suggestions for improvements:
1. use extruded plastic (eg, borosilicate). as mentioned above, eddie currents are not your friend. example: take a magnet and drop it through a length of copper pipe…..THAT is eddie currents at play.

2. consider using simple mechanical triggers for your coils eg, two bare wires inside the plastic tube that are bridged by the projectile.

3. plan for acceleration; use more than three coils spaced with <= logarithmic spacing. once you pass a certain velocity – maybe past the third coil – you’ll have to use electronics to manage the coils, and the 2n3055 idea above will do the job. you can use an MC to detect the initial coil’s trigger rate and deduce the velocity of the projectile. if you know the velocity, you can calculate the trigger times and pulse width for the subsequent coils.

4. consider higher current/voltage sources, but DO BE CAREFUL. 30V at 1ma can kill you.

I put a welding rod through genuine .mil armor plating using an approach similar to your own; a simple charge circuit, and five discarded radar delay line caps. make sure you can stop the projectile…

And oh,it’s a really nice hack,I watched your video on youtube man,your awesome ^^

Using relays for switching can introduce large mechanical delays, so using a solid state approach is much faster if you can get it implemented correctly. Using BJTs, like the 2n3055 that localroger suggested, could probably work but if significant amps are required then it will not be able to source all of that current. Plus, BJTs are current-controlled current sources, and when used as a switch, their amplification is very small (approx 40 perhaps), compared to > 1000 when used as an amplifier.

The best option IMHO is to use a low-voltage MOSFET. The diode for suppressing inductive voltage spikes is built into the FET, plus since the FET can only block say 35-ish volts, you can get some SUPER FAST switching, like in very small microseconds. These are the devices used in high-frequency switching power supplies, like the CPU power supplies on the motherboard that switch in the 100’s of kHz.

Also it’s important to know that during the inductive voltage “kick,” current is still free-wheeling through the diode and coil, so the coil is not yet turned off. Maybe that was said above but I missed it. You’ll probably want to shut the coil off before the projectile hits the middle of the coil or you may end up slowing down the projectile. That is, of course, assuming the switches/FETs/etc can be controlled accurately enough to do so.

These are by no means the only way of doing this, just my overeducated thoughts on the subject.

Again, Andrew, this is a really cool hack and I echo strider_mt2k’s thoughts on your work!

The best I’ve done on a rainy day are some simple electromechanical robots
This is extremely cool given the materials you had to work with.

I like how it’s getting the creative and technical juices flowing in the comments too!

You can always tell a good project on hack a day by the quality and content of the comments.
They are always a great read.

Kudos andrew!

My conclusion was you really need some sensors to combine with the timing. One after each coil would probably suffice and the rest of the timing can be done in a pic. My ideas for sensors were light gates (ir led and detector pairs) or metal brushes that connect with a metal projectile. Timing accuracy is probably only important to ~500 µS, so pretty much any kind of sensor would work. I ran out of time so all those improvements never got done.

One thing that seemed to limit it was the de-energisation time of the coils – effectively we couldn’t switch them fast enough due to arcing in the relays.

Watch out having sensors near the coils, because the magnetic fields from the coils could induce currents in the sensor wires and fry your sensor or controller.

The coils were wrapped with 22 gauge magnet wire. I used a milling machine to spin the tube while I wrapped it so I don’t have the exact number of turns.

Most of these parts were made without specific measurements so aside from the coil resistance I don’t think much many of these are crucial.

Ben:
I never wrote a schematic for this circuit because it seemed pretty self explanatory.
Is there a particular part that is unclear?

@ Jim: I’m not sure why that would help maybe the posted code will clear things up. I did start the second and the third coil timers at the same time but that was due to the slow reactions of the relays. Its worth noting that the coils due technically draw more power than the PS can handle so the pulse times had to be shorter than the time it took to drain the PS’s caps or it would shut off.

As requested youtube:
http://www.youtube.com/watch?v=nsjQfXr7ll4