Chris writes –
I am the author of the original Beakman’s Electric Motor page. Over the years, I have received dozens of e-mails asking me how to measure the speed of the motor. I got one such e-mail last week on the same day my Arduino Diecimila board arrived from the Make Store, so I had an idea for my first Arduino project. I built an optical tachometer for the Beakman’s Motor using the Arduino, an IR LED/photodetector, a red LED, two resistors, and some KNex pieces. It sends the speed in RPMs back to the computer over the serial port. I wrote it up on Instructables and posted more pictures up on Flickr and in the Flickr Make pool.
Arduino-Based Optical Tachometer – Link.
Arduino Diecimila – Link.