A bridge rectifier and 12-volt regulator powers the MOSFET sensing circuit. The unregulated output of the bridge rectifier operates the drain solenoid via switch SI. The sensor itself is built from two electrodes, one made of copper, the other of lead. In combination with the liquid trapped by the sensor, they form a miniature lead-acid cell whose output is amplified by MOSFET Ql. The maximum output produced by our prototype cell was about 50 Âµ. MOSFET Ql serves as the fourth leg of a Wheatstone bridge.
When acidity causes the sensor to generate a voltage, Ql 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.