The SST Liquid Level Sensor provides single point liquid detection via a TTL compatible push/pull output. This sensor has been built without moving or mechanical parts that can be jammed and stop working. An infra-red LED and phototransistor accurately positioned at the base of the sensing tip ensure good optical coupling between the two when the sensor is in air. When the sensing tip is immersed in liquid, the infra-red light escapes from the cone, causing a change in the amount of light present at the phototransistor, which makes the output change state.
More information is available on the website of Sparkfun.