The operation of the circuit is based on the difference in the primary impedance of a transformer when its secondary is loaded and when it is open-circuit. The impedance of the primary of Tl and resistor Rl are in series with the load. The triac's gate-control voltage is developed across parallel resistors Rl and R2. When the water level is low, the probe is out of the water and SCR1 is triggered on. It conducts and imposes.a heavy load on transformer Tl's secondary winding. That load is reflected back into the primary, gating triac TR1 on, which energizes the load.
If the load is an electric value in the water-supply line, it will open and remain open until the water rises and touches the probe, which shorts SCRl's gate and cathode, thereby turning off the SCRl, which effectively open-circuits the secondary. The open-circuit conditionâ€”when reflected back to the primary windingâ€”removes the triac's trigger signal, thereby turning the water off. The load may range from a water valve, a relay controlling a pump supplying water for irrigation, or a solenoid valve controlling the water level in a garden lily pond.