Ignition circuit for explosive devices and the like

An ignition circuit for explosives with a monostable multi-vibrator controlling a transistor in circuit with a battery and the primary winding of a switching transformer, with the secondary winding directly connected to spaced electrodes at the explosive.

BACKGROUND OF THE INVENTION 
This invention relates to electrical circuits for generating sparks for 
firing explosives and the like and, in particular, to a new and improved 
monostable multi-vibrator circuit for generating an electric spark between 
spaced electrodes for igniting a solid explosive, such as would be used in 
a firearm for propelling a projectile. 
Battery powered oscillator circuits have been used in the past for 
producing electric arcs between electrodes. See U.S. Pat. Nos. 3,671,805; 
3,731,144; and 4,141,297. An improved circuit is shown in U.S. Pat. No. 
4,355,263. However during operation and testing of the circuit of the 
later patent, some disadvantages have been noted. While the circuit does 
operate, it has been difficult to obtain the desired high output voltage. 
Also problems were encountered with component burn out, and the operation 
was sometimes erratic. 
Accordingly, it is an object of the present invention to provide a new and 
improved ignition circuit. It is a particular object to provide such a 
circuit which has reduced I.sup.2 R losses and one which will provide 
output voltages in the order of 12 kilovolts at an electrode spacing of 
0.06 inches. 
Another object of the invention .sbsp.it to provide such a circuit 
utilizing a conventional integrated circuit chip and a conventional 
switching transformer. In particular, it is an object of the invention to 
provide such an ignition circuit utilizing a monostable multi-vibrator 
rather than a free running multi-vibrator, and a circuit which provides 
for setting of pulse width as desired for the particular material to be 
ignited. 
Other objects, advantages, features and results will more fully appear in 
the course of the following description. 
SUMMARY OF THE INVENTION 
The ignition circuit is switch operated and provides an electric spark 
between spaced electrodes for igniting an explosive, with each switch 
actuation. A switching transformer has its secondary winding directly 
connected to the spaced electrodes and its primary winding connected in 
series with a battery and transistor emitter and collector electrode 
connections. An integrated circuit multi-vibrator has an input connected 
to the battery through the ignition switch, and an output connected to the 
transistor circuit. The battery is connected through the ignition switch 
to the reset and Vcc terminals of the integrated circuit. The threshold 
and trigger inputs of the integrated circuit are connected to circuit 
ground through a first capacitor, and the two inputs of the integrated 
circuit are interconnected by a variable resistor which provides for 
setting the pulse width of the monostable output. The reset and Vcc input 
is also connected to circuit ground through a second capacitor and 
parallel resistor. With this arrangement, the control switch is positioned 
outside the primary winding of the switching transformer and the 
integrated circuit is operated as a one shot or monostable multi-vibrator.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The ignition circuit of FIG. 1 includes a battery 11, a switching 
transformer T1, and a transistor circuit Q1. The transformer T1 has a 
primary winding Nl and a secondary winding N2, with the battery 11, the 
primary winding N1, and the collector and emitter electrode connections of 
the transistor circuit Q1 connected in series. Typically the transistor 
circuit Q1 comprises a Darlington transistor. The secondary winding N2 is 
connected across spaced electrodes 13, 14, which typically are spaced in a 
range of 0.005 to 0.010 inches (?). The battery 11 typically is a 12 volt 
battery rated at 1 ampere hour. 
The ignition circuit also includes an integrated circuit U1 which is a 
conventional multi-vibrator circuit 
having terminals 1-8. The integrated circuit is shown in greater detail in 
FIG. 2 and typically may be a Texas Instruments NE 555 or a Motorola 
C6130P or a J4-1555 timer-oscillator chip. 
The multi-vibrator terminals 4 and 8 are connected together and are 
connected to the battery 11 through a diode D1 and an ignition switch 15. 
In the circuit of the present invention, terminal 4 is a reset input and 
terminal 8 is a control voltage input, functioning as a first input. The 
diode D1 is not an essential element in this circuit, functioning to 
provide over voltage protection. 
Terminals 2 and 6 of the integrated circuit are connected together and 
serve as a second input, with terminal 2 being the trigger input and 
terminal 6 being the threshold input. This second input is connected to 
circuit ground through a capacitor C1. A resistance circuit, preferably 
comprising a variable resistor R1 and a fixed resistor R2 in series, is 
connected across the two inputs, that is, across terminals 2 and 4 of the 
integrated circuit. Another capacitor C2 and another resistor R5 are 
connected in parallel between the first input and the circuit ground. 
The output terminal 3 of the integrated circuit is connected to the base of 
the transistor circuit through resistor R3. A diode D3 is connected across 
this base and collectors of the transistor circuit, to provide over 
voltage protection. Another resistor R4 and a light emitting diode D2 are 
connected in series between the integrated circuit output 3 and circuit 
ground, to provide a visual indication of operation of the circuit. The 
resistor R4 and the diodes D2, D3 are not essential but are desirable. 
In operation, switch 15 is closed, connecting the battery to the 
multi-vibrator circuit. The capacitor C1 is charged through the resistors 
R1, R2. Terminals 2 and 6 of the integrated circuit are inputs to 
comparators, as seen in FIG. 2. The comparators are set at 0.33 Vcc and 
0.66 Vcc, respectively. During the time period that the voltage across the 
capacitor C1 is less than 0.33 Vcc, the flip flop of the integrated 
circuit is set, causing the output at terminal 3 to go to approximately 
Vcc. This supplies current through the current limiting resistor R3 and 
drives the transistor circuit Q1 into saturation. The transistor circuit 
Q1 provides a return path for the battery through the primary winding N1, 
building a magnetic field in the transformer core. 
When the voltage across capacitor Cl increases to 0.66 Vcc, the flip flop 
of the integrated circuit is reset, turning transistor Q1 off. When this 
occurs, the magnetic field in the transformer collapses rapidly as 
compared to its build up time, and this field collapse creates a high 
potential across the secondary winding N2. This high potential at the 
electrodes 13, 14 produces the desired spark. 
The capacitor C2 and the resistor R5 serve to insure that battery power is 
supplied to the integrated circuit for a period of time long enough to 
complete the multi-vibrator cycle even if the ignition switch is closed 
for less than the total cycle time. 
In the operation of the circuit of FIG. 1 a spark at the electrodes 13, 14 
is obtained at each closure of the ignition switch 15. The multi-vibrator 
circuit is operated as a monostable circuit, with one cycle per ignition 
switch closure. The transformer T1 is a switching transformer in which the 
magnetic field in the core is built up relatively slowly, while being 
discharged relatively quickly to produce the desired high voltage at the 
secondary winding. 
The use of a switching transformer with one pulse per spark permits a 
substantial reduction in size of the overall circuit. Excluding the power 
source, typical circuit based on the prior art design requires a volume of 
about 12 cubic inches, while a corresponding circuit of the new design 
requires a volume of about 4-6 cubic inches. 
By way of example, a circuit constructed as shown in FIG. 1 has a charge 
time of about 50 milliseconds and provides an output voltage of about 12 
kilovolts at 0.06 inch spacing between the electrodes. The peak power 
input is about 40 milliamperes average and the energy is about 0.19 
joules, with a cycle time of about 100 milliseconds. The switching 
transformer discharges in about 400 microseconds to about 50 percent of 
the power level achieved during the charging portion of ignition cycle.