Inflammable gas detector with prearranged action
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inflammable gas detector with prearranged action. 
     2. Description of the Prior Art 
     An inflammable gas detector is known from European Pat. No. 82830111.9 consisting of a detecting element in the form of a spiral with close windings and composed of a thin platinum-alloy wire inserted in a resistance bridge circuit, so that a variation of the detector resistance as a function of the density of the gas surrounding it will cause in the bridge circuit an imbalance that enables a system, which sets off an alarm and/or produces an action, to obtain a control signal. U.S. patent application Ser. No. 372,658 filed Apr. 28, 1982 and now abandoned corresponds to European Pat. No. 82830111.9. 
     Numerous tests have shown that the detecting element employed exhibits a variable sensitivity by varying the operating voltage, the level of said voltage depending upon the type of gas in which the detector is located. More specifically, it has been demonstrated that the DC supply voltage for maximum sensitivity and, therefore, for optimum response of the detector is 2 V for methane gase in a 2% concentration, and 1.6 V for propane-butane gas in a 0.8% concentration, and that the variation of the voltage measured at the terminals of the detector so supplied changing from the condition around it without gas to that with gas in the above-mentioned percentage is 0.2 V. 
     SUMMARY OF THE INVENTION 
     The major object of the present invention is to enable an inflammable gas detector of the platinum-alloy wire type, inserted in a resistance bridge circuit, to operate in a condition for maximum sensitivity. 
     Due to the above result, the present invention adopts the idea of preadjusting the voltage at the terminals of the detecting element inserted in an electrical bridge circuit to the level for maximum sensitivity depending upon the desired type of gas to be detected and, therefore, of utilizing the signals supplied by the aforementioned circuit in the presence of gas in order to bring the cathode of a diode to the threshold limit value and, at the same time, to make two diodes conductive and to activate a signal and/or control relay. 
     The advantages obtained by making use of the present invention resides essentially in the fact that the apparatus produces an optimum response for any type of gas that is to be detected, that the response is also optimal for any predetermined minimum value of the percentage of the gas to be detected, that said minimum value of the percentage of the gas to be detected can be adjusted quickly and accurately as required, and that the apparatus is very compact and very light in weight. 
    
    
     The invention will now be described in greater detail in conjunction with the accompanying drawing representing one specific embodiment of the invention. 
     BRIEF DESCRIPTION OF THE DRAWING 
     The one and only FIGURE represents the electrical diagram of an inflammable gas detector with prearranged action as taught by the invention and is designated FIG. 1. 
    
    
     DESCRIPTION OF THE BEST MODE AND PREFERRED EMBODIMENT 
     Reduced to its basic structure and with reference to the accompanying drawing, the apparatus of the invention includes: 
     a bridge circuit comprising two fixed resistors R1, R2, and two variable resistors P1, P2. Resistor P1 is capable of adjusting the voltage for maximum sensitivity of a detector R in relation to the desired type and concentration of the gas to be detected. Variable resistor or potentiometer P2 is capable of adjusting the potential at the cathode A of diode DS1. The prespecified voltage for maximum sensitivity is controlled by and supplied to terminals of detector R which is of the platinum-alloy wire type, and is inserted in a branch of said circuit. The electrical bridge circuit is capable of supplying at the terminals of detector R a prespecified voltage for maximum sensitivity. 
     an electronic circuit comprising a first diode DS1, two other diodes DS2, DS3 in series, two cascaded transistors TR1, TR2, four biasing resistors R3, R4, R5, R6, and a relay RL or similar device, the excitation of which enables one or more signal and/or control units to be activated: all said components are connected as shown in the circuit depicted in FIG. 1; the diode DS1 controls the operation of relay RL. When diode DS1 is non-conducting as a result of absence of gas to be detected because the potential at the cathode of diode DS1 is lower than the threshold limit value corresponding to the prespecified voltage for maximum sensitivity of the detector R, the transistor circuitry TR1 and TR2 are rendered inoperative and relay RL is deactivated; 
     a DC power supply circuit section stabilized with circuits of known construction and denoted with V+ in the drawing. 
     DESCRIPTION OF OPERATION 
     The operation is as follows: 
     In the rest condition, i.e., in the absence of the gas around the detector R that is to be detected, one brings as a first step, and working with the variable resistor or potentiometer P1, the prespecified voltage at the terminals of the detector R to the experimentally determined level for its maximum sensitivity, depending upon the desired type of gas to be detected, then, working with the variable resistor or potentiometer P2, the cathode potential at A is such that the diode DS1 is fixed at a voltage which is predetermined to be lower than the threshold limit value by a predetermined amount corresponding to the prespecified voltage for maximum sensitivity depending upon the desired type and concentration of the gas to be detected, so that, owing to the pressence of the diodes DS2 and DS3, and in the absence of the gas to be detected the base of the transistor TR1 is reverse-biased, thereby cutting off the transistors TR1 and TR2 and deactivating the relay RL. 
     In the presence of gas around the detector R in the concentration provided, the signals supplied by the electrical circuit bring the cathode of diode DS1 to the threshold limit value rendering the transistors TR1 and TR2 conductive. Specifically, a voltage variation occurs at the terminals of the detector R which manifests itself at point A of the circuit, and this causes the cathode of the diode DS1 to be brought to the threshold limit value: Thus, the transistor TR1 switches quickly and is rendered conductive which, in turn, renders the transistor TR2 conductive and causing the relay RL to be operative. This operating condition is maintained during the time that diode DS1 is rendered conductive in which the detector R detects a percentage of the gas provided which is higher than that provided for a prespecified level of safety because the potential at the cathode of diode DS1 is higher than the threshold limit corresponding to the prespecified voltage of maximum sensitivity of detector R. 
     Diodes DS2 and DS3 are in series with but oppositely directed from diode DS1 and therefore are capable of reverse-biassing the base of transistor TR1 so that the potential at the cathode A of diode DS1, the common series connection between diodes DS1 and DS2, is lower than the threshold limit value. 
     EXAMPLE OF OPERATION 
     Below are shown the values of the essential device components used in a specific operation: 
     R1: 4.7 Ohm 5 W 
     R2: 100 Ohm 1/4 W 
     R3: 390 Ohm 1/4 W 
     R4: 560 Ohm 1/4 W 
     R5: 6.8 KOhm 1/4 W 
     R6: 150 Ohm 1/4 W 
     P1, P2: 100 Ohm 
     TR1: BC 208 
     TR2: 2N1711 
     V+: 5 V CC. 
     INFLAMMABLE GAS DETECTOR WITH PREARRANGED ACTION 
     In order to detect the presence of an inflammable gas of prespecified type and concentration by means of a detecting element R consisting of a spiral with close windings and made of a thin platinum-alloy wire inserted in a resistance bridge circuit that operates in a condition for maximum sensitivity, a bridge circuit is utilized having the fixed resistors R1 and R2 and the potentiometers P1 and P2, and an electronic circuit with the diodes DS1, DS2, DS3, the transistors TR1, TR2, the biasing resistors R3, R4, R5, R6, and the relay RL in the circuit assembly illustrated in the drawing,in which the voltage across the terminals of the detector R is preadjusted with potentiometer P1 to the experimentally predetermined value for maximum sensitivity with respect to the desired type and concentration of the gas to be detected, and the potential at the cathode A of the diode DS1 is fixed with potentiometer P2 at a level that is lower than the threshold limit value by an experimentally determined amount in relation to the aforementioned voltage for maximum sensitivity of the detector R. In the absence of gas, the base of the transistor TR1 is reverse-biased so that the transistors TR1, TR2 and the relay RL are cut off; in the presence of gas in the predetermined concentration, the variation of the voltage across the terminals of the detector R brings the cathode A of the diode DS1 to the threshold limit value, making the transistors TR1, TR2 conductive and energizing the relay.