> [Phillip says] Seems to me that if the propagation time for each sensor
> was the same/equal i.e. the sensors were made with the same type
> devices and you used the same amount of wire etc. the propagation
> time would not matter because you only care about the difference

That's quite true. As you say though, the trick will be to balance the
whole thing. For example, if one wire is passing over damp or wet
grass and one is over asphalt. It would be fairly easy to match a pair
of sensors with a 'scope or logic analyser, or at least characterise them

Have fun. I'm thinking of a little project myself to add a digital readout
for the speedo in an older car, but I'll be doing that on a wheel or the
transmission, not carring a laser jig around

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