Fully automatic water heater control circuit

The working principle of the circuit: As can be seen from the figure, close the switch S and press the button SB, the contactor
KM2 is closed, and the solenoid valve receives power and water supply. When the water level reaches a certain height (reaches
the position of the resistance sensor composed of a and b), VT2 is turned on, and the relay K. When it is powered on,
its normally closed point KM1-3 is disconnected, contactor KM2 loses power, and the solenoid valve is closed; at the same time, K. 1:
Closed, the contactor KM is closed, and the heater is powered to work. When heating, K, one: closed,
the LED lights up to indicate that the heater is working. RT in the picture is
a thermistor composed of a triode ce junction (practice has proven that the performance of ce junction is better than that of be junction). Different water
temperatures can be obtained by adjusting RP. When the water temperature reaches the set temperature, VT. On, VTz off, relay K.
When power is lost, K1-2 is disconnected, contactor KM1 is powered off, and the heater stops heating; K1-1 is disconnected
, and the LED goes out, indicating that the water is also heated. When there is no water in the water tank, VT2 is cut off,
K1 does not work, so that KM1 does not work, and the electric heating tube does not heat, thus avoiding
the phenomenon of dry burning without water.