Differential Temperature Controller.

This circuit measures the temperature difference between 2 sensors and activates a relay when they are different. It is also possible to detect a temperature change, even when the two measured temperatures are normally different.

Dimmer / AC motor speed controller

If you want to control the light level of a room or if you want to control the speed of a drill or a fan (devices that use AC motors), this is the circuit you are looking for

Water Level Controller Circuit

This simple Water level controller circuit is useful to control the water level in a tank. It does this by turning On and Off a water pump depending on the status of the sensors.