Figure shows the water level automatic control circuit. The control circuit consists of step-down circuit, the water level measurement and control switches, bistable triggers, and so on. Buck rectifier circuit 555 provides the power supply voltage VDD=12V. Bistable mode using the 555 as RS flip-flop. BG1 and upper level probe A as a reset trigger switch; BG2 and median probe set B as the trigger switch; C is connected to the lower level probe. Using the characteristics of RS flip-flop, the control 555 of the set and reset, the relay J pull or release, to control the operation of pumping motor D, so that the water level to maintain a given between the upper and lower limits.