For a boat project a water sense power-on circuit was needed. The power supply is a small solar with a LiPo battery cell as back-up.
The circuit needed to have zero power consumption during stand by ( no water sensed ) and should go to stand by via a digital controlled signal from a microcontroller output pin. In the circuit below, a MOSFET transistor is used as 1 bit memory cell:
When water ( sea water !) conducts current between the WATER_SENSE and the 10VDC pins, the gate of the HEXFET is charged , which pulls Drain low , opening Q2 to provide power at PWR_OUT for the microcontroller circuit. When the microcontroller has finished the task , it pulls CLOSE low , discharging the gate of the HEXFET , which via Q2 turns off the PWR_OUT for the microcontroller.
In stand-by ( waiting for water to trig) , the circuit consumes zero power.