PHOTOELECTRIC COMBUSTION PRODUCTS DETECTOR WITH LOW POWER CONSUMPTION AND IMPROVED NOISE IMMUNITY

摘要

Abstract of the DisclosureA photoelectric combustion products detector forperiodically sampling the ambient air includes samplingmeans capacitively coupled to an AC source, so that thecoupling capacitor produces at the sampling means asource current 90° out of phase with the AC sourcevoltage, which is rectified to provide a supply volt-age. Sampling is controlled by a NAND gate having atits inputs a varying threshold level which is propor-tional to and in phase with the supply voltage. A rampsignal generator connected to one gate input terminalenables the gate when the ramp signal exceeds the vary-ing threshold level. The other input terminal of thegate is connected to a timing circuit comprising aZener diode, a capacitor and a discharge resistor whichproduces a short trigger pulse at or near each positivezero crossing of the AC source voltage, the coincidenceof a trigger pulse with an enabling period causing thegate to actuate the sampling means.