16 of terminal block 10 about 120 v a c or 24 v a c volts alternating current and outputs 15 v a c on its secondary output diode 30 serves as a rectifier to allow only the positive half of the a c voltage to charge polarized filter capacitor 32 capacitor 34 filters higher frequency voltage spikes from the transformer 28 and filter capacitor 32 in a preferred embodiment capacitors 32 and 34 have capacitances of 470 uf microfarads and 0 1 uf respectively zener diode 36 acts as a voltage regulator to prevent the d c direct current output voltage across terminals 38 and 40 from rising above 14 volts d c probe 41 is positioned in the boiler and is coupled to terminal 42 when water contacts probe 41 the water acts as a resistor between terminal 42 and chassis ground of transformer 28 thereby creating an a c voltage across the boiler water between probe 41 and chassis ground diode 44 is forward biased only on the positive peak of the a c voltage resistors 46 and 48 act as a bridge to charge capacitor 50 when the resistance of the water is less than about 4 000 ohms when capacitor 50 is charged it provides a logic level 34 high 34 at terminal 52 of transistor.