laser trip wire

Thread Starter


Joined Sep 8, 2007
I wondered if knowledgeable participants of this forum would help with suggestions on the transferring of a signal from a laser trip wire to a computer.

Trying to understand how, if a laser trip wire with an approx 15 foot range when tripped by a vehicle could send that signal to a computer database.

What type of interface would I require to achieve this.

The end result is to have a program that we will design to interpret the database info to determine how frequent and fast a vehicle trips the wire.

(It is for an underground parking lot to control traffic)

I know very little of electronics but am comfortable once the info is sitting in the database as I'm a Programmer/Developer by trade.

Many thanks.



Joined Apr 20, 2004
There are several possibilities for do this. How far away is the computer? I assume the 15 foot distance is the separation between the emitter and detector of the light beam.

Also, are pedestrians likely to use the same pathway? It's a bit difficult to tell if it's a car or a person breaking the beam.


Joined Jul 22, 2007
I'd think that if you put the tripwire close to the ground, and use the time the tire has the light blocked to estimate speed, would be better then by the vehicle chasis. The tire diameter will vary a few inches on the average, where the car body could be several feet.

The output of the tripwire should be simple to interface with a computer, simple on/off, binary 1's and 0's. You never mentioned if you were looking for a wireless (tough underground), or running a cable.

If you position your tripwire near the enterance/exit, shouldn't be much foot traffic...


Joined Jul 17, 2007
Have you considered using a wire loop instead?

Lasers and optics (such as phototransistors or photocells) would certainly work, but would be problematic as far as installing them in an out-of-the-way location, proper alignment, contamination by dirt/fumes/etc, the curious fiddling with or stealing components, etc.

Instead of something so visible, you could embed a wire loop of arbitrary size in the surface of the concrete, and use the loop as the L in an LC oscillator circuit; an op-amp chip should work fine. A large metal object (such as a car or truck) passing over the loop would effect a large change (drop) in the output frequency, whereas a person (or groups of persons) would have little effect.

You could monitor the oscillator's output frequency directly, using the line input of your computer's sound card - as long as you kept the frequency within the sound card's practical limits (say, 50Hz-20KHz). Setting the threshold frequency is up to you.

Car on, car off.


Joined Sep 8, 2007
just a thought, you wont be able to accuratley get a speed of the cars with only one trip laser, as all cars have different wheel base differences, you could do two lasers a known distance apart to accuratley determine speed.

for the input... i work as a tech at a haunted house, and i know that you can get cheap laser trip circuits that simply close a relay, from there use that relay as a switch that goes through a unit gain amplifier (a pass gate pretty much, to not overload the mother board with current and blow it) into a parallel port, and from there you can simply use c/c++ (to be able to access the port directly) to do the programming end, which i am assuming you dont need help in, seeing your a programmer...