> In theory it is just a clock but it needs to be able to count time
> very accurately. We were thinking to add an external clock to
> make it more sensitive. We need to be able to count down to
> 0.0001 sec

Example :

100.0kph => 27.7778 m/s => 2.7778mm per 0.0001s (100us)
100.1kph => 27.8055 m/s => 2.7805mm per 0.0001s

If the timing lasers were 10m apart, using 0.0001s resolution

10m @ 100.0kph = 0.3599s
10m @ 100.1kph = 0.3596s

0.0003s difference => 3 * 100us

So you'd be able to differentiate between 100.0 and 100.1, but
it's very close at 10m and 100us slice

Using a 40MHz 18F you've got a 250ns time slice, or 400 times
finer than 100us. An external clock will not be necessary. CCP
module would be the way to go

If all went well, using 250ns (0.00000025s) resolution

10m @ 100.0kph = 0.36000000s
10m @ 100.1kph = 0.35964035s

0.00035965s difference => 1439 * 250ns

Putting the lasers further apart would help, however, finer resolution
and longer connections brings its own problems

Propagation (wire time, sensors reaction time etc) of signals from lasers
to the PIC for example, but they can be calculated and factored out.
Micro-fluctuations in speed, maybe not so important or even avoidable

With a good set-up under controlled conditions, you should be able
to do speed measurement quite well. At 100m you could add another
kph decimal place with some confidence

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist