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Bodom
2011.11.24, 04:07 AM
Hello everyone,

I am on my way of experimenting, trying to create cheap homemade lap counter capable of tracking three cars as a start. Purpose of use is a small home track. It is way too expensive to buy a real lap counting system :rolleyes:

The idea is simple.
The transponder would be simple NE555 timer IC (http://en.wikipedia.org/wiki/555_timer_IC) based signal generator with an IR diode at the output. Different transponders would work on different frequencies.
The receiver would be any kind of IR sensor with an amplifier.
The signal from the receiver is plugged into a computer's sound card microphone in. Then a software would do sound processing in order to detect the cars passing through the start/finish line and record the times.
This is the basic idea.

Now I've done a test gear and am working on the software right now. And this is where I have problems :)
First the IR receiver captures some constant noise.
Second the 555 generator produces square shaped signal which has harmonic noise. I don't know for sure, but I think that sine wave would have less harmonic noise ?!
When the receiver is capturing signal from only one transponder - it is pretty clear which one it is. But when I turn on two or three transponders simultaneously - it becomes pretty messy :)

So far I've implemented Blackman–Harris window function over the input signal and then FFT transformation.
I don't know if this is gonna work at all.

Any comments and ideas are welcome.
If anyone has signal processing knowledge would be a great help.

Does anyone know haw does the real IR based lap counting systems work? What kind of modulation do they use? How do they detect the transponders. Does it happen all in the hardware? What information do they pass to the computer?

If I manage to make something that works - I'll publish all the schematics and software source code.

Thanks for reading. :cool:

LED
2011.11.24, 06:57 AM
I think you would be better of looking at some other stuff people made so far.
Like the webcam.
But if this is for experimenting, then go wild :p

Cant help with your question, all sounds like japanese to me :)

z3zinho
2011.11.24, 09:17 AM
This looks like a nice project.

Hardware wise, you could probably loose most of the noise. Most of it will be 50/60Hz flicker from all the artificial light around the bridge.

I have no experience with IR systems whatsoever, but If you google there are some projects revolving around Pico-Z helis and stuff like that that user IR remote controllers. That should be a good place to start ;)

z3zinho
2011.11.24, 09:20 AM
Altough I don't know much about the IR stuff, my guess is that you would be better off choosing a carrier frequency, and then modulate it so that different transponders have different signals. This way you could get rid of most of the noise in your analog front end ;)