Abstract:
A fire detecting system can mix fire detectors, such as smoke, flame, or, thermal detectors on a common loop, or zone, with gas detectors. Signals indicative of gas can have one amplitude while those indicating fire can have a different amplitude. Duty cycles can be varied to indicate other conditions, such as trouble conditions.

Description:
FIELD 
       [0001]    The application pertains to detectors and alarm systems where different types of detectors can be readily installed on a common loop or zone. More particularly, the application pertains to such detectors and systems wherein differing signal amplitudes and durations are used to indicate the presence of different conditions. 
       BACKGROUND 
       [0002]    Installation problems exist at times where a need develops to retrofit a system connected gas detector, for example, a CO detector, into an existing alarm system. An installer may have to pull a multi-conductor cable from the installation point back to the control panel. This process can be both expensive and inconvenient. 
         [0003]    It would be useful to be able to install gas detectors, or combination gas and fire detectors on an existing zone or loop with previously installed fire detectors without needing to pull additional wires. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a block diagram of an embodiment hereof; 
           [0005]      FIG. 2A  is a graph indicating a first signaling condition; and 
           [0006]      FIG. 2B  is a different graph indicating a second signaling condition. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    While embodiments can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles hereof, as well as the best mode of practicing same. No limitation to the specific embodiment illustrated is intended. 
         [0008]    In embodiments hereof, gas detectors can be added to existing fire, wired, detection systems without having to add additional wiring to take into account different signal values associated with detected gases, or trouble indicators from the respective gas detectors. In accordance herewith, the respective gas detectors, or the gas detection portion of a new, or additional, combination gas/fire detector, will clamp, or assert, an open circuit voltage on the loop, for example 12 volts, to a lower voltage, perhaps 10 volts, which is above the expected minimum operating voltage of the fire directors, which may be on the order of 8.5 volts. 
         [0009]    Advantageously, both gas alarm signals, and trouble signals associated with gas detectors can be indicated to a respective alarm system control element, or panel, by asserting a single predetermined voltage. A singular amplitude level can be combined with varying duty cycles to indicate either the detected presence of a gas or a trouble condition. Where the selected voltage amplitude is between the loop&#39;s normal open circuit voltage and the minimum operating voltage of the associated fire detectors, the gas detector related signaling is transparent to the operation of the associated fire detectors. 
         [0010]    In one aspect hereof, additional gas or combination gas/fire detectors can be added to a system of previously installed fire detectors using the same signaling loop. Separate wiring for the gas detectors, or the gas sensing portion of combination gas/fire detectors is not needed. Visual or audible signaling devices, or sounders, coupled to the loop can be activated independently as needed by reversing the polarity of the voltage applied to the loop. 
         [0011]      FIG. 1  illustrates an embodiment  10  of a system in accordance herewith. System  10  includes a fire alarm system  12  of a conventional type to which can be coupled a two wire loop  14  which extends into a region R being monitored. Fire detectors along with gas detectors, or combined gas/fire detectors, indicated at  16   a,  and audible or visual alarm indicating signaling devices, indicated at  16   b,  can all be coupled to the loop  14 . 
         [0012]    A representative combination fire and gas detector  20  is illustrated coupled to the loop  14 . It will be understood that neither the number of detectors, such as  16   a,  or  20 , nor the number of signaling devices such as  16   b  which are coupled to the loop  14  are limitations hereof. Further, neither type of fire sensors such as smoke, heat or the like or the type of gas sensors, such as electrochemical, or semiconductor based, are limitations hereof. 
         [0013]    Representative detector  20  includes a housing  22  which encloses and carries components such as a gas sensor  24   a,  and an optional fire sensor  24   b.  It will be understood that multiple gas or fire sensors can be carried in housing  20  without departing from the spirit and scope hereof. 
         [0014]    Control circuits  26  can receive signals from the sensors  24   a, b  for local processing or analysis. Control circuits  26  can include interface circuitry  26 , which could be bidirectional. Connectors  28  coupled to the interface circuits  26   a  can couple the detector  20  to the loop  14 . 
         [0015]    The control circuits  26  can be implemented, at least in part, with a programmable processor  26   b  and associated control software  26   c  stored on non-volatile memory or storage units in the circuitry  26 . 
         [0016]    Signaling patterns associated with detector  20  are illustrated in the graphs of  FIGS. 2A ,  2 B. An open circuit voltage V 1  can be coupled to loop  14  by the alarm system  12 . The particular value of V 1  is not a limitation hereof so long as V 1  exceeds a minimum voltage Vsmk needed to activate the smoke detectors on the loop  14 . 
         [0017]    For example, where V 1  is on the order of 12 volts and the minimum smoke detector operating voltage is 8-8.5 volts, the voltage amplitude associated with gas sensing functions, Vco can be on the order of ten volts. 
         [0018]    Where a fire condition is sensed, at sensor  24   b,  or at any other smoke detector on the loop  14 , the interface  26   a,  or corresponding circuitry in the detector sensing the fire condition, clamps the voltage on the loop  14  to a value below the smoke indicating threshold at the system  12 , for example, 3 volts or less. The presence of the gas sensor  24   a  and its signaling has no effect on detection of smoke, heat, flames or the like. 
         [0019]    At detector  20  gas sensor related signaling asserts a common voltage amplitude Vco, on the loop  14 , where either a selected gas such as carbon monoxide is sensed, or the gas sensing portion of detector  20  is exhibiting at trouble condition. Different duty cycles are used to distinguish the presence of a gas trouble condition, from a gas present condition. In  FIG. 2A  a gas related trouble condition can be indicated to system  12  by pulsing the loop  14 , via interface  26   a  for a time interval less than a predetermined value. Where the duty cycle on the loop  14  is greater than zero, and less than a predetermined value, such as Dcotrb, the system  12  can detect a gas, or CO trouble condition. To assert a gas, or CO alarm, the detector  20  asserts the CO indicating voltage Vco continuously, as in  FIG. 2B . 
         [0020]    In summary, as described above and illustrates in the figures, a common amplitude value, in combination with predetermined duty cycle thresholds can be used to indicate the presence of gas, or a gas related trouble condition at a detector without impacting operation of the fire detectors present on the loop. Additional gas detectors, such as detector  30 , can thus be added to an existing alarm system using the existing loops, and installed fire detectors without needing separate loops, or cable runs for the later installed gas detectors. 
         [0021]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope hereof. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.