Abstract:
A pulsed or strobed smoke detector operable periodically in a pulsed mode. A strobe generator having minimum power consumption limits the time of power consumption by other elements of the detector to a fraction of continuously operating time. A storage circuit is also included to limit transient or start-up consumption of power.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to smoke detection and in particular to the operation and supplying of power to smoke detectors. 
     Recognition in recent years that deaths and injuries from fires, particularly in residential buildings, are far more frequently due to smoke rather than from actual flames has accelerated the development of smoke detectors of various kinds. Work has also been spurred on by the passage of laws and regulations requiring the use of smoke detectors in residences being newly constructed or renovated. 
     The most common types of smoke detectors presently in use perform the actual detection of smoke by either optical or ionization devices. The present invention is concerned with both types of detectors in several of its aspects but, as is noted below, may best be understood in connection with optical detectors and will be so described. 
     The basic principle underlying optical smoke detection is the reflection or scattering of light by smoke particles. Simply stated, the absence or presence of a certain amount of light from a source is continuously monitored by a photosensitive element. Smoke, if it appears, is permitted to pass between the light source and the photosensitive element and its effect on light transmission changes the response of the photosensitive element in one way or another to trigger an alarm. Commonly, it is the reflection of light by smoke particles to the photosensitive element which initiates the change of response. 
     In recent years, the most notable improvement in optical smoke detectors has been the substitution of light-emitting diodes for incandescent light bulbs as the light source in the detectors. Although the light-emitting diode (LED) is a reliable and durable light source, it has certain disadvantages, not the least of which is that it is an inefficient source of light. Consumption of power, which is especially important in battery-operated systems, is unfortunately high. Also, because the light output is relatively low, ultra-sensitive receiver devices and circuits are needed. 
     One other area of investigation to improve efficiency of optical smoke detectors has involved the use of pulsating light sources. In such arrangements, the use of a duty cycle which cuts the time of operation of the light without sacrificing smoke detecting capacity has indeed resulted in reduced power consumption, but much room for improvement remains. 
     SUMMARY OF THE INVENTION 
     The present invention also relies upon pulsing (or strobing) techniques to reduce power consumption and also to improve the sensitivity and reliability of the smoke detector. Basically, however, even greater advantages are derived by resorting to an indirect pulsing or strobing of the light source. Specifically, the power supply, or even more desirably, the regulator commonly used with the power supply for the light source and the associated circuitry is strobed. A further step of storing voltage between pulses further minimizes consumption of power by the smoke detecting apparatus by avoiding instantaneous start-up demands. These concepts are applicable to any type of smoke detector, including optical smoke detectors and ionization detectors as well as other lesser known detectors. 
     Accordingly, the major object of the invention is to increase the sensitivity and reliability of smoke detectors while simultaneously reducing power consumption. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the annexed drawing, a number of figures are presented as an aid in understanding the detailed description of the invention which follows. In the drawing: 
     FIG. 1 is a block schematic of a simplified form of the present invention in which the regulated output of a power supply is strobed; 
     FIG. 2 is a simplified block schematic of a detector circuit similar to that of FIG. 1; 
     FIG. 3 is a block schematic of a detector circuit in which the unregulated input to a voltage regulator is strobed; 
     FIG. 4 is a detailed diagram of the components making up the strobe generator, the gated regulator and storage circuit of the invention; 
     FIG. 5 is a block schematic of a battery-operated and supervised system incorporating the present invention; and 
     FIG. 6 is a detailed showing of the supervisory circuit of FIG. 5. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1, there is shown a regulated power supply 12 across which there is directly connected an alarm latch 14 which, when triggered, connects the power supply to the alarm in conventional fashion until the latch is reset. In parallel with the latch are a strobe generator 16 and a light-emitting diode (LED) 18 in series with the parallel combination of a light-receiving transducer 20 and an amplifier 22. The output of the transducer 20 is connected to the amplifier 22 and the output of the latter is connected to a trigger 24. The output of the trigger 24 is connected back to the latch 14. 
     In most known optical smoke detectors, the supply voltage is typically 12 or 24 volts DC. Because the LED typically has a voltage drop of about 1.5 volts DC, a series ballast resistor is commonly employed and 80% or more of the power consumed is wasted. By placing the LED in series with the associated circuitry as shown here, such waste is avoided, it being necessary only that the regulated voltage from the supply 12 less the turn-on voltage of the LED 18 divided by the circuit impedance be equal to or greater than the turn-on current of the LED 18. 
     More importantly, however, by providing a strobe generator 16 it is possible to limit the drain upon the output of the regulated power supply to brief intermittent periods. For example, these periods may be of the order of 20 milliseconds every 5 seconds and it is only during these periods that the LED 18, the transducer 20 and the amplifier 22 are operative and consuming power. Such a period is merely exemplary; the only limitations are the response time of the circuit elements, the duty cycle of the strobe generator 16 and an acceptable sampling time to ensure smoke detection. 
     In the example given, current consumption on a steady state basis might reasonably be of the order of 50 milliamps. With the strobed circuit, the current consumption becomes: 
     
         50 × 20/5 × 1000 ma = 200 μa 
    
     In FIG. 2, a simplified version of the circuit of FIG. 1 is illustrated. Here, the output of the regulated supply 12 is still applied directly to the latch 14, is strobed by the generator 16 and applied in pulsed form to the separated transmit circuit 19 and receive circuit 21. In this instance the LED 18 and the transducer 20 with their associated circuitry are comprehended in the transmit circuit 19 and the amplifier 22, the trigger 24 and their associated circuitry are comprehended in the receive circuit 21. 
     A further improvement in efficiency is obtainable by arranging the circuit elements as shown in FIG. 3. In this instance, unregulated input power is applied directly to the strobe generator 16 and the latch 14. The voltage regulator 13 is then operative only when strobed or gated into operation as are the transmit circuit 19 and the receive circuit 21. 
     All regulators, of necessity, require a certain amount of current to maintain a regulated output voltage. In a commonly used regulator, input voltage, unregulated, is applied across a series combination of a resistor and a Zener diode. Regulated voltage is then tapped across the diode from its junction with the resistor. There is thus a continuous Zener current drain, but if, as in the circuit of FIG. 2, a current of about 12 milliamps is periodically demanded by the circuits being strobed, a Zener current of even higher value must normally flow to achieve regulation. In the circuit of FIG. 3, however, the regulator 13 draws current only for a short period of the order of milliseconds every few seconds. Parenthetically, and as will be detailed hereinafter, the strobe generator itself requires very little current. 
     FIG. 4 illustrates in some detail a suitable low current pulse or strobe generator 16 for supplying pulses to a switching transistor 25. 
     DC voltage is applied to a relaxation oscillator made up of the resistors R 1  and R 2 , the capacitor C 1 , and the diode D 1 . The element P 1 , which may be a so-called programmable unijunction transistor, or other switching device has an applied reference voltage derived from the voltage divider made up of resistors R 4  and R 5 . Each time the charge on the capacitor C 1  reaches a value in excess of the breakdown voltage of the element P 1 , output pulses are produced across the resistor R 3 . As in the previously described example, assuming a 10 millisecond pulse being applied every 5 seconds to the transistor 25, current flows through a considerable impedance of the transmit and receive circuits, here lumped and shown at 27 and real values of the other elements such as the resistor 29. 
     During the periods when the transistor 25 is gated on by the strobe generator 13, a storage capacitor 31 paralleled by a Zener diode 33 discharges through the impedance represented by the circuits 27. This discharge is, however, only partial. Between pulses, when the transistor 25 is gated off, the capacitor 31 charges towards the breakdown voltage of the Zener diode 33. However, the resistor 29 is of a value sufficiently high that current drain is maintained at a low level. 
     With other values as indicated above, a supply voltage of 12 volts DC and a Zener voltage of 10 volts DC, a capacitor 31 of 100 μF and a resistor 29 of 30 kilohms will operate satisfactorily with a constant current of 100 microamps. 
     In such conditions, the capacitor 31 will discharge only to the extent of 10%, for example from 10 volts to 9 volts DC during operation and a recharge of a single volt will restore equilibrium. 
     The invention as described can indeed be used with ionization smoke detectors and other lesser known systems. Moreover, it is applicable to battery operated systems as well. A typical arrangement for battery operation is shown in FIG. 5. 
     A battery 41 has an internal resistance 43 which varies as the battery discharges even though voltage remains generally constant. There being no regulator needed, the strobe generator 13 is connected directly across the battery 41. As in previously described embodiments, the generator 13 pulses or strobes a gate 25 to cause periodic operation of the circuits 27. In this instance, though, the reservoir or storage capacitor 31 merely serves to compensate for the increase in battery resistance as the battery discharges. 
     Battery supervision may also be included to provide a warning of low battery voltage. To accomplish this, a supervisory circuit 45 is added, and it also is strobed for periodic operation to conserve current. FIG. 6 gives detail on the supervisory circuit which is simply one form of a comparison circuit. 
     As the gate 25 energizes the circuit, a voltage V s  is developed at the junction of the voltage dividing resistors 51 and 53. An adjustable reference voltage V R  is developed at the tap of the potentiometer 58 which, with resistors 55 and 57 constitutes a second voltage divider across the Zener diode 33, the total voltage being V Z . Circuit values are chosen so that with a fully charged battery V s  is greater than V R  and a high output is derived from a comparator 59, the inputs to which are V R  and V s . With decreasing battery voltage, V Z , the Zener voltage because of its series connection between a resistor 61 and the strobe generator and the nature of Zener operation tends to remain constant, and V R  accordingly also holds constant. On the other hand, V s  drops in value until it ultimately falls below V R  to reverse inputs to the comparator 59 and thereby trigger an alarm. The capacitor 63 is across the resistor 57 and a tapped portion of the potentiometer 58. This is to assure that V.sub. s reaches its stable value before V R  and prevents false actuation of the comparator 59.