This photoelectric counter uses a groove IR Optocoupler to detect if there is anything between the IR emiter and IR receiver. With this method, this module can detect the number of obstruction in a given time thus to detect the speed of the rotation.

When the opto sensor didn't covered , the D0 output logic LOW, the signal indicate LED light, when the opto sensor covered, the D0 output logic HIGH and the LED off, adjust the potentiometer can adjust the sensitivity of the sensor.