I've had dozens of e-mails over the years asking how to measure
the speed and I've always suggested using a broken light beam
and a counter, but I've never built one myself. I even suggested
to one person that they use a slot-car lap counter since I'd
seen on one sale at Toys 'R Us for 99 cents and she said that it
worked perfectly, but not everyone can find a good deal like
that. It just so happened that I got one of these "how do I
measure the motor speed?" e-mails on the same day my new Arduino
Diecimila microcontroller board arrived from the Make Store, so
I thought that would make a great weekend project. Here is the
result, an optical tachometer for Beakman's Electric Motor using
an IR emitter/detector pair and a Arduino board. With a few
modifications to the programming, you can use this tachometer
for measuring other things such as fan or propeller speed. Notes
are included on what to change for different applications. How
to have fun with Arduino is a good source of basics on how to
setup and use the