Camera Flash Ignition
Introduction
There are many ways in which to initiate the combustion reaction in your spud gun/cannon, but the best methods are going to involve an electronic spark. The piezoelectric BBQ ignitor is one very popular example. This method certainly beats sticking a lit match into a gaping hole in the side of the chamber! Electronic ignition allows for the most control and reliability, greatly contributing to overall performance, and of course safety. However, it wasn't very long before my BBQ igniter showed signs of mechanicle wear, which meant increasingly frequent mis-fires. This prompted me to search for something better.
The common piezoelectric BBQ igniter. Cheap, but rather short-lived.
I decided that I wanted a "quick-fix" to this problem, as I was eager to get back to the joy of shooting my cannon. The solution: the flash circuitry in the common disposable camera. The camera's flash is essentially a high-voltage discharge, just like what we need for our electronic ignition in our cannon. Furthermore, the circuitry has been professionally designed to be compact, being powered by a mere 1.5 volts AA-type cell. Many of the other high-voltage designs on the Internet require relatively large flyback coils and large, bulky power-supplies. Thus, this solution is an extremely attractive one. Also, flash-circuitry from other types of cameras can be used, even the external flash-unit designed for some high-end cameras. Then, this is an excellent way to "recycle" that old camera that nobody uses, providing a great excuse to your environmentally-concious parents. Just don't mention anything about combustion or cannons!
Today's victim: Fuji disposable camera with flash.
WARNING - This circuit operates with high-voltage, to the tune of 311V. This is potentially lethal, and it will hurt! This is not a "fool's guide", and fools should not be playing with high-volage anyways. Carefully study the circuitry, and have respect for high-voltage.
Details of Operation
When the charge switch is closed, an inverter (based around T1) boosts the battery's 1.5 volts to over 300 volts. A large-sized capacitor (about 100 microfarads in this example) is charged with this high-voltage output, storing up a considerable amount of energy. The neon lamp glows when the voltage across the capacitor has risen past a certain point, indicating a ready-for-fire status. This capacitor is directly hooked up to electrodes at either end of the xenon-filled flash tube, but the tube's impedance is much too high to allow for a rapid discharge. When the fire switch is closed, another capacitor is discharged through the trigger transformer (T2), producing a very high voltage (~4-5 kV) pulse on the output. Applied to the flash tube's trigger electrode (surrounding the tube), it ionizes the xenon gas inside the tube, greatly reducing the impedance to current flow through the gas in the tube. The result is a rapid discharge of the energy stored in the larger capacitor into the tube, and a brilliant white flash of light. Immediately, the capicitors begin to recharge, the cycle repeats and the circuit is ready to fire within seconds, provided that the charge switch is still closed.
The schematic of the flash unit from a Fuji disposable camera, professionally engineered for our fun :). Of course, yours may differ. For more on that, check this page.
Basically, we want the electric discharge that occurs inside the flash tube to instead occur inside the combustion chamber, fully exposed to the fuel-air mixture. Besides the lack of a glass tube, the main difference is that the gas inside the flashtube is at a much lower pressure than the fuel-air mixture inside the combustion chamber, which is at atmospheric pressure. In the combustion chamber, the relatively higher gas pressure means a higher impedance to current flow. To overcome this higher impedance the electrodes need to be much closer together for the desired discharge to happen. Of course, the electrodes cannot touch each other, but in my experience, they need to be really close. The spacing is very critical, and this becomes a major issue when bits of the electrode get vapourized with each shot! Later, in the Modifications and Improvements section, I will cover solutions to this problem. However, some electrode materials may have significantly better performance than mine did, and I would like to experiment further...
Refering to the following photo, there are the same three connections for the ignition electrodes as there was for the camera's flashtube. At either end are electrodes connecting to 300+ volts from the large capacitor. In the center is the trigger electrode, connected directly to the output from the trigger transformer. All three electrodes should almost meet at a single point.
A small PCB holds the stiff electrodes steady in place, as spacing is critical.
Extract the circuit
WARNING - Please keep in mind that the camera's circuitry is designed to hold a potentially lethal charge at high-voltage. Use a suitable conductor (preferably a screwdriver with an insulative handle) to discharge the large capacitor, at the first opportunity. As a precaution, remove the AA cell from its holder, to prevent the circuit from recharging itself.
I had to access the flash circuitry from the front of the camera. The cardboard was skin ripped off without issue. Next, you need to remove the plastic shell from the front. There may be plastic tabs along the sides of the camera, which need to be pried apart with aid of a screwdriver. If you must use a hammer, just bear in mind the delicate circuitry you are trying to rescue! Now before you get your eager hands on this circuit, this is an excellent time to discharge that fat storage capacitor, which can hold a potentially lethal charge. Also, study the circuitry and the different switches built-in to the camera. Most likely, these switches are really cheap stuff that you will want to desolder and replace with more reliable ones.
After removing cardboard and plastic cover, our treasure is clearly in view.
Wire it up
Our precious little treasure.
The board is now safely out. Desolder the flashtube's three connections. Replace these with wires that will connect to your electrodes. The trigger wire is the only one that may require significant insulation. It comes out of the top of the trigger coil, and formally connected to the flashtube by the chrome-plated reflector or some wire set around it. I desoldered the cheap trigger switch and wired my own push-switch. I did the same for the cheap battery holder, which no longer functioned outside of the camera case. Finally, I replaced the power switch and neon lamp indicator so that I could operate this circuit from within a nice new case. Try having all of these "bells and whistles" with a mere BBQ igniter!
Inside my cannon's new ignition case, all wired-up. The insulative case protects me from the circuit and the circuit from me :). I have a couple of extras: igntion switch, charge switch, and neon charge level indicator. The three wires on the left go straight to the electrodes within the combustion chamber.
Sparks produced are bright and loud, evidence of the high amounts of current involved. This in and of itself is enough to strike fear into most people! Perfect for our ignition needs. See more photos of the project in operation!
Modifications and Improvements
As mentioned, the electrodes need to be very close for this thing to work. Coupled with the fact that bits of the electrodes get vapourized with each shot, further increasing the gap distance, this setup proved rather unreliable. I had to keep reaching into my combustion chamber to re-adjust the electrodes. Very disappointing. Not to mention, getting zapped once or twice! I'm sure that a stronger material (such as high-grade steel or tungsten) can be used for better results, but I suggest you try one of the following two things to really make this thing work.
- Mechanically short the storage capacitor. I noticed that discharging the capacitor (C1) by shorting its leads produces quite a spark. I then hooked up wires to the leads of the capacitor, and tested this in an open jar of aerosol fumes. The prompt ignition of said fumes was very satisfying, and I decided to take this into my combustion chamber by building a simple mechanical switch.
- Beef up the trigger transformer. The existing trigger transformer (T2) was well suited to the low-pressure xenon flashtube, but at atmospheric pressures it struggles, as the impedance is significantly higher, and the voltage is no longer high enough. The solution: replace the trigger transformer for another that will produce higher voltages with ample current. I decided to try winding my own, and was delighted at how well it worked the very first time! This is now my prefered setup.
