On our kit car, we have a speedo using an old mechanical setup (spinny magnetic disk) but as we are going to be changing gearboxes, diffs and so on, I want to turn it into an electronic one.
I basically envisage a reed switch operated by a magnet on the propshaft that feeds a signal to a circuit that will then drive a stepper motor/servo to turn the dial. Obviously it will need some form of calibration, but this can be as simple as a tweakable variable resistor combined with trial and error comparing to a GPS, I don't need a fancy device where you set diameters of wheels and it works it all out (although if its simple then great!)
So, I assume the pulses will feed a freq-voltage convertor and this will then operate the motor to turn a set amount from rest for a rising voltage, presumably some sort of feedback required there? I think i have the theory down OK, but I could do with some circuits- any on the net that people have found that I can work from?
Also, replacing our mechanical bits will involve losing our odometer... I imagine this is more complex to make so not worried immediately, but any examples of these will be handy also.
Free ride in the car to all who help! (If you live nearby....) :o)
TIA, James