Using an Arduino, it is easy to make a speedometer/tachometer for virtually any man-powered vehicle. I made one for my skateboard that used a small magnet to count revolutions and utilized an LCD display screen. This Instructable will show you how you can do the same step by step.
You will need a few things before we get started.
1) A man-powered vehicle (I will be using a skateboard) 2) An Arduino 3) A plastic Arduino protective box (pictured) 4) A small magnet (mine is 1/4 inch in diameter and about 0.2 inches tall) 5) A magnetic relay switch 6) A 9V battery and an adapter that connects this battery to the Arduino plug (pictured above plugged into an Arduino) 7) A small LCD screen (16 x 2 character display) 8) 10K and 47 Ohm resistors 9) Wires 10) A soldering iron and solder 11) Gorilla glue, super glue, crazy glue, or some adhesive of the sort 12) 10K Potentiometer
The first thing I did was embed the small magnet in my rubber skateboard wheel. I drilled a small hole (1/4" diameter to fit the magnet snugly) on the inside of one of my back wheels toward the edge of the wheel. I then dropped some gorilla glue in this hole and fit the magnet in it so about half of the magnet's height was imbedded in the wheel and half was sticking out of the wheel. Later this magnet will allow the magnetic relay switch to count the wheel's revolutions when the relay switch is near the spinning magnet.