| 摘要 |
A includes a first capacitance plate, a dielectric covers a first capacitance plate, second capacitance plates are formed by moisture in an area. The capacitance value, dependent upon the second plate formation, controls the frequency of signals produced by an oscillator. This frequency controls the opening and closing of a first switch. A second switch is in a circuit with the first switch and a solenoid. The closing of the second switch is dependent upon the operation of circuitry energized by an alternating voltage. The alternating voltage is modified to extend the zero voltage level period which occurs at the zero cross-over in the negative direction. When this condition occurs, a pulse is generated by the detection circuitry and this pulse causes the second switch to remain closed. When the first switch operates to indicate no need to water the area, the solenoid is energized and an associated relay is opened. This causes a valve providing, in the open state for the watering of the area, to remain closed. The solenoid becomes de-energized to close the relay when either the second switch becomes opened to indicate a failure to produce pulses at the zero crossover or the first switch becomes opened to indicate a need for watering the area. When a button is manually operated, it prevents the pulses from being produced at the zero crossover time of the alternating voltage. This causes the second switch to open, the solenoid to become de-energized and the relay to close. The valve then becomes opened to obtain a watering of the area.
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