The LL105100 liquid level sensor is a voltage switch that provides a digital output indicating the presence or absence of liquid. It operates based on the principle of total internal reflection, with an LED and detector housed within a plastic dome. When no liquid is present, the LED's light is fully internally reflected to the detector. When liquid covers the dome, the change in refractive index causes some light to escape, reducing the detector signal and changing the switch state to indicate liquid presence. This sensing method is rapid and suitable for water detection. The LL Series requires external circuit protection, which is integrated in the LLE Series.