Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1975-04-14
1976-03-23
Borchelt, Archie R.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250221, 340237S, 250214R, G01N 2126
Patent
active
039462412
ABSTRACT:
A detector of the type utilizing photo-electric detection of reflected light which is almost completely immune to false alarms from changing ambient light and random electrical noise, with a sensitivity that is independent of ambient light, with a power consumption low enough to permit battery operation for a period of over 12 months. The light source is a light emitting diode which is pulsed at a low repetition rate, such as one pulse every two seconds, by an extremely short pulse, such as 20 microseconds. Voltage pulses generated when the reflected pulsed light is received by the photo-generative cell is amplified and applied to a level detector, the output of which is applied to the "set" terminal of a flip-flop circuit. The amplifier is on continuously, however the level detector is pulsed to the on condition simultaneously with the on pulse to the light emitting diode, and for the same period of time. Simultaneously with the application of the pulse to the light emitting diode and the level detector, a shorter pulse is applied to a bi-stable switching device such as to the `re-set` terminal of the flip-flop circuit. The output of the flip-flop circuit may be applied through an integrator to an alarm energizing switch. The integrator has a time constant that is longer than the pulse time, so that more than a single pulse from the flip-flop must be applied thereto to activate the alarm energizing switch. The photo-voltaic cell is capacitor coupled to the amplifier, so that constant or changing light, having a rate of change below that to which the amplifier responds, cannot affect the amplifier to cause a false alarm. Since the level detector is on only about 1/100,000 of the total time, a false alarm can be caused only by an extremely fast change in ambient light or a random noise pulse, that occurs at the exact instant the level detector is on, in two consecutive pulse times.
REFERENCES:
patent: 3534351 (1970-10-01), Harnden
patent: 3563661 (1971-02-01), Charlson
patent: 3710365 (1973-01-01), Barnes
patent: 3719938 (1973-03-01), Perlman
patent: 3727207 (1973-04-01), Missio
patent: 3749918 (1973-07-01), Jones
Borchelt Archie R.
Nelms D. C.
Pyrotector, Incorporated
Ross Robert E.
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