laser ends up reflecting onto the photoresistor.
4.  Turn on the laser and plug in the siren and trigger.
5.  Test it out!

6.  Once triggered, the siren will sound until the push break button is pressed. This will reset          the circuit.


  • Cut some PVC (or an empty pen case) and hot glue it to the photoresistor so that the laser goes into the tube and hits the photoresistor.
  • Use flux solder, it makes life easier.
  • Make sure the mirrors are clean, otherwise the laser will become distorted.


  • Soldering irons are hot! Don't burn yourself.
  • Don't electrocute yourself!
  • Make sure you use the correct adapter. 
  • Ensure polarity is correct.