Simple circuits made of up the different types of logic gates – NOT, OR, AND NOR, NAND, XOR, XNOR – can be used to perform simple and useful tasks. Because the circuits are simple and the logic gates are reliable and mass produced, the circuits are reliable.
One such is the circuit used to make a safety thermostat. The circuit is shown below.
The contacts X and Y and thermister are immersed in water. Input 2 to the AND GATE is then 1. If the water is cold then the thermister has high resistance so the voltage across it is high and input 2 to the AND GATE is high. The electromagnet attracts the spring loaded switch to turn on the heater.
The temperature at which the heater turns on can be varied by changing the variable resistance R.