Specialized detonator firing circuit
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     The present invention relates generally to condition responsive electronic firing circuits and more particularly to an electronic circuit responsive to a predetermined change in supply voltage for producing a pulse of firing energy for firing a detonator. 
     The present invention has particular application in the sterilization of mines which, in the past, has been carried out by mechanisms of a mechanical or electrochemical nature. These devices normally operate on a time basis and the mine may or may not be still active when the sterilization takes place. An active mine life equal to the life of the battery power supply associated therewith is usually desirable. Since precise battery life is impossible to predict under the wide range of operating conditions encountered, some means of sensing battery voltage continuously is necessary. The present invention is designed to accomplish this function by providing a means for sterilizing a mine at the end of the power supply life. 
     While the circuit of the present invention has particular application as a means for continuously sensing the battery voltage until it falls to a predetermined threshold level, the invention could nevertheless have application in situations where the supply voltage is manually controlled. 
     Accordingly, an object of the present invention is to provide a reliable electronic firing circuit which will insure detonator and mine firing upon the application of a predetermined threshold voltage thereto. 
     Another object is to provide a novel electronic circuit operable to fire a detonator when the power supply associated therewith falls to a value below which is insufficient to fire the detonator. 
    
    
     Other objects and features of the present invention will become more fully apparent in the following description in the sole embodiment thereof. 
    
    
     Referring to the drawing there is shown a first normally conducting switch means Q1 coupled to the power supply through the Zener diode D3. A second switch means Q2 which is normally nonconducting is connected to the output of the first switch means Q1 and has a pair of storage means C1, C2 connected thereacross. When the second switch means Q2 is biased into conduction as switch means Q1 is cut off, capacitors C1 and C2 are allowed to discharge through the detonator to provide the necessary firing energy for mine sterilization. 
     Referring to the drawing in somewhat more detail there is shown a controlled rectifier Q2 which is biased in the off position by resistors R3, R4, R5 and R6. A capacitor C3 is connected between the gate electrode and output electrode of control rectifier Q2 to provide a by-pass path for the rectifier to prevent actuation thereof by spurious signals. 
     The capacitors C1 and C2 are connected in parallel and each have one plate connected to one side of the detonator and the other plate connected to the output electrode of controlled rectifier Q2. Capacitors C1 and C2 will charge with the polarity as shown when switch S1 is closed and the 12 volts is applied thereto. This causes no adverse effect on controlled rectifier Q2 and the associated detonator so long as Q1 is conducting. Q1 is a silicon transistor and is biased normally conducting by Zener diode D3 and resistors R1 and R2. The base of Q1 is biased through the Zener diode, D3, and provides a precise voltage drop thereacross sufficient to bias Q1 cutoff once the supply voltage falls to a desired threshold value. 
     In normal operation, the 12-volt power supply is connected to the circuit through switch S1 and diode D1. Transistor Q1 immediately becomes conducting as previously described, and capacitors C1 and C2 charge to approximately the supply voltage through R5. Once capacitors C1 and C2 have charged to the supply voltage, the circuit is armed for firing. If the supply voltage is removed, becomes shorted out, or falls to a predetermined value, Q1 ceases to conduct, allowing the voltage at the midpoint of resistors R3 and R4 to become approximately equal to the supply voltage. This turns on Q2 and C1 and C2 then discharge through the detonator and through Q2. 
     As the battery supply begins to fail, its voltage level decreases. D3, however, provides a constant voltage drop thereacross and this enables Q1 to become cut off when the supply voltage becomes equal to the Zener voltage of D3. For this reason D3 should be selected so that sufficient energy remains in capacitors C1 and C2 to fire the detonator as the battery supply voltage drops. 
     The following is a table of values for the circuit parameters shown in the drawing. These were used in the design of one circuit connected as shown in the drawing and are in no way intended to limit the scope of the present invention. 
     
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Circuit Parameter      Value                                              
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R1                      15K                                               
R2                     270K                                               
R3                      1K                                                
R4                      33K                                               
R5                      10K                                               
R6                      68K                                               
C1                     100 ufd                                            
C2                     100 ufd                                            
C3                     .033 ufd                                           
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     The present invention provides a compact, reliable detonator firing circuit which incorporates a means for sterilizing a mine when its useful life has been expended. Furthermore it affords a means for sterilization in the event of mooring line failure. It does so without the use of at least three separate mechanisms which are now required to perform sterilization and flooding functions. Standard electronic components are used throughout in the design of the new and novel circuit described above. 
     The circuit of the present invention has a very low current drain which is in the vicinity of 0.67 milliamperes and this factor contributes to a long battery life. It is, however, possible to operate the present invention with much less current drain without degrading the performance thereof. 
     Although the circuit described is intended to fire a detonator, it may have a wide variety of applications anywhere a pulse of energy is utilized to perform some function in response to a given condition. The circuit shown may be readily redesigned to operate from nearly any practical power supply voltage without loss of the novel features described herein. 
     Accordingly it should be understood that the foregoing is only a description of an illustrative embodiment of the invention, the scope of which is limited only by the following appended claims.