Automatic irrigation system or soil humidity sensor circuit. Schematic is taken from http://s.eeweb.com/members/samir_guessmi/answers/1334155085-Soil_Sensor.png
Lower left POT simulates sensor's contacts. The light in the right side of scheme replaces the actuator such as mini-pump. Mini-pump is automatically activated (LIGHT ON) when the soil moisture is reduced to a predetermined value and deactivates (LIGHT OFF) once moisture level reaches its target value.
In the heart of circuit is Operational Amplifier. Easy but handy
