 The process of controlling the water level of a storage tank is demonstrated in the PLC ladder logic program. Three modes of operation are programmed in this example, manual mode, automatic mode, and off mode. In manual mode, the program controls the water level by monitoring the low-sensor switch input. In this mode, the pump motor turns off when the low-sensor switch is off and turns on when the low-sensor switch turns on. The automatic mode of operation is selected by turning the switch to auto. The program will control the discharge pump motor automatically by monitoring the high and low-sensor switch inputs. As the water level rises, the low-sensor switch turns on, the red indicator lamp turns off indicating the level is no longer low, and the internal bit is unlatched. When the water level reaches the high-sensor switch, the yellow high-level lamp turns on indicating the level is now high and an internal bit is latched, beginning the pump motor cycle. As water is discharged from the tank, the high-sensor switch and high-level lamp is turned off. Water falling below the low-sensor switch stops the discharge pump motor and turns on the low-level red lamp. The automatic cycle waits for the tank level to reach the high-sensor switch to begin the cycle again. The last mode of operation is off mode. By turning the on-off switch to the off position in man or auto mode, pump operation is disabled.