A reflective optosensor counts revolutions as the turbine spins in an HVAC duct, and with a little calibration the rig produces good results. The limitations are obvious: duct turbulence ...

Ultrasonic proximity sensors emit and receive sound waves. The carrier signal is a high frequency, inaudible sound wave. They detect the presence of the target object in one of two configurations.

Proximity sensors are common enough in automation ... The first try at this used an OBP732 reflective transmitter and an ATtiny202 microcontroller and had three pads on the PCB for programming.

Proximity or diffuse (use the target as the "reflector") sensors incorporate the emitter and receiver packaged together. Presence is detected when any portion of the diffuse reflected signal bounces ...