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How it works: The product contains a near-infrared light-emitting diode and a photosensitive receiver. The light from the LEDs is directed into a lens on top of the sensor. When the liquid immerses the lens of the photoelectric level switch, the light is refracted into the liquid so that the receiver receives no or only a small amount of light. If there is no liquid, the light emitted by the LED is reflected directly from the lens back to the receiver, thus judging the change of liquid level and activating the external alarm or control circuit.