Camera flash
A camera flash can be used for spudgun ignition in several ways, either by itself or by feeding the high voltage from the flash capacitor through a step-up transformer (commonly an ignition coil, flyback transformer or a mains transformer in reverse).
The main advantage of camera flash-based ignitions is that it can be built using cheap, readily available components. The spark is more powerful than that of a piezoelectric ignitor. A camera flash is usually powered by one or two AAA or AA cells. Disadvantages include several seconds of recharge time before the ignition can be triggered again, and significant shock hazard from the storage capacitor if built improperly. Special care should be taken when used in metal launchers, as both sides of the storage capacitor can have significant voltages between them and other parts of the circuit, such as the battery poles and the trigger switch.
For an excellent description on how these disposable camera photo circuits work see [1].
The camera flash is most easily obtained from a disposable camera, many places that develop films from these will give the discarded camera shell away for free if asked. The flash capacitor can store charge for a long time, and should be discharged before handling the circuit.
Contents
Circuits
There are several ways to connect a camera flash to a spark gap, which all have their own advantages and drawbacks. These are the most common circuits.
Ignition coil
An ignition coil (or other step-up transformer, such as a flyback or a mains transformer connected in reverse) will provide a high voltage spark when the flash capacitor is discharged through it. Most Ignition coils has a 100:1 winding ratio, meaning that for every 1 volt that is put into the coil, 100 volts come back out.
In order to modify a camera flash circuit for use with an ignition coil, follow these 6 steps:
Step 1 - Removing the Flash Bulb
Step 2 - Drilling the Holes for the Ignition Coil Wires
Step 3 - Before Adding the Relay
Step 5 - Adding the Ignition Coil and Spark Strip/Spark Gap
Step 6 - Providing Power to the Relay
The flimsy battery holder can also be replaced with a plastic one, which is easily attained at any electronics store. Also, you can add a "kill switch" in series after the battery holder to cut off the power when the circuit is not in use. Be aware that the circuit is not "powered down" when the kill switch is in the off position. The circuit will not be able to charge, but the capacitor will still hold the electricity for a long time. The cannon will need to be "dry fired" after the kill switch is turned off. Be sure to point the cannon away from everyone when doing this.
Step 7 - Removing the Battery Contacts
Step 8 - Adding a Battery Holder and Kill Switch
On a final note, the flash charging button on the front of the camera circuit board can also be replaced with an external one to add a finishing touch and an extra degree of safety. This step may be the hardest of all, as the margin for error when drilling the holes may be small. However, in the larger disposable flash cameras such as the Kodak Funsaver or Kodak Max cameras, the circuit board is larger and this is not an issue.
Step 9 - Replacing the Charging Button
The relay used in the diagrams is a standard 5-pin automotive relay, which can be purchased at any auto parts or electronics store. It is advisable that you also use a relay socket; it will make the relay easily removable in case of a wiring problem or to be used in another project.
If a mains transformer connected in reverse is used, the spark gap will be electrically insulated from the rest of the curcuit, an advantage when used in metal cannons.
The output transformer from a stun gun can also be used, which can be useful if the charging circuit in the stun gun has died. These circuits are in fact quite similar to that of a stun gun.
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Tube Switched Coil
The ignition that requires the least modification of the camera consists of cutting one of the main leads to the flash tube, and connecting the ignition coil in between. Triggering the flash will produce a spark from the output of the transformer. The advantage to using the flash tube itself to switch the main discharge, is that almost any small switch can be used to trigger the circuit. If the trigger switch is replaced by a thyristor (SCR), the ignition can be triggered by an electronic circuit. The flash circuit itself is mostly intact and not driven out of spec, which means this circuit is quite reliable.
Manually Switched Coil
The flash tube can also be replaced by a heavy duty switch for triggering, though if the switch is too weak it may become welded shut. This switch will also be carrying the full voltage of the capacitor, and must be properly insulated to prevent electric shock. If the trigger switch is held down for a long time the charging circuit in the flash may break since its output is shorted for the duration.
Triggered spark gap
Another ignition method that does not require an external step-up transformer, consists of removing the flash tube, and taking out the trigger lead as well as the two main leads. These are then connected to three electrodes, which are placed together to form a spark gap of a millimeter or less. Triggering the flash will ionize the air in the gap and discharge the main capacitor. This produces a more intense spark than the first method, but it requires a more complex spark gap. The electrodes can also erode over time, requiring readjustment.
Trigger transformer spark
The trigger transformer for the flash tube produces a few kV on its own, and can be connected to a spark gap. The advantage to this method is that the main capacitor is not dumped, so recharge time is eliminated and it uses less battery power. The spark is very weak however, and this method is generally not recommended.
Short circuit gap
A camera flash can be used with a short circuit gap, which has movable points that can be brought in contact from outside the chamber. The flash capacitor is connected directly to the gap and creates a spark when it's shorted. This spark gap is more difficult to make, and may have isues with welding and erosion, but is easy to hook up and requires little electronics knowledge.
Others
A camera flash can also be used to trigger sprinkler valve solenoids built for mains voltage, however the opening time will be brief since the flash capacitor is drained quickly.
