G - Physics
08
B
340/155, 340/140
G08B 17/10 (2006.01) G01N 21/53 (2006.01) G08B 17/107 (2006.01)
Patent
CA 1056931
ABSTRACT OF THE DISCLOSURE A smoke detector of the type utilizing photo-electric detection of light reflected from smoke particles, 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 repeti- tion rate, such as one pulse every two seconds, by an extremely short pulse, such as 20 microseconds. Voltage pulses generated by the photo-generative cell when smoke is present in the housing are 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 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 energiz- ing 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. In one embodi- ment of the invention, to prevent power supply voltage changes from affecting the operation of the amplifier, a capacitor is provided across the power leads to the amplifier, and means is provided between the capacitor and the power supply for electric- ally isolating the amplifier and capacitor from the transient voltages from the power supply, which are caused by the turning on of the light generating device, or from other causes. In one form of the invention said means comprises a choke coil in the line to the capacitor and a second choke coil in the line to the light generating device. In another embodiment of the invention the isolating means may be Zener diodes in place of choke coils. In a third embodiment of the invention the means comprises a switch in the power line between the capacitor and the power supply, with means being provided to open the switch when the light generating device is energized. In each embodiment, the amplifier is isolated from transient voltage changes during the time the light generating device is energized, so that the pulse of energy required to power the amplifier is provided by the capacitor.
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