snoz

Well here’s the first baby step. I can approximate the signal that the spedo gets. I’m off by 7 or 8 mph from the calculations I have for what it should be at 200 Hz. That could be that the spedo is off, my function generator actual frequency is off, or the fact that I based my calculations on some data from tire rack is faulty. I used the revolutions per mile off of a 255/40/17, with a value of 832, to come up with my estimate of Hz vs. mph. At any rate, it’s close enough for me to proceed. I can easily adjust for inaccuracy later. Next step is to use some code to turn the analog signal into something meaningful to a digital display. More to come.

Edit: Youtube imbed didn't work.

danglerb

Measure the pulse width, smooth, convert.

snoz

The signal is FM. You are saying that measuring it by PW is better? I'm not arguing. I'm just curious. It's been many years since I've messed with this stuff. I'm having fun anyway.

danglerb

Signal is I think from a proximity detector type sensor and rotating toothed wheel with a edge up/down from each tooth. I would square up the edges and look at the time between positive going pulses which should correspond to speed of rotation. Years here too, and it does sound like fun. A simple reasonableness test should remove a lot of noise as well.

Counting pulses works too, but you would need a LONG sampling time for smoothing I would think, due to the coarseness of pulses, same speed might get 27 pulses one cycle and 28 the next etc.