Photoelectric combustion products detector with low power consumption and improved noise immunity

摘要

A photoelectric combustion products detector for periodically sampling the ambient air includes a sampling circuit capacitively coupled to an AC source, so that the coupling capacitor produces at the sampling circuit a source current 90° out of phase with the AC source voltage, which is rectified to provide a supply voltage. Sampling is controlled by a NAND gate having at its inputs a varying threshold level which is proportional to and in phase with the supply voltage. A ramp signal generator connected to one gate input terminal enables the gate when the ramp signal exceeds the varying threshold level. The other input terminal of the gate is connected to a timing circuit comprising a Zener diode, a capacitor and a discharge resistor which produces a short trigger pulse at or near each positive zero crossing of the AC source voltage, the coincidence of a trigger pulse with an enabling period causing the gate to actuate the sampling circuit.