Abstract:
A circuit for controlling an electro-explosive device. The circuit includes a heating element, an input circuit and a load control circuit. The load control circuit is responsive to an electrical input provided by the input circuit for energizing the heating element. The heating element selectively causes ignition of the electro-explosive device when its level of energization reaches a firing level. The load control circuit substantially limits current in the heating element when the electrical input is less than a predetermined threshold for maintaining energization of the heating element at a level less than the firing level to prevent the ignition of the electro-explosive device. In contrast, the load control circuit applies substantially all of the electrical input to the heating element when the electrical input signal is greater than the predetermined threshold. This maintains energization of the heating element at a level greater than the firing level to cause the ignition of the electro-explosive device.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to pyrotechnic squibs and, particularly, to a pyrotechnic squib having an improved bridgewire circuit that addresses both all-fire and no-fire requirements. 
     In an electrically controlled explosive system, including a pyrotechnic system, an electro-explosive device translates an electrical signal into a pyrotechnic signal for selectively beginning the detonation of an explosive material. Depending on the particular industry in which they are used, such devices are referred to by various names including squibs, initiators, ignitors and electric matches. Moreover, the pyrotechnic signals provided by the devices may take several forms (e.g., gas pressure, a flame front or a shock wave) depending on the particular use. As used herein, the term “squibs” refers to electro-explosive devices collectively. 
     A conventional squib includes a bridgewire that heats up in response to an electrical current. In turn, the heat generated by the bridgewire initiates detonation. The bridgewire is a resistive component, such as a wire or filament, coated or otherwise in contact with a flammable or explosive composition. This pyrotechnic composition is typically the first in a sequence of compositions of decreasing sensitivity, increasing mass and increasing energy (i.e., a pyrotechnic or explosive train). Typically, the bridgewire components are enclosed in a metal, plastic, or paper housing for maintaining the proper juxtaposition of the components. The housing also protects the pyrotechnic composition from humidity and other environmental effects. A squib operates when the current heats the bridgewire until it reaches a temperature high enough to start a chemical reaction (e.g., burning or exploding) in the first composition of the pyrotechnic train. It is to be understood that the bridgewire need not be in the actual form of a wire and may be a metal, an alloy (e.g., nichrome or tungsten) or another conducting material (e.g., semiconductor). 
     One shortcoming of presently available squibs is their inability to simultaneously meet fairly precise all-fire and no-fire specifications. All-fire requirements specify a minimum current and duration at which all squibs of a particular design are expected to fire (i.e., ignite to begin detonation). On the other hand, no-fire requirements specify a maximum current and duration that can be applied to the particular squibs without causing them to activate. A squib is needed that is sensitive enough to meet the former all-fire requirements but insensitive enough to meet the latter no-fire requirements. The problem is complicated by the fact that the response of a conventional squib varies with temperature, pressure, acceleration and other environmental factors. Presently available squibs can be, for example, sufficiently sensitive to meet all-fire specifications at a minimum required operating temperature but too sensitive to meet no-fire specifications at a maximum operating temperature. 
     Squibs are useful in ignitors, pin pullers, pin pushers, wire cutters, exploding nuts, explosive bolts, detonators, explosive bolts, rocket motors, gas generators, thermal batteries, signal flares, safe-and-arm apparatus, pressure cartridges, pyro switches, pyro valves, bellows actuators, piston actuators, perforators, air bag inflators, seat belt tensioners, and the like. 
     SUMMARY OF THE INVENTION 
     The invention meets the above needs and overcomes the deficiencies of the prior art by providing a bridgewire circuit that improves the all-fire/no-fire characteristics of a squib. Among the objects and features of the present invention may be noted the provision of an improved bridgewire circuit that limits the current through the bridgewire to a relatively small fraction of the squib input current when the input current is below a desired firing current level and passes substantially all of the input current through the bridgewire when it is above the desired firing current level; the provision of such a bridgewire circuit that determines whether the desired firing current level has been reached; the provision of such a bridgewire circuit that permits squibs to have more precise no-fire and all-fire characteristics over a wider temperature range than conventional squibs; the provision of such a bridgewire circuit that permits squibs to have improved speed and reliability; the provision of such a bridgewire circuit that permits squibs to have improved performance at low temperatures; the provision of such a bridgewire circuit that is compatible with conventional squib firing circuitry; the provision of such a bridgewire circuit that permits continuity testing of the bridgewire; and the provision of such a bridgewire circuit that is economically feasible and commercially practical. 
     Briefly described, an electrical system embodying aspects of the invention is for use with an electro-explosive device. The system includes a heating element and an input circuit supplied by a power supply for providing an electrical input to the system. The system also includes a load control circuit receiving and responsive to the electrical input for energizing the heating element. The heating element selectively causes ignition of the electro-explosive device when its level of energization reaches a firing level. The load control circuit substantially limits current in the heating element when the electrical input is less than a predetermined threshold. This maintains energization of the heating element at a level less than the firing level to prevent the ignition of the electro-explosive device. In contrast, the load control circuit applies substantially all of the electrical input to the heating element when the electrical input is greater than the predetermined threshold. This maintains energization of the heating element at a level greater than the firing level to cause the ignition of the electro-explosive device. 
     Another form of the invention is directed to a component of an electro-explosive system. The component includes an explosive material and a heating element for causing ignition of the explosive material. An input circuit supplied by a power supply provides an electrical input to the component. The component also includes a load control circuit receiving and responsive to the electrical input for energizing the heating element. The heating element selectively causes ignition of the explosive material when its level of energization reaches a firing level. The load control circuit substantially limits current in the heating element when the electrical input is less than a predetermined threshold. This maintains energization of the heating element at a level less than the firing level to prevent the ignition of the explosive material. In contrast, the load control circuit applies substantially all of the electrical input to the heating element when the electrical input is greater than the predetermined threshold. This maintains energization of the heating element at a level greater than the firing level to cause the ignition of the explosive material. Further, the component includes a housing for the explosive material, heating element, input circuit and load control circuit. 
     In yet another form of the invention, an electrical system is for use with an electro-explosive device. The system includes an input circuit for providing an electrical input to the system in response to a signal supplied by a power supply. A semiconductor device is connected to the input circuit for causing ignition of the electro-explosive device. The semiconductor device heats in response to the electrical input for selectively causing the ignition of the electro-explosive device when the electrical input applied to the device exceeds a firing level. The system also includes a load control circuit, which includes the semiconductor device, connected to the input circuit for controlling the electrical input. 
     Yet another form of the invention is directed to a method of testing continuity of a bridgewire in an electro-explosive device. The method includes the steps of connecting the bridgewire to a positive rail and a negative rail for providing an electrical input thereto and defining a forward polarity of the electrical input applied to the bridgewire with respect to the positive and negative rails. The method also includes inserting a diode in the negative rail electrically in series with the bridgewire. The diode has its cathode connected to the bridgewire. The method further includes the steps of applying a reverse polarity current to the bridgewire via the diode and measuring a voltage across the positive and negative rails to determine continuity in the bridgewire. 
     Alternatively, the invention may comprise various other methods and systems. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a bridgewire circuit according to a preferred embodiment of the invention. 
     FIG. 2 is a schematic diagram of the bridgewire circuit of FIG. 1 having additional conditioning circuitry. 
     FIG. 3 is a schematic diagram of a bridgewire circuit according to another preferred embodiment of the invention. 
     FIG. 4 is a cross-sectional view of a pyrotechnic squib according to a preferred embodiment of the invention. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 illustrates a squib bridgewire circuit  10  for use in firing an explosive or pyrotechnic train to detonate an explosive. The bridgewire circuit  10  includes a bridgewire  12  that heats up when energized by an electrical current. The bridgewire  12  constitutes a heating element, such as a resistive wire or filament, coated or otherwise in contact with the first flammable or explosive composition of the squib&#39;s explosive train (see FIG.  4 ). In turn, the heat generated by sufficient current in bridgewire  12  initiates detonation. Advantageously, bridgewire circuit  10  is compatible with conventional squib firing circuitry, which has a power supply for supplying an electrical input to bridgewire circuit  10  via an input circuit of positive and negative rails  14  and  16 , respectively. 
     In addition to bridgewire  12 , one preferred embodiment of the electronic bridgewire circuit  10  includes a first semiconductor device such as a transistor  20 , a second semiconductor device such as a silicon controlled rectifier (SCR)  22  and an electrical sense resistor  24 . As shown in FIG. 1, the collector and the base of transistor  20  are connected across bridgewire  12 . These connections define nodes  28  and  30 , respectively. The resistor  24  is in the negative rail  16  and connected electrically in series with the emitter of transistor  20 . As shown, resistor  24  is connected between a node  32  at the emitter of transistor  20  and a node  34  at its opposite end. As described below, the value of resistor  24  determines the firing threshold of the squib conditioned by bridgewire circuit  10 . 
     According to the invention, the SCR  22  cooperates with transistor  20  and resistor  24  to selectively route current to bridgewire  12 . In the illustrated embodiment, the anode of SCR  22  is connected to the base of transistor  20  at node  30 ; the cathode of SCR  22  is connected to the negative rail  16  at node  34 ; and the gate of SCR  22  is connected to the emitter of transistor  20  at node  32 . The voltage across resistor  24  is imposed on the gate-cathode junction of SCR  22  and, thus, controls conduction in SCR  22 . 
     In operation, bridgewire circuit  10  receives a signal from an external firing system in the form of an electrical input current I IN . If the input current I IN  is below a predetermined threshold, SCR  22  remains open and the current passes through two parallel paths, one being bridgewire  12  and the base-emitter junction of transistor  20  and the other being the collector-emitter junction of transistor  20 . By the operation of transistor  20 , the current in the base-emitter junction and, thus, the operating current in bridgewire  12 , is limited to a fraction of the total current (i.e., the reciprocal of the current gain h FE  of transistor  20 ). The balance of the input current I IN  bypasses bridgewire  12  through the collector-emitter junction of transistor  20 . In particular, transistor  20  conducts but SCR  22  does not when I IN  is less than the threshold. Thus,          I   E     =         I   C     +     I   B       =     I   IN                 I   C     =       h   FE          I   B                       I   IN     =         h   FE          I   B       +     I   B                   =       I   B          (     1   +     h   FE       )                       I   B     =         I   IN       (     1   +     h   FE       )       ≈       I   IN       h   FE                                
     If I B  is approximately equal to I IN /h FE , then the collector-emitter junction of transistor  20  conducts a relatively large amount of the input current I IN . This effectively bypasses bridgewire  12 . For a typical silicon bipolar npn transistor, h FE  is in a range of 10 to 100 and, thus, the current fraction (i.e., the operating current) actually heating bridgewire  12  is approximately {fraction (1/10+L )} to {fraction (1/100+L )} of the total input current I IN  to the bridgewire circuit  10 . The small fraction of current provides the relatively high margin of protection of bridgewire  12  under no-fire conditions. Since SCR  22  is not conducting at this time, substantially all of the current I IN  passes through resistor  24 , which is in series with the two parallel circuit paths described above. 
     In the illustrated embodiment, the emitter current I E  generates a voltage V R  across resistor  24 . When SCR  22  is open, substantially all of the input current I IN  flows through resistor  24  and, thus, I E =I IN  and V R =I IN R. The voltage V R  is imposed on the gate-cathode junction of SCR  22  and, thus, controls conduction in SCR  22 . According to the invention, the value R of resistor  24  is selected to keep the voltage V R  below the gate threshold of SCR  22  so long as I IN  is less than the desired firing threshold. When the input current I IN  reaches the threshold, however, the voltage V R  developed across resistor  24  exceeds the gate threshold of SCR  22 , which turns on SCR  22 . The anode-cathode junction current latches SCR  22  in its conducting state and essentially robs the base drive of transistor  20 . In other words, I B  is approximately zero and the collector-emitter junction of transistor  20  reverts a high impedance condition. At this point, bridge circuit  10  connects bridgewire  12  directly to the positive and negative rails  14 ,  16  of the input current source. Since SCR  22  is latched on, whether it robs itself of gate drive does not determine its operation. When SCR  22  is conducting, substantially all of the input current I IN  passes through bridgewire  12 . The bridgewire current exceeds the firing level necessary for heating bridgewire  12  to a temperature sufficient to ignite or initiate the first pyrotechnic composition in the train for firing the squib. Thus, transistor  20 , SCR  22  and resistor  24  constitute a load control circuit. Although illustrated as a bipolar npn transistor, it is to be understood that transistor  20  may be embodied by another type of transistor (e.g., a field effect transistor having a gate, drain and source). 
     As described above, the squib bridgewire circuit  10  operates with precision over a wider range of temperatures than conventional squibs. The precise threshold of SCR  22  is a function of temperature but it is possible to compensate for the change in the threshold of SCR  22  by fabricating resistor  24  from a material or sub-circuit having a negative coefficient of resistance with temperature. Advantageously, this keeps the firing current threshold of bridgewire circuit  10  within a relatively narrow range over temperature and further improves the performance of circuit  10  with respect to temperature. 
     As understood by one skilled in the art, a pair of transistors may be used instead of SCR  22 . This may simplify implementing the bridgewire circuit  10  as an integrated circuit (IC). In a discrete application, it is also possible to use SCR  22  itself as the electrical heater element and eliminate the resistive filament or wire, bridgewire  12 . 
     Referring further to FIG. 1, the bridge circuit  10  of the improved squib also provides simple and accurate continuity testing of bridgewire  12  with an external continuity verification circuit. If bridgewire  12  fails and becomes an open circuit, no current is present at the base-emitter junction of transistor  20 . This condition causes transistor  20  to turn off, which presents an open circuit across nodes  28  and  32  (i.e., between positive rail  14  and negative rail  16 ). Thus, bridge circuit  10  permits detection of an open circuit bridgewire  12  by inputting a continuity testing current via rails  14 ,  16  to detect an open circuit condition at the input terminals of circuit  10 . According to the present invention, bridge circuit  10  also permits use with different continuity testing systems. In this regard, when transistor  20  is saturated, the combined voltage drop across transistor  20  and resistor  24  does not have exactly the same response as a purely resistive bridgewire. Nonetheless, bridgewire circuit  10  may be tailored so that the voltage response is within the design specifications of conventional continuity testing systems. 
     An alternative testing method may be implemented by inserting a diode  36  between nodes  30  and  34 . In particular, the cathode of diode  36  is connected to the base of transistor  20  and the anode of diode  36  is connected to the cathode of SCR  22 . With the diode  36  added, bridgewire  12  may be tested through diode  36  by applying a reverse polarity test current to the input terminals of circuit  10 . The reverse polarity condition turns off the current-bypassing function of transistor  20  but turns on the added diode  36 . This allows test current to flow only through bridgewire  12 . Preferably, the former method of continuity testing described above is employed when the no-fire specifications of the squib must be met independently of polarity. 
     The present invention permits yet another method of testing bridgewire  12 . In this alternative embodiment, an input pulse of current triggers SCR  22 . Although the pulse has an amplitude sufficient to turn on SCR  22 , its length is short enough in duration to avoid excessive heating of bridgewire  12 . After the pulse, a steady-state, low-level input current maintains SCR  22  in the conducting state without excessive heating of bridgewire  12 . In this test condition, substantially all the test current passes through bridgewire  12  and the voltage drop across SCR  22  is largely independent of slight variations in current. Therefore, slight variations in the test current induce variations in the terminal voltage of circuit  10  substantially proportional to the resistance of bridgewire  12 . 
     FIG. 2 illustrates an added voltage divider circuit  38  for the squib bridgewire circuit  10 . When circuit  10  is in the firing state, the anode-cathode saturation voltage of SCR  22  minus the voltage V R  across resistor  24  appears on the base-emitter junction of transistor  20 . Since the saturation voltage of SCR  22  may be higher than the base threshold of transistor  20 , the resistive voltage divider  38 , including resistors  40  and  42  connected to the base of transistor  20 , ensures that transistor  20  turns more completely off when SCR  22  turns on. Resistive divider  38  also facilitates tailoring of the continuity-test response of the improved squib. An alternative embodiment includes a diode junction inserted in series with the base of transistor  20 . Either embodiment serves to increase the fraction of input current that is impressed upon bridgewire  12  to substantially the full input current I IN . 
     Silicon controlled rectifiers such as SCR  22  are sensitive to firing from a high rate of change of the anode/cathode voltage. This defect might allow the improved squib to fire at currents below the desired firing threshold when the input voltage increases rapidly. This problem is minimized by selecting a small area silicon controlled rectifier die in the squib. Bypassing rapidly changing currents around the squib by the addition of a capacitor  46  across the input also provides protection. The capacitor  46  also improves the immunity of circuit  10  to radio frequency (RF) signals. Adding an inductor (not shown) in series with the input leads, preferably before capacitor  46 , provides further protection from rapidly changing currents and RF signals. Although the individual application will dictate the best choice for the inductor, in most cases bulk ferrite material surrounding the input lead wires performs satisfactorily. For maximum RF immunity, the entire squib, including both electronic and pyrotechnic components may be enclosed in a magnetically and electrically conductive housing and the lead wires brought in through a filter comprising feed-through capacitors and ferrite inductors. Such RF protection measures may also be applied to the alternative embodiment described below with respect to FIG.  3 . It is to understood by those skilled in the art that additional passive electrical elements may be added to reduce the sensitivity of circuit  10  to high frequency radiated signals or to fast rise-times on the input. 
     FIG. 3 illustrates a squib bridgewire circuit  50  according to another preferred embodiment of the invention. The bridgewire circuit  50  is also compatible with conventional squib firing circuitry, which provides the input current via rails  14  and  16 , for heating bridgewire  12 . In addition to bridgewire  12 , electronic bridgewire circuit  50  includes first, second and third transistors  52 ,  54  and  56 , respectively, and first, second and third electrical resistors  60 ,  62  and  64 , respectively. As shown in FIG. 3, the base of transistor  52  is connected to one end of bridgewire  12 . The other end of bridgewire  12  is connected to positive rail  14  and to the collector of transistor  54  at a node  68 . The resistor  60  is in the negative rail  16  and connected electrically in series with the emitter of transistor  52 . As shown, resistor  60  is connected between a node  70  at the emitter of transistor  52  and a node  72  at its opposite end. As with resistor  24 , the value of resistor  60  determines the firing threshold of the squib conditioned by bridgewire circuit  50 . The emitter of transistor  54  is connected to the collector of transistor  52  via the resistor  62 . In other words, when transistors  52  and  54  are on, they form a conduction path between the positive rail  14  at node  68  and the negative rail  16  at node  70 . With respect to the transistor  56 , its base-emitter junction is connected across resistor  60  at nodes  70  and  72  and its collector is connected to the base of transistor  54  at a node  76 . The resistor  64  is connected to the positive rail  14  at a node  78  and to the base of transistor  54  at the node  76 . 
     The transistor  52  operates in a manner similar to transistor  20  of FIGS. 1 and 2 for bypassing the input current from bridgewire  12 . As described above, switching on SCR  22  robs transistor  20  of base current for turning it off. In this embodiment, however, switching off transistor  54  in the collector circuit of transistor  52  causes transistor  52  to turn off. In operation, bridgewire circuit  50  receives the electrical input current I IN  from an external firing system. If the input current I IN  is below the predetermined threshold, transistor  56  remains open and the current passes substantially through two parallel paths. In this instance, the first parallel path is bridgewire  12  and the base-emitter junction of transistor  52  and the second is the collector-emitter junctions of transistors  54  and  52  between nodes  68  and  70 . By the operation of transistor  52 , the current in the base-emitter junction and, thus, the current in bridgewire  12 , is limited to approximately I IN /h FE . Therefore, the collector-emitter junction of transistor  52  conducts a relatively large amount of the input current I IN  to effectively bypass bridgewire  12 . 
     Since transistor  56  is not conducting at this time, substantially all of the current I IN  passes through the sense resistor  60 . In FIG. 3, the emitter current I E  of transistor  52  generates a voltage V R  across resistor  60 . When transistor  56  is open, substantially all of the input current I IN  flows through resistor  60  and, thus, I E =I IN  and V R =I IN R. The voltage V R  is imposed on the base-emitter junction of transistor  56  and, thus, controls conduction in transistor  56 . According to the invention, the value R of resistor  60  is selected to keep the voltage V R  below the base threshold of transistor  56  so long as I IN  is less than the desired firing threshold. When the input current I IN  reaches the desired firing threshold, however, the voltage V R  developed across resistor  60  exceeds the base threshold, which turns on transistor  56 . The conducting state of transistor  56  essentially robs the base drive of transistor  54 . In other words, I B  for transistor  54  is approximately zero and the collector-emitter junction of transistor  54  reverts a high impedance condition. At this point, bridge circuit  50  connects bridgewire  12  directly to the positive and negative rails  14 ,  16  of the input current source. When transistor  54  turns off, substantially all of the input current I IN  passes through bridgewire  12  and heats it to a temperature sufficient to ignite or initiate the first pyrotechnic composition in the train for firing the squib. 
     To ensure reliable operation, transistor  52  is able to pass the full input current through its base-emitter junction without opening the circuit. Since squibs are single-use devices, it is not necessary for transistor  52  to handle the relatively large base current without failure, only to do so without failing as an open circuit. As is well known in the art, the most common failure mode of a transistor is as a closed circuit so the performance of transistor  52  is satisfactory even in failure. Further, the sense resistor  60  remains in series with bridgewire  12 , even after circuit  50  fires. This is necessary to keep transistor  56  turned on and, therefore, transistor  54  turned off, when circuit  50  is in the firing state. 
     In a preferred embodiment of the invention, resistor  62  absorbs excess input power instead of transistor  52  performing this function. This allows transistor  52  to be embodied by a less powerful device and provides the possibility of using the transistor chip itself (i.e., transistor  52 ) as bridgewire  12 . Transistor  52  will heat with power approximately equal to 0.6 volts times the current when transistor  54  in the collector circuit of transistor  52  opens. This is possible, even in an IC implementation, as long as the bypassed-current absorbing resistor  62  is external to the IC. 
     Preferably, the electronic bridgewire circuit  10  or  50  of the present invention is implemented as a hybrid including discrete transistor and SCR chips and thick-film resistances assembled on a common substrate. It is further contemplated to implement the signal conditioning circuitry as a monolithic IC. Due to the nature of integrated circuit manufacturing, the embodiment of FIG. 3 provides further advantages for implementation as an IC. The threshold programming resistance can be external to the IC implementation of the squib conditioning circuit components to allow a single type of IC to be employed in a variety of applications. As well as tailoring the all-fire/no-fire response of the squib, electronics embedded in the squib can be used to provide other functions currently provided with pyrotechnic technology. For instance, a time delay may be precisely and reliably implemented by electronic means such as an RC network  82  added to the base circuit of transistor  54  in FIG.  3 . 
     The bridgewire circuits  10 ,  50  are preferably built into the headers and/or casings of the squibs themselves. For high-reliability applications, the respective bridgewire circuit  10  or  50  may be isolated from the pyrotechnic train by an hermetic glass seal. FIG. 4 illustrates an exemplary squib  84  including advantageous features of the present invention. As shown, the squib  84  has a housing or casing  86  for enclosing a pyrotechnic charge  90  (i.e., the first composition in the explosive train). A printed wiring or circuit board  92  carrying the conditioning circuitry of bridgewire circuit  10  or  50  may be used as the bulkhead that closes the charge cavity of casing  86  for lower cost applications of the present invention. In this embodiment of the printed wiring board  92  being used as a bulkhead, the electronic component serving as bridgewire  12 , whether resistor or semiconductor device, is mounted on the charge side of printed wiring board  92  and in intimate contact with or even coated with the first composition of the explosive train (i.e., the pyrotechnic charge  90 ). Moreover, the electronic components that dissipate the bypassed current in the no-fire state (e.g., transistors  20  or  52  and/or resistor  62 ) are preferably located on the side of printed wiring board  92  that is opposite charge  90 . Printed wiring board  92  is, for example, an epoxy/glass laminate or other insulative material to further protect charge  90  from initiation under no-fire conditions. A pair of lead wires  94  project from a potting compound  98  (e.g., epoxy) which seals the cavity of squib  84 . The lead wires  94  define rails  14 ,  16  of the respective bridgewire circuit  10 ,  50  and provide connections to the input power source. If no-fire tests are of a relatively long duration, it may also be advantageous to ensure that the potting compound  98  have low thermal resistance so that it conducts the dissipated heat into the casing  86 . 
     It is to understood by those skilled in the art that other embodiments of the present invention are contemplated with an understanding of the above-described principals of bypassing a portion of the current to bridgewire  12  with a transistor and either shorting the transistor&#39;s base or opening its collector to fire the squib. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.