Speedometer Measurement Using Reed Sensors

Introduction

In today’s motor vehicles and motorcycles the driven distance is measured through an actuating shaft with gears connected to one wheel and indicated on the dashboard. Additional electrical signals are necessary for the on board computer to present details on specific parameters.

The rotational speed (RPM’s) of a wheel is transferred to a ring-magnet with multiple poles through the actuating shaft. The ring-magnet is covered by a copper casing and has a stiff shaft with indicator needle and a coil spring mounted to it.

Depending on the speed of the vehicle the magnet rotates and induces eddy currents into the copper casing.

The shaft with the indicator needle on the coil spring moves in a linear manner indicating the actual speed of the vehicle. Accordingly, the current speed of the vehicle is indicated on the speedometer.

The multiple poles of the magnet can also be used to generate square pulses. A Reed Sensor can detect this rotation without touching the actuator. Depending on the number of magnetic poles, the Reed Switch will close and open generating pulses that can be processed by the on board computer. This data is used to control motor specific parameters such as RPM’s, ignition, fuel injection, indicate speed, and much more.