The Schmitt trigger, named after the American inventor Otto Schmitt, is a comparator that has hysteresis at its input. A comparator is a device that compares two inputs, and reports which input is larger at the output. Hysteresis is created by adding feedback from the output to the positive input so that the switching threshold at the input changes when the output state changes in a manner that creates a positive shift in the switching threshold when the output goes positive, and a negative shift when the output goes low. This simple circuit will introduce the basic concept of a resistor-capacitor (RC) oscillator that is based on charging and discharging of a capacitor through a resistor. Overall, the circuit has three blocks; the A/D converter, the display decoder and the LED display, and the RC square wave generator. The charging/discharging cycle of the capacitor between one third and two thirds of the power supply voltage continues indefinitely.