Jeff's Electromagnetic Coil-Gun: Assembly Instructions

(Draft 03/10/2001, corrections 9/1/02: was 22 gauge wire, not 24, and 3/4" coil, not 1". 06/17/03 added diagram. 8/9/04 added coil dimensions)

Enough people have asked for instructions on how to build the coil-gun that I reluctantly decided to post the details. Please don't make me regret it.

Warning: This is a High-Voltage device. Please be extreemely careful and realize that there may be dangers that I haven't properly described. Attempt this at your own risk.

The coil-gun is simply an electromagnet coil powered by a short high-voltage pulse generated by the flash unit of a disposable camera.

Parts:

Kodak disposable flash camera

40 feet (12.2m) of 22 Guage Enamel-Coated Magnet wire (part of a 3 spool set from Radio Shack part number: 278-1345)

SPST lever Micro switch (rated to 250v is ok, even though we are using 300v but only for a short time) Radio Shack: 275-016

soda straw

2 steel washers 1/4" center hole (or whatever fits nicely around the straw), by 1" outer diameter

1" piece of small tubing to plug end of straw

6" piece of wood dowel to fit down center of straw (ok if it is a bit loose, just use masking tape to make it larger diameter to fit snugly)

12" x 6" piece of corrogated cardboard for the gun chasis

Masking tape

Tools:

Small flat blade screwdriver to help pry camera case apart

Soldering Iron & solder

Electric screwdriver or variable speed drill to wind coil (optional)

Scissors to cut cardboard chasis







Step 1 : Dismantle a disposable flash camera.

This can be done after the film is used up. There is a small door on the bottom that allows you to remove the film cannister and send it in for processing, or you can have the photo lab do it and give the camera back to you (normally they keep the camera and send it back to the company for reloading).

>>>>>>>>>>>>>>>>>>> >>> WARNING <<<<<<<<<<<<<<<<<<<<<<<<

While dismantling the camera it is difficult to extract the PC card without shocking yourself !!!

The large capacitor is charged to 300v and stays charged for a long time so always treat it as though it is fully charged.

Always discharge the capacitor (see below) before handling the card.

>>>>>>>>>>>>>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<<<<

To discharge the capacitor:

Remove the battery, then repeatedly press the shutter release and film advance to discharge the capacitor. Then when the case is half open,

reach in with a plastic handled screwdriver and physically short out the leads of the large cylindrical capacitor.

(Note: there is a small risk of the capacitor exploding at this point if you haven't sufficiently discharged it. But this is the only way to be sure it is completely discharged. The correct way to do this would be to discharge through a resistor, but it is difficult to do with out increasing the shock risk).

Attach a shorting wire or resistor across the leads to make sure it stays discharged while you are working on it. (don't forget to remove the wire later for operation)

Step 2: Assemble the barrel and coil.

The barrel of the gun is a plastic soda straw. Two steel washers are fitted over the straw 7/8ths of an inch (2 centimeters) apart to make a spool to wind the wire. The steel helps concentrate the magnetic field within the barrel. Disks of paper or tape should be placed on the insides of the washers so the metal does not scrape the insulation. At least 1" of straw should protrude from the short end, and maybe 3 or 4" from the long end. I put a wooden dowel down the center of the straw to keep it stiff while winding the wire, then chucked it in a portable drill so it would spin easily. Then an assistant held the spool of wire, and I spun the spindle to wind the wire onto the straw. Try to keep the windings tight and smooth by tilting the spindle a little so that the wire winds from side to side evenly. The final coil is approximately 1 inch ( 25mm) in diameter.

Don't forget to leave a long enough lead from the first end of the wire (I brought it out under the washer and taped it to the straw so it wouldn't be in the way. After it is all wound, wrap it tightly with electrical tape. It will help the performance if you can surround the coil with a wrap of sheet steel (maybe cut from a soup can) to confine the magnetic field, but mine isn't.

The next step is to make an end-plug. I used a small piece of rubber hose that fit tightly into the end of the straw, then a plastic pen tube through the center of it and extending down the straw to just before the first washer.

When loading the projectile in the barrel, it is critical that the front of the projectile is even with the end washer. Any further in, and it might get shot backwards or not move at all. Not far enough, and it won't get pulled in.

Step 3: Mount in an enclosure and design trigger switch

I mounted the circuit card inside of a cardboard box (because I was in a hurry, plastic or wood would be nicer). I made two holes in the box, one to push a wooden stick in to start the charging of the flash assembly (only need to push it if the gun hasn't been used for a few hours, after that it auto recharges). The second hole is to view the neon bulb that shows when it is recharged.

Step 4: Wiring

One lead of the coil is soldered to the negative terminal of the large capacitor. The other end of the coil is soldered to a micro switch which acts as a trigger. The other end of the switch is attached to the positive lead of the capacitor. Put a piece of tape between the normal flash switch contacts since they aren't needed. The rest of the flash card is unaltered.

Operation:

Loading of ammo is critical. The front of the projectile (BB or small nail) must be exactly positioned at the entrance of the coil's aft end plate. I used a small rubber hose as an adjustable breach plug so I could vary how far it pushes the ammo into the barrel. Mine doesn't have any means of holding the ammo from falling out the front, so you just have to hold the barrel slightly elevated when it is loaded.

Make sure to remove any shorting wire that may have been installed across the capacitor leads during construction.

To start the unit charging, there is a "click" switch on the circuit card that you push with a plastic stick. After that the unit will recharge automatically after each shot. Takes about 20 seconds to recharge. I cut a hole in the box so I can see the neon bulb that signals a full charge.

Always wear glasses when firing this gun since the projectile can ricochet unpredictably. Performance varies so even though one shot was a dud, the next one could be a real zinger. Be especially careful when reloading since the unit is usually charged and could fire at any time.

Then just point at a safe target (soft styrofoam works well) and shoot.

Performance:

BBs go about 20 feet, and small nails will push about 1/4" into a piece of styrofoam. Not bad for just a single AA battery.

It recharges in about 20 seconds. Try firing 2 BBs at the same time - surprise, they hit harder. 3 BBs even harder. Aparently the additional metal makes it more efficient. So then I went to using finishing nails and cut up pieces of 8 penny nails.

There's a lot of complicated physics that goes into the design of these. I just eyeballed it and got lucky. The critical factors are charge voltage, capacitor discharge rate, and coil dimensions. The capacitor needs to completely discharge before the projectile has left the coil or the coil will end up holding it back, or even just oscillating it in place. My first test firing blew the breach plug and projectile backwards into my chest.

To see if addtional capcitance would help, I tried putting a variety of large capacitors in parallel with the existing one. They caused it to recharge much, much more slowly but didn't seem to help projectile velocity much if any.

Fancy Options:

A piece of clear tubing is attached to the back of the straw (breech) to hold BB's. A system of two small nails pushed through the tubing allows me to select one BB at a time to feed into the barrel.

The laser pointer helps to aim above the target to counter the drop due to slow velocity.



Be SAFE, LEARN, and have FUN.

