A simple bridge rectifier and a 12-V regUlator powers the MOSFET sensing circuit. The unregUlated output of the bridge rectifier operates the drain solenoid via switch S1: The sensor itself is built from two electrodes: one made of copper, -the other oflead. In combination with the liquid trapped by the sensor, the electrodes form a miniature lead/acid cell whose output is amplified by MOSFET Ql. The maximum output produced by our prototype cell was about 50 p.A.
MOSFET Q1 serves as the fourth leg of a Wheatstone bridge. When sensed acidity causes the sensor to generate a voltage, Q1 turns on slightly, so its drain-to-source resistance decreases. That resistance variation causes an imbalance in the bridge, and that imbalance is indicated by meter Ml.