Before I go any further Ill just give the usual disclaimer.
-This project deals with high and potentially lethal voltages, the main capacitor has enough charge to deliver a painful shock, after it has gone through the spark coil the voltage is increased to anyware from 20000 to 75000 volts, probably with enough current to do some damage.
-The capacitor will retain it's charge for quite a long time even once the circuit has been switched off.
-If you are new to working with electronics, or are unsure about working with high voltages do not attempt this, or get someone else who is, to help you.
-Do not adjust wires or anything while the system is charged up, always turn off and discharge.
-I will not be held responsible for any damage or injury caused by this device.
Ok, this project uses circuits from disposable flash cameras. If you don't have any of these then go to a photo developing store and ask for some used ones, they should give you some for free.
Different cameras have different circuits but they all do basically the same thing. As long as you can turn it on and off and it charges up the capacitor then you're sweet.
The circuit I have used is from an older Fuji brand disposable. Not sure if their still around but they might be, I use it because it is the most simple and easy to work with.
Theory
It's quite simple how it works. All your doing is charging up the main capacitor to about 330 volts, then using a SCR (Silicon Controlled Rectifier or Thyristor) to discharge the capacitor through an ignition coil.
This boosts the voltage to an extremely high level, enough to make a spark. The advantage of the SCR is that it is solid state, it will never ware out and operates efficiently and safely.
The SCR is trigged by a switch connected to the capacitor and a high value resistor. After it has discharged it will charge up again and so on.
Originally there is only one cell used to power the circuit, I have used 2 AA size cells in series to produce 3 volts. This makes the circuit charge faster.
In doing so though, I have to replace one of the resistors with one of a slightly higher resistance to limit current to the transistor. Although it should work without doing this I like to be safe as I have burnt a few of them out by drawing too much current.
Parts
-One SCR, I used a "TYN816" It has an 800volt limit and 16 amp (continuous) 200amp (surge) limit. Plenty for this application. It cost around 5 bucks. I bought it at "Dick Smith Electronics" in New Zealand but any electronic hobby store like Radioshack should have them or be able to order some in.
-2 AA battery case with switch. The inbuilt switch just simplifies things. You would just add your own switch if putting this in a custom case of some sort.
-Push-on Pushbutton switch.
-Lengths of wire for connecting the triggering switch and some that connect to the ignition coil.
-33Kohm (33000) ohm Resistor
-390ohm Resistor (optional)
-Soldering iron and solder. (and a solder sucker if you got one) You need to be pretty handy with a soldering iron to make a good job of this project.
Although this post deals with one type of camera flash circuit. The SCR and triggering device can be added to ANY camera flash circuit out of a disposable. Just connect the appropriate leads to the positive and negative sides of the main capacitor(330volt@100-160µF).
Once you have built it, before you turn it on check to make sure nothing is going to short out. Check the legs of the SCR especially, covering with heatshrink tubing helps. Check that you haven't accidently soldered two different pads together and that there are no dry welds.
When you turn it on, most likely you will hear a faint high pitched whine as it charges up. When it is fully charged the built in neon light or LED will start to flash indicating its ready.
You don't need to use a car ignition coil for this, if space is limited you can use one off a lawnmower engine or a go cart engine, ofcourse the size of the spark will be smaller.
Feel free to ask questions..
Now some pics
