Circuit and PCB Designed by Phil Townshend

Updated: 11-10-05

This speedometer is based upon magents activating a reed switch and the speed of the pulses being displayed on an LED display. The unit has up to 11 LED's and can measure from 1-11 metres per second (m/s) or 2-22 m/s depending on whether you have 1 or 2 magnets on the wheel.

When first turned on each LED will light up in turn to show it is activated. The unit will display the speed while you pedal. If you stop for longer than 16 seconds, the display will repeat the light sequence to remind you it is on and functioning correctly. If the unit is idle for more than 5 minutes it will go into low power mode to save battery. You will need to switch the unit off and on again. Average current consumption is about 11mA while functioning and about 1.5mA when idle so a PP3 battery will last quite a while.

The reed switch was secured using cable ties to the front forks. The magnets were purchased from a cycle shop and were the screw on type to the spokes. When connecting the wires to the reed switch take care as the connections protruding from the reed switch are easily broken off fracturing the reed. A suggested way of connecting to the reed is shown left. Cover the reed and wires with heatshring sleeving, a couple of times over to help protect it. When securing it to the front forks take care not to put cable ties around the glass body of the reed or its connections.

Below is a drilling template for the LED's. A more accurate template for both 3mm and 5mm LED's with the circuit and PCB details.

The base was constructed from MDF but a more waterproof material should be used. The cover was attached to the base using self taping screws. The holes must be pre-drilled.

The unit runs from a PP3 9volt battery secured inside.