The Miniature Coil Gun Project

Or... How to build a working model gauss gun for virtually nothing

This quick tutorial will show you how to build a small working model of a gauss gun, for very little money using mostly things found around the house(at least my house). The completed project, although it falls quite a bit short of administering a "euphoric orgy of plasmariffic destruction", is still a pretty neat toy and is a good demonstration of the principles behind a coil gun.

Approximate statistics for my version.

Pulse energy	About 100J
Pulse voltage	320V
Coil resistance	1.4 ohms
Pulse current	229A
Pulse wattage	73,142 watts
Pulse duration	.001367 seconds

Basic design

There really wasn't so much "design" to this project as there was "throwing together". I built this in high school, specifically one day when I didn't really have much to do during networking class. Someone brought in some old disposable cameras (thanks Omar!). After repeatedly discharging one of the capacitors with a pair of dikes got boring, I decided to do something more interesting with them. That was when I built the mini-coilgun.

A coil gun consists of one or more magnetic coils wrapped around a barrel. A piece of ferrous metal is placed in one end of the barrel and high energy pulses are applied to the coils in sequence, each right before the projectile enters it. This creates an electro-magnetic pulse that pulls the projectile into the coil and quickly drops off, allowing the projectile to continue out the other side of the coil at a significantly higher velocity than it entered at. With several coils, they could be triggered either with a preset timer or, more reliably, with a sensor to detect when the projectile passes it. It is somewhat similar to a linear motor or a solenoid.

This version is very simple, consisting of only one coil, timed only by the size of the capacitor bank and the resistance of the coil. It is nowhere near as efficient as it could be. The capacitor bank is about the size of a deck of cards, despite the large size of the project box it is in. There are no precision machined parts, only duct tape and wire ties. Despite this, it can easily chuck a piece of steel wire across the room across the room. However, if you really want to put some work into it, I suggest you go with a much larger design and multiple stages, or build a railgun instead. If you want chaos and destruction, you will be extremely disappointed with this.

Parts - What you will need:

The most difficult parts to come by are the disposable camera circuit boards. You will need at least one intact circuit board and several more if you don't want to buy the photoflash capacitors online. The project box is nifty, but not required.

1 or more: disposable camera circuit boards.
1: Switch capable of handling pulse energy. I used a key switch I had in my parts box for this. An old wall switch might also work. If the pulse energy is too high for the switch, it can weld the switch closed(this happened to me once), so don't use anything you don't mind being damaged for this.
1-2: Small switch to control charging and/or main power(depends on flash circuit).
Copper wire Unfortunately, I can't remember how much I used for this. I used wire from some Cat-5 cable. A few tens of feet should be enough.
1-2: Large paperclips with heavy wire.
1: Plastic drinking straw or other small tube for the barrel.
1: Battery holder.

And, of course:

Duct tape!

Assembly:

Assembly is fairly simple. You will need to know how to solder and understand some basic electronics(you need know what you're looking at).

Step 1: Taking the camera apart:

The first step (aside from gathering the parts) is to take apart however many disposable cameras you want to use(I used 4). Most disposable cameras actually rewind the film as you "advance" it. If you want to keep the pictures that are on it, you will probably want to use up all the available pictures to rewind it, so that the last picture won't be damaged if it is not completely rolled up.

Next, open the camera case. On some cameras, you pry back two tabs and it opens like a clamshell. Most disposable cameras use tabs to hold the case together. Since you won't be using the camera again, you could also just break it open if needed, as long as the circuitry isn't damaged.

Once you have the case open, remove the film and put it aside so you can have it developed later. Remove the battery and short the terminals of the capactor to make sure it is discharged. The capactor will hold around 300V at full charge and can put a fairly large pulse of energy through whatever you short it with. Use something heavy enough to handle the current, like a screwdriver or dikes/wirestripper, not a small wire(can explode). If the capacitor was charged, there will be a spark and a very loud pop when you discharge it. Next, remove the circuit board and flash assembly. Unfortunately, there is no standard way this is mounted, so the method for this will depend on your cameras. On one of mine, there were tabs, on another, there were screws.

Step 2: Examining the circuit board:

The circuit on a disposable camera is usually fairly simple. While charging, an oscillator converts the DC battery current into AC, which is fed through a transformer to step up the voltage to around 300V to charge the capacitor. The flash tube is connected to the capacitor and will have the 300V potential across the electrodes when fully charged. However, 300 volts is not enough to jump across the electrodes in the tube. When you press the shutter button, a flyback transformer is connected to the circuit. This provides over 1000V, which is enough to ionize the gas in the tube and allow the 300V to discharge through it, creating the flash of light. For the purposes of this project, only the charging circuit and the capacitor itself are necessary.

So far, I have come across two types of charging circuits. One kind has a button that you have to hold do until it is charge, and the other kind has a button that you just press once and it will charge. While either kind will work for this project, they will require slightly different wiring setups. The actual type of switch used also varies, so you will have to figure out how and where to connect the wires for the switch. The circuit I used had two tinned pads that were connected by a piece of metal on the back of the button so I just soldered the wires onto the surface of the pads.

Note the ratings on the capacitors. The voltage may vary somewhat. While you can use several capacitors with different voltage, only use the circuit board with the lowest voltage.

Step 3: Modifying the circuit board and wiring components:

You will need to do several modifications to the circuit board in order to use it for this project.

Wiring the battery:

Most disposable camera flash circuits are powered by one or two AA or AAA batteries. The coil gun will probably drain these fairly quickly, and you may want to have the battery in a more easy to change location if you are using a project box, so you will probably want to wire up a battery holder.

Disposable cameras are not engineered to have their batteries periodically changed during use. Polarity will likely be critical with this and, in some cases, could damage the circuit if not properly connected. Make note of the positive and negative terminals on the circuit board and make sure you match these with the corresponding terminals on the battery holder. Typically, the current battery holder will consist of a couple pieces of copper. Desolder the copper pieces and solder wires in their place. Connect these to the battery holder with proper polarity and install a power switch if needed.

You can buy a suitable battery holder at Radio Shack or build your own. You can use "D" sized batteries, but make sure the total voltage is the same. If the circuit used 2 AAs, use 2 Ds.

Wiring the switches:

As mentioned earlier, there are two types of camera flash circuits that I have seem. Some have a button that you have to hold down while charging, and some have a button that you press once to charge. If you have a circuit that requires the button to be held down, I suggest you use a toggle switch, since the charging time is usually to long to comfortably hold down a momentary push button. A momentary push button will work with the second type of circuit, though. Be aware that the second type of circuit may automatically recharge after each shot, so a real power switch may be needed. These switches do NOT need to be rated for a high current(unlike the trigger switch).

Wiring the capacitor bank:

Start by marking which pin on the capacitor connect to which terminal on the board. Wiring the electrolytic capacitor with reverse polarity can damage it and, if there is enough power to electrolyze and/or evaporate the water in the electrolyte, cause it to explode. Next, desolder the capacitor from the board. Repeat for all the cameras you want to use. Make sure that you have discharged the capacitor first! Exploding molten solder in your face would be bad.

Next, straighten all the legs on the capacitors and lay them next to each other on the table. Straighten out the paper clips and cut off two segments of wire that are long enough to run across all the capacitor leads. Leave space at the end to connect wires. Solder the capacitors to the paperclip sections, using them as bus bars. Make sure that all cathodes are on one bar, and all anodes are on the other.

Lastly, connect one end of the bus bars to the terminals where the capacitor was on one circuit board using some wire. Again, it is critical to use the proper polarity. The other side of the bus bars will connect to the trigger switch and coil.

Wiring the trigger switch:

On the other side of the bus bars, connect two wires. Install a switch rated for at least 300V and a considerable current(10A should work as long as the coil has a high enough resistance. Since the pulse is only momentary, the biggest problem will be the terminals being welded together, not heating over a longer period of time. Wire the trigger switch and coil(or connector for the coil) in series. The switch should probably be momentary.

The coil:

Without this, it wouldn't be much of a coil gun. The solid copper wire in Cat5 cable seems to work well for this. The ideal size for the coil depends on many factors, including the voltage in the capacitor bank, the resistance of the wire, the weight of the projectile, the length of the projectile, the friction between the projectile and the barrel, and, of course, the capacity of the capacitor bank. However, since this is not exactly a precision design, it is probably okay to omit these calculations and determine the ideal size experimentally. Start out with maybe 100-200 turns of wire. You can change this later to improve the power. Keep in mind that too few turns will lower the resistance and may risk damaging the trigger switch.

Wind the wire around the barrel. I used a drinking straw for this. Make sure that it is not too loose and wrap it in duct tape to prevent the winding from moving too much when it fires. Connect this either in the circuit with the trigger switch, or to a connector(I used a dual bannana plug) if you want the coil to be removable.

When positioning it on the barrel, make sure it is close to one end, but leave enough space so that the projectile can be placed slightly into the coil without falling out.

Finishing touches:

Look over the setup and make sure that there are no bad or shorted connection. Verify the polarity on the battery holder and capacitor bank again. Make sure that everything is connected where it needs to be/

Internal wiring(click for original size)

Step 4: Testing:

Now comes the time to actually test it and see if it works. Cut a straight 0.5-1" piece of heavy paper clip and place it in the short end of the barrel so that it just enters the coil. Make sure that the barrel is pointed away from you, although, even at full charge, the projectile probably won't be capable of much damage(unless you used a LOT of capacitors). Also, move any disks and credit cards away from it, since the magnetic pulse can erase them. Put the batteries in and turn the power and charge switches on. You will have to wait a while for it to charge up fully, but since this is just the first test, wait about 5-20 seconds and close the trigger switch. If the projectile moves a bit and/or the coil makes a "thunk" sound, and there isn't any smoking or popping, congratulations, the power supply works!

If you have a DMM, and opted to use a plug to connect the coil, you can remove the coil and connect the DMM to the power supply. This will allow you to check the voltage on the capacitor bank. When it gets up to around 300V, it will be fully charged. When it is fully charged and the trigger switch is closed, the wire should be shot at least a few feet. At this point, it will require a lot of experimentation to get the maximum distance from it. Mine can easily shoot across the room as long as the paper clip is properly positioned and the batteries are fresh.

Loaded and ready to fire.

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Copyright (C) 2005 Luminous Data Labs. All Rights Reserved. You agree to accept any injury or damage that may occur from or during construction of this device. For questions, comments, or boredom, E-Mail projects (AT) ldwp (DOT) com. No spamming - prosecutors will be violated!