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An input control system having a sensing circuit, a switching circuit and a source of power isolated from the sensing circuit and the switching circuit. The sensing circuit includes a sensor having a variable electrical characteristic, a detector for detecting variations in that characteristic and f

An input control system having a sensing circuit, a switching circuit and a source of power isolated from the sensing circuit and the switching circuit. The sensing circuit includes a sensor having a variable electrical characteristic, a detector for detecting variations in that characteristic and for producing a representative output, a signal producing circuit for producing a predetermined signal in response to the detector output achieving a selected value, and coupler responsive to the predetermined signal to produce a coupling output. The switching circuit which is isolated from the sensing circuit produces a switching signal in response to the coupling signal to operate a switch for connecting an electrical load to the power source.

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In a refrigerated unit, a dew point sensing system for controlling operation of electrical heating means for the refrigerated unit to preclude formation of condensation on the exposed surfaces of said refrigerated unit comprising: resistive transducer means affixed to an exposed surface of said refr

In a refrigerated unit, a dew point sensing system for controlling operation of electrical heating means for the refrigerated unit to preclude formation of condensation on the exposed surfaces of said refrigerated unit comprising: resistive transducer means affixed to an exposed surface of said refrigerated unit and having a surface temperature which varies with the temperature of said unit, said transducer means comprised of an electrically insulated member mounted on a thermally conductive member in contact with said refrigerated unit, and a plurality of spaced apart electrically conductive members supported by said electrically insulated member and exposed to the ambient air surrounding said refrigerated unit; the resistance of said transducer varying as a function of condensation formed on the surface thereof; a source of alternating current; means coupled to said source for applying an ac voltage across said resistive transducer means and for electrically isolating said resistive transducer means from said source; operational amplifier means connected to said variable resistive transducer means for producing a detection output having an amplitude representative of the maximum amplitude of the voltage across said resistive transducer means; differential amplifier means connected to receive said detection output and producing a signal having a constant amplitude in response to the amplitude of said detection output achieving a selected value, and terminating said signal in response to said detection output achieving a second value lower than said selected value light emitting diode means connected to the output of said differential amplifier for emitting light in response to said constant amplitude signal; phototransistor means electrically isolated from said light emitting diode for producing a control signal in response to light emitted from said light emitting diode; a semiconductor switch having its main electrodes connected in series between said source of electrical energy and electrical heating means and having a control electrode connected to the output of said phototransistor means for connecting said source to said electrical heater means in response to said control signal; whereby said electrical heating means in energized.