Pulse Counting Type

Speedometer pulse counting — Pulse counting type application

When an application requires the volume of a fluid, or a distance, or the RPM of a machine tool measured, such an application can be classified as a pulse counting application.

The basic principle of pulse counting is to use a reed switch or a reed sensor on a fixed surface, with a multi-pole ring magnet attached or embedded to the center of the wheel or counter. The best practice is to use ring magnets with two poles for high speed applications and to use ring magnets with 4 or more poles for low speed applications, for better accuracy. When the sensing circuit is low in the range of milli-amperes, the basic reed switch does the job for up to a billion operations.

Reed Switches works so well in such applications as they have a very high operating frequency. The Reed Switches used generate a square wave that can then be translated to the actual consumption of fluid, or energy, or the measurement of wheel speeds. As the wheel rotates, the multi-pole magnet’s alternating North and South poles turn the reed switch on and off continuously and this generates the square wave pulse. This pulse output is used by control circuits to display the actual measurement. The translation of number of pulses to the actual unit of measurement is achieved initially, by conducting various trials. More information regarding magnet actuation of reed switches is available here.

For larger wheels, where the diameter of the magnet is more and the pole moves away far enough to open the reed switch, any standard reed switch with a differential of 40-50% can be used. But for very tiny wheels in water and gas meters, it is important the reed switches with a close differential or 70% minimum differential are used. This is to ensure that the reed switch opens immediately as the magnetic pole moves away, so that the reed switch does not remain closed at any point.

Reed Switches are hermetically sealed, they are immune to dust and moisture, and can safely be used in environments with extreme dirt and moisture. Pulse counting for applications can also be achieved by using active devices like Hall effect sensors, but Reed Switches and Reed Sensors, are passive, have more advantage as no power is consumed for operation, and have no leakage current across the contacts, that need to be compensated for. Some of the pulse counting applications are listed below.