Light detector with pulsed light source and synchronous data gating

• US 3,946,241 A
• Filed: 04/14/1975
• Issued: 03/23/1976
• Est. Priority Date: 11/26/1973
• Status: Expired due to Term

• Current Assignee: Graviner Limited
• Original Assignee: Pyrotector Incorporated
• Inventors: Malinowski, William J.
• Primary Examiner(s): Borchelt, Archie R.
• Assistant Examiner(s): Nelms, D. C.
• Application Number:
  US05/568,015
• Time in Patent Office: 344 Days
• Field of Search: 250/554, 250/221, 250/222, 250/206, 250/574, 250/214 B, 250/214 RC, 340/237 R, 340/237 S, 340/258 B
• US Class Current: 250/574
• CPC Class Codes: G01N 21/53 within a flowing fluid, e.g. smoke
• CPC Class Codes: G08B 17/107 for detecting light-scattering due to smoke

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.

Claims

1. In a detector of the type utilizing a pulsing light source and means for producing a signal pulse in response to the pulsed light under prdetermined conditions, the improvement comprising a level detector having an input connected to the output of the signal pulse producing means, said level detector producing an output signal only in response to an input signal pulse above a predetermined value, the output of the level detector being connected to an alarm actuating device, means rendering said level detector incapable of producing an output signal when the light source is de-denergized and rendering said level detector capable of producing an output signal only when the light source is emitting light.

2. A detector as set out in claim 1 in which a bi-stable switching device is connected between the level detector and the alarm actuating device, said bi-stable switching device being normally in a first condition in which the alarm actuating device is not actuated and is responsive to a pulse from the level detector to shift to a second condition in which the alarm actuating device is actuated, and means periodically returning the bi-stable switching device to the first condition.

3. A detector as set out in claim 2 in which means is provided for applying simultaneously a periodic pulse to the light source to cause it to emit light, to the level detector to render it capable of producing an output signal and to the bi-stable switching device, the periodic pulse to the bi-stable switching device being of substantially shorter duration than the periodic pulses to the light source and the level detector and being so applied to the bi-stable switching device as to insure that it is in the first condition during an initial portion of each periodic pulse to the light source and the level detector, whereby the occurrence of a signal pulse from the level detector to the bi-stable device in response to an input signal pulse will shift the bi-stable switching device to the second condition after the termination of the periodic pulse to the bi-stable device.

4. In a detector of the type utilizing a pulsing light source and means for producing a signal pulse in response to the pulsed light under predetermined conditions, the improvement comprising a level detector having an input connected to the output of the signal pulse producing means, said level detector producing an output signal only in response to an input signal pulse above a predetermined value, a flip-flop circuit having set and re-set terminals, the level detector output being connected to the set terminal of the flip-flop circuit, whereby the flip-flop output is turned on when the level detector produces an output signal, means periodically applying a signal to the re-set terminal of the flip-flop to turn off the flip-flop output, the output of the flip-flop being connected to an alarm actuatin unit, said level detector being rendered incapable of producing an output signal when the light source is de-energized and is rendered capable of producing an output signal only when the light source is emitting light.

5. A detector as set out in claim 4 in which the level detector is energized only when the light source is emitting light.

6. A detector as set out in claim 4 in which the signal lead of the level detector is connected to ground when the light source is de-energized so as to be incapable of producing an output signal to the flip-flop and said signal lead is disconnected from ground only during the time that the light source is emitting light.

7. In a detector of the type utilizing a pulsing light source and means for producing a signal pulse in response to the pulsed light under predetermined conditions, the improvement comprising a level detector having an input connected to the output of the signal pulse producing means, said level detector producing an output signal only in response to an input signal pulse above a predetermined value, a flip-flop circuit having set and re-set terminals, the level detector output being connected to the set terminal of the flip-flop circuit, whereby the flip-flop output is turned on when the level detector produces an output signal, the output of the flip-flop being connected to an alarm actuating unit, and a pulse generator for simultaneously applying a periodic pulse to the light source to cause said light source to eimit light, to the level detector to render it capable of producing an output signal, and to the re-set terminal of the flip-flop to insure that the flip-flop output is off during an initial portion of each periodic pulse to the light source and the level detector, said periodic pulse to the flip-flop being of shorter duration than the pulse to the light source, whereby an output signal pulse from the level detector will turn on the flip-flop output.

8. A smoke detector of the type utilizing a pulsing light source and means for producing signal pulses in response to the pulsed light under certain condition and for utilizing signal pulses above a predetermined magnitude to actuate an alarm, said light source being a light emitting diode, said means for producing signal pulses being a photo-voltaic device capacitor coupled to an amplifier, said amplifier having a frequency response such that no appreciable output occurs below input frequencies below 1,000 cycles and a pulse generator applying individual pulses to the light emitting diode, said pulses having frequency components greater than about 1,000 cycles, a duration of about 20 micro-seconds and a repetition rate of between about 1 to 2 seconds, said means for utilizing the signal pulses to actuate an alarm having means requiring a signal pulse from at least two consecutive pulses to actuate the alarm.