Optical Liquid Level Sensors Application Note
An optical liquid level sensor uses an infra-red LED and phototransistor accurately positioned at the base of the sensor’s tip. When the tip is air, infra-red light reflects internally round the tip to the phototransistor providing good optical coupling between the two. When the sensor’s tip is immersed in liquid, the infra-red light escapes from the tip causing a change in the amount of light at the photo-transistor which makes the output change state.