Thyristor type liquid level automatic control circuit 2 (withdrawable type)

It uses two anti-parallel unidirectional thyristor control methods. It is a withdrawable liquid level automatic
control circuit. The characteristic of
    this circuit is that the voltage on the detection electrode
is very low, only a few volts, which is very safe.     In the figure, the purpose of connecting a resistor Ri to the lower limit electrode
is to ensure that the relay KA can be released quickly when the water level reaches the lower limit.