Solar water heater water level automatic control circuit

Working principle of the circuit: When the water level is at normal level, since the normally
closed (dynamically open) contact KA of the relay is in a closed state, the power supply passes through the resistance of the water body, and then through the wind,
Rz is VT. Provide base current to make VT1 reach saturation conduction (general water resistance
R water = 7~30 kΩ), current Ib1 is equal to the power supply voltage minus the base-emitter voltage
drop divided by the sum of R1 and R water, That is, Ib1=(12 -0.7)/(33+30)≈
0.17 mA, under the premise that the collector load resistance R3 of VT, =6.2 kfl,, VT1
has reached saturation and is turned on, and VT2 is cut off without base current . KA remains released.