This simple circuit structure, drawing easy, reliable, easy to operate.
How it works: Suppose there is no water inside the tank, you need warm water, as long as K1 is closed (see below), the green indicator light, the electromagnetic valve by the J1-2 normally closed inlet valve is energized and open, to be rose water tank when the plane S1, BG1, BG2 was a composite tube and drive forward bias relay J1-1 J1 action will pull the same time, J1-2 action, power off the water valve, AC contactor J2 was electric pull, electric water heating pipe R1, red light. Bath process, the water level falling, the water level below the surface S2, the composite pipe failure J1 forward bias to the release, the electromagnetic valve is energized to open the valve water, electric heater failure protection, the red indicator light turns off.

Sometimes the surface needs to be located in the plane between the S1 and S2 plane, as long as the water level reached the plane, click to complete the function of K2, will encounter when the plane has been located in the surface plane S1 and S2, between And then when you need to raise the water level of the water, as long as the linked K1 twice. When the water temperature to meet the requirements, you can stop off K1, but the bath water temperature is generally the level of water from the tank with the regulator valve and water valve, so the tank water temperature not controlled circuit.
  Components Selection: K1, K2, respectively, the pull switch and use an ordinary doorbell button, AC contactor, fuse, electric tube size selection based on the use of power, two water wand best selection of stainless steel, the plastic can also be copper line sets of head-end 5 cm strip of leather for, J1 for the two contacts of JQX-4 12V voltage relays. Manufacture of glass tank with water level indicator is best, and ask around in the box wrapped with asbestos materials, from insulation effect. Solenoid valve is not used, could be right in the picture into the water left over for alarm, as long as the picture right in the A, B, C access to the corresponding A ', B', C '(not found) can be.