The reed switch is commonly used as a speed sensor or position sensor. It consists of a set of contacts that open when adjacent to a magnet. In the speed sensor application, the magnet is attached to the speedometer cable and rotates with the cable. Each time one of the poles of the magnet passes the switch the contacts open and then close. A voltage is supplied to one contact on the switch and the other contact is connected to ground. Each time the points close, the voltage is pulled down to 0 volts, just like the phototransistor speed sensor.
Was this article helpful?