lap counter, with atmel rf 125kHz ??

Hi everybody, I'am a newbe 8)

I want to make a lap-counter, the only problem is how.
What I do know is that most professional systems work with radio frequencies. Now that’s not a big problem for me, I’ve got a scope and I did build some transmitters. The biggest problem is how to electronically identify every car…
And even a bigger problem: what if two cars pass at the same time ???

At first I was thinking about doing the transmission whit IR; that way I could build a singlechip IR remote control chip. That way I could give each car specific code.
The only problem with IR is that most IR led’s work at a maximum frequency of 56khz, and that’s to slow. And off course the biggest problem, that if two cars pass at the same time.

and then I saw this on atmel.com: http://www.atmel.com/dyn/products/devices.asp?family_id=644#695
A read only chip, but will it be fast enough ?

or are there other options available?

Hi hilbren,

I can imagine that professional RF lap counter systems have a unique transmitting frequency for each car.

In IR you would need different wavelength transmitters or receivers. Unfortunately this is not cheap and their passband has slow roll offs, so separation between differen wavelength will not work very nice.

Another method coul be creating a correlation pulse, unique for each car, and then searching for this pulse in the received time signal.....
For this you will need a DSP :-(((.
This depends on the time the car can transmitt it's ID to the receiver, the data rate and the number of identifiers (length of the ID).

I think easiest will be RF transmission with a unique frequency and for each car a receiver that just reacts on this certain cars frequency.

okay, and how can I seperate those signals again? ne567 or is there something better ??