The next project that I’ve been working on (with a lab partner) for my electromagnetics course is an “EM Buffet Lab.” In other words, we get to choose what we want to build, as long as it is EM related, and then do a write-up about why and how it works. What my lab partner and I are building is a coil gun. We have actually already finished it, but we are still preparing our presentation and report for Wednesday.
Note: Be sure to also read the follow-up post to see the projectile velocity and some other goodies.
What is a coil gun?
A coil gun uses a solenoid (coils of wire wrapped closely together) to create a magnetic field. This magnetic field attracts a ferromagnetic material during a strong, brief current discharge (usually produced by a capacitor.) The projectile wants to go to and stay in the middle of the solenoid due to the induced magnetic field, but by the time the projectile gets to the middle of the coils, the capacitor is fully discharged and no magnetic field exists anymore. Therefore, the projectile does not stop, but continues traveling through the solenoid instead of stopping in the middle of it.
Parts needed to build a coil gun
Coil guns are surprisingly cheap and easy to build. Here are the minimum parts needed:
- Disposable camera with flash.
- Lots of magnet wire (magnet wire looks bare, but actually has a very thin insulator coating on it.)
- Wire to connect components together.
- BB’s or steel rod (with a diameter that fits into the pen casing.)
- a pushbutton switch (“pushbutton” = only on when you are pressing it.)
That’s all you need! If you want to get a few extra goodies, then you can also get the following (if you don’t have them already):
- 2 washers, where the hole is barely larger than the diameter of the pen casing.
- Nut and plastic bolt, where the threaded end of the bolt fits easily into the pen casing, and the nut is larger than the pen casing.
- Electrical tape.
- Hot glue gun (and glue)
- Piece of wood.
- Various brackets and screws to secure coil gun components to the piece of wood.
- 2 opto-interrupters (for determining speed of projectile as it exits the coil gun.)
- Some resistors.
- An on-off switch.
- Pizza (work is always more fun when you have some food nearby!)
Building the coil gun
To start, disassemble the disposable camera. We need the disposable camera for the circuitry inside that controls the flash. You can get a disposable camera for around $3 at most stores like Wal-Mart (make sure you get a camera with a flash on it!) You can usually pry the camera open with a screwdriver. If you haven’t used the camera before, then the capacitor in it probably won’t have any charge on it. However, you should try to discharge it anyway to be safe. The capacitor is designed to hold about 300 V, which can be discharged at very high currents. Take something metal that has a plastic or rubber grip (like a screwdriver) so you don’t get shocked, and touch the metal part of it simultaneously to the two leads on the capacitor. If the capacitor had no charge, then nothing will happen. If the capacitor is fully charged, then you will hear an unbelievably loud BANG! as the capacitor almost instantly discharges. It is kind of fun to make it discharge when it is fully loaded, but it isn’t very safe and you will start making black marks on the capacitor leads.
It is interesting to know how the flash circuit works. What I had not realized before was that the 300 V on the capacitor is always connected to the flash. The flash does not go off, though, until its backplate (the reflecting part) has a high voltage introduced to it (greater than 4000 V.) Read all about it at Howstuffworks. The flash circuit uses a small transformer to attain the 4000 V for the back plate, but it is useless to us because the 4000 V has almost no current. We need a high current to make our coil gun work well, which is why the capacitor is so important.
At this point you should unsolder the flash from the rest of the flash circuitry. The next step will involve connecting the coil gun directly to the capacitor, with a switch in series with them. The switch will serve as the trigger for the coil gun. To do this, solder some wires where the flash was just removed. In this picture, the brown and green wires are connected where the flash was. There is a white wire in the picture, but it is not connected to anything; it is just secured to the board for convenience (you don’t need it and can ignore it.)
To make the coil, begin by disassembling the pen and keeping only the hollow pen shaft. Next, hot glue one washer in the middle of the pen shaft, and then hot glue the other washer about one inch from the first washer. The washers will keep the coil together.
Now, start wrapping the magnet wire around the pen beginning at one washer and ending at the other. The wrappings should be very tight and only one layer thick. After you have completed one layer of coils, put some electrical tape over the completed wrappings and start making a second layer of coils on top of the tape that is covering the first layer (like in the picture to the right.) Continue doing this until you have around 10 layers of coils between the washers. Make sure you have left long leads at both ends of the magnet wire so that you can easily connect the solenoid to your flash circuit.
After you have completed the wrappings, put some electrical tape around them so you do not scratch the magnet wire and short it out. Next, hot glue the nut to the end of the pen shaft that is closest to the coil. Make sure that the center of the nut is aligned with the hollow shaft of the pen so that the plastic bolt can be screwed into the pen shaft. The bolt will be used to adjust where the projectile is positioned and to keep the projectile’s position consistent. This picture shows what our finished coil looks like.
Next, hook up the coil to the flash circuit by connecting one wire lead to the pushbutton switch, and the other wire lead to the capacitor lead that is not connected to the switch. I recommend using a “pushbutton” switch – where the switch is engaged only while you are pressing it, and is disengaged the moment you stop pushing it. This helps ensure you don’t accidentally leave the coil gun on and drain your battery.
That’s it! You can test your coil gun by inserting the projectile into the coil gun barrel, turning on the flash circuit, waiting for the LED on the circuit to light up (which signifies that the capacitor is charged up,) and then launching the projectile by depressing the pushbutton. Here is a video of us firing our coil gun:
We also used some of the items in the “extra goodies” shopping lists above to make testing easier and to measure the projectile speed, but I will describe this in another post later (click here for the follow-up post).