secondary winding of the transformer 24 the resistance sensing electrodes 14 are connected in series with a zener diode 30 to develop a voltage which is applied to the inverting input of a voltage comparator 27 the noninverting input of the comparator 27 is impressed with a reference voltage obtained from a junction between series connected resistors 28 and 29 this reference voltage corresponds to the dryness factor a in fig2 the comparator output is coupled through a resistor 31 to a light emitting diode 33 which forms a photocoupler 32 with a phototransistor 34 the phototransistor 34 has its emitter connected in series with a diode 39 and a resistor 37 to one terminal of the mains supply and has its collector connected in series with a resistor 36 to the other terminal of the mains supply when the phototransistor 34 is illuminated by the led 33 a voltage is developed across the resistor 36 and applied to the input port of the microcomputer 35 a parallel combination of a capacitor 41 and a zener diode 42 is connected to the diode 39 to stabilize the dc voltage supplied to the phototransistor 34 microcomputer 35 and multivibrator 43 since the electrical resistance.