When I was 18 and just about to graduate from high school, my buddies and I first heard about potato guns and decided we had to build one. This was way before MAKE, Mythbusters, or the modern DIY movement as we know it, and potato guns were, at least on my radar, new and scary. They had just been made illegal in Texas, and so our furtive project to secretly construct one in my bedroom felt comically sinister, in retrospect.
We went to the hardware store and came home with lots of PVC bits that we assembled into a barrel and a chamber. Then we waffled for some time about how to hack together an electric ignition system. Our thoughts went immediately to using an automotive spark-plug, but none of us really understood enough about electricity to know how to separate the spark-plug from the car. We were pretty sure you couldn’t just take a spark-plug and wire it up to a car battery and expect results, but beyond that we were clueless. We ended up using a piezoelectric barbecue grill igniter, which gave satisfactory results but often required several “clicks” to actually fire the thing.
Anyway, what we needed at the time, but did not have, was this tutorial over at Popular Science, which is exactly about how to fire a spark plug without, you know, having to keep a whole car around to do it.
From the pages of MAKE:
The Night Lighter 36. Launch potato projectiles 200+ yards with this stun-gun triggered, high-powered potato cannon with see-thru action, from MAKE 03.
4 thoughts on “How-To: Make high-voltage sparks using a spark-plug”
The circuit described in the article is pretty simple but text descriptions of circuits suck. In fact, they didn’t even need an article; they should have just posted a schematic and called it a day. A picture is worth a thousand words after all…
Also, the circuit “may” work. Relying on the switching frequency to be “as fast as the relay will switch” is not good enough. It may not be on for a long enough time to energize the primary coil of the ignition coil to produce a spark strong enough or even a spark at all.
Apply the tiniest bit of engineering here. Figure out the frequency you know a ignition coil can operate. A 4-cylinder 4-stroke engine can rev say 1000 RPM (a fast idle). So, in 2 revolutions there have been 4 ignition events or ((1000RPM/2)*4cylinders)/60seconds =~ 33Hz.
Now, go build a 555 circuit that puts out a square wave at 33Hz and have its output connected to the base of say a 2N3055, add a flyback diode etc. For a spud gun you would be fine to use a %50 duty cycle. But if you are having fun, look up some “dwell” times for engines and modify the 555 circuit waveform to match that high time. And if your still having fun, add a potentiometer to the 555 to adjust the frequency.
I remember when Popular Science used to be fun to read and educational.
I think the circuit he is describing is from a secondary link. The schematic is pretty fuzzy but here: http://www.oldmarineengine.com/discus/messages/3457/3079.html
I agree with you though, more pictures!
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