Page 28 - 15 Dangerously Mad Projects for the Evil Genius

Chapter 1 ■ Coil Gun 9

Figure 1-15 Measuring the charge voltage

Put the switch to the “charge” position. If the Measuring the Projectile Speed
LED does not light, put it immediately back to the
You can tell if the firing was a good one, and the
center off position and check your wiring. Also,
projectile was going fast, just by observation, but
check that your switch is wired the correct way
it’s better to have a more precise way of measuring
around. You can test this last point by just putting
the speed. The way to do this is with a humble
the switch to the momentary “fire” position. If the
computer. I am indebted to the excellent Barry
LED lights, then your switch is probably the
Hansen for his web site at www.coilgun.info that,
wrong way around (see the earlier section titled
amongst many other useful things, describes the
“Step 5. Fitting the Batteries and Switch”).
decidedly Genius (I would not say Evil) way of
Note that, during charging, R1 will get hot.
measuring the speed with nothing more than a
As soon as the multimeter indicates about 10V, computer with a microphone input.
put the switch back to the center position.
Here is how it works. You just record the sound
Watching the multimeter, you should see the
of your test and then use some sound software to
voltage decrease very slowly. If it drops down to
examine the waveform and measure the time
0V quickly, then something is wrong, so put the
between the coil firing and the projectile hitting its
switch back to off and check everything.
target. I think it’s safe to say that our projectiles
Assuming this test passed, we can now push the are slow enough that we can ignore inaccuracies
projectile into the firing tube until the end is just due to the speed of sound.
level with the edge of the coil on the far side of the
First of all, you will need to download some
coil. Now put the switch to fire and the projectile
sound recording software. I used Audacity
should move, hopefully traveling through the tube
(http://audacity.sourceforge.net) because it is free
and emerging at a modest speed.
and available for most operating systems,
Congratulations! Since everything seems in including Windows, LINUX, and Mac.
order, it’s time to try a full-power test. This time,
You then need to set up your coil gun a
you can just let the gun charge until the LED turns
convenient distance away from a target that will
off, which should be after 10 or 15 seconds.
make a noise when the projectile hits it—say,
about 3 feet (1m). I use a plastic shopping bag
hung from a door handle that is conveniently the