DylanJ said:
Thanks man, the accuracy I was asked for was +- 5%, and I was thinking
of using a LCD display, the displays not too much of a problem though.
I don't know how hard that would be to obtain using an all analog method,
but getting that accuracy with a micro should be easy. The first problem to
solve is how to get pulses from the prop. Another poster pointed out (from
personal experience) that a light source was not necessary. He says that
enough ambient light will be reflected from the prop to do it. At this
point, I would start experimenting with a phototransistor and an op-amp.
Something like this might be a good starting point:
http://www.coilgun.eclipse.co.uk/optical_trigger_module.html
I just quickly googled up that page, I have no idea if the circuit works or
has fast enough response time for your needs, but it is a good example of
amplifying and detecting pulses from a phototransistor. The output of the
op-amp could be fed to a LM-393 comparator to further clean up the pulses so
they can be fed into digital logic of some sort if you like. Personally
this is the approach I would take if it were my project.
Or, as I said before, you could take those pulses and use them to trigger a
one-shot timer to set the pulse width to a consitent size and then integrate
these to come up with a voltage proportionate to the pulse rate. I would
think this could be calibrated to within 5%, just.