How does pulse inputs work?

Pulse outputs are common on meters that detect flow, such as gas meters, water meters and also electrical meters (detecting the flow of electricity).

Usually the pulse is simply a mechanical contact that closes for every turn of a sensor wheel. One pulse corresponds to one turn of the wheel. If the flow increases the wheel will turn faster, so the time between each pulse shortens.

The electronics

To detect the switch closing, the ezeio supplies a very small current through the switch.

As long as the pulse switch is open, the input voltage will be close to 5V as supplied though the resistor.

When the switch closes, the input voltage drops to close to 0V.

The ezeio input detects a pulse transition when the voltage goes from >1V to <1V.

ezeio handling of pulses

Each time the input transitions from high-to-low, the ezeio starts an internal timer. When the next high-to-low edge is detected, the timer is fed to the input value, and the counter register is increased by one.

Thus, the input value when in pulse mode reflects the time between the two most recent pulses, in milliseconds. The counter value shows the total number of pulses detected.

Gotchas

Some meters provide a voltage output instead of a switched output. To use these types of meters with the ezeio, configure the input jumper as 0-10V. This disables the pull-up resistor in the ezeio, and should allow the pulse to be detected.

Battery operated electronic meters sometimes accumulate pulses internally to conserve battery. At some interval they 'wake up' and emit all the pulses collected since the last 'wake up'. These kinds of meters cannot be used for flow sensing, but only to monitor total volume.