The LLE series Liquid Level Sensor incorporates a photo-transistor trigger which provides a digital output that denotes the presence or absence of liquid. The mode of operation is derived from the principle of total internal reflection. An LED and photo-transistor are housed within a plastic dome at the head of the device. When no liquid is present, light from the LED is internally reflected from the dome to the photo-transistor, when liquid covers the dome, the effective refractive index at the dome-liquid boundary changes, allowing some light from the LED to escape. Thus the amount of light received by the phototransistor is reduced and the output switches, indicating the presence of liquid. This method of liquid level sensing is very fast and almost instantaneous for water.