Product information "Gravity: Non-contact Liquid Level Switch"
This non-contact liquid level switch can be triggered when the liquid level fall below the sensor attached to the outside surface of a non-metal container.
Unlike traditional liquid level sensors, it does not need to be immersed into liquid. So, in this way, liquid will not be contaminated and the life of the sensor will not be reduced so fast because of the contact between liquid and sensors.
The sensor is very sensitive and stable, and the output current is upto 100mA which can directly drive a relay.
You can use this switch to monitor the condition of your water machine or make a water-drinking reminder by attaching it to the surface of your cup.