Schneier on Security

Sparky • August 16, 2007 3:32 AM

@AM: I find it very unlikely that a charged flash capacitor from a cheap camera would kill somebody, assuming it is used with two pins touching the skin, not piercing it, or used with large electrodes with conductive gel, or administered to very sensitive parts of the body, like the optic nerve.

I have messed a lot with electronics in the past (and still do), and as such, have gotten more than my fair share of jolts, including accidental discharges of large strobe light capacitors.

Some years ago, a friend and I where pretty fed up with getting our bicycles stolen. I live in the Netherlands. Bicycles seem to be regarded as public property by some people. I believe the figure is about 10% of all bikes that are reported stolen every year, and most people don’t even report it if it wasn’t brand new or insured.

Anyway, we had a free afternoon and a lot of junk parts, including an old mountain bike, and we build a nice little circuit in a small saddle bag on the bike, basically a wave rectifier, a fly-back transformer circuit, flash capacitor, and a cute little ignition coil from a motorbike. The whole thing was powered from a wheel-mounted generator, normally used for lights.
Two thin wires ran through the steering bar, and in a loop around the rubber handles that you could barely see.

We parked this bike on a busy street in the city, grabbed a beer on the terrace of a nearby pub, and waited. It didn’t take too long before some shady guy walked past our bike, noticed it wasn’t locked, walked past it again, looking around if the owner might be near, and finally took the bike and rode off on it. After about 15 meters, we could hear the distinctive whining of the flyback circuit, meaning the current through the capacitor was decreasing as it build up it’s charge. A few seconds later, we heard a sharp snapping sound, someone screaming (guess who), and the sound of a bicycle crashing. The guy ran away and we went outside to get the bike and put it back for the next victim. After about 4 of them, and a whole lot more beers, we got tired of having to go get the bike every time.

Point being: we shocked 5 people that day, which included me to test the circuit, and nobody died. The shock hurts, but only for a short time. The victims always crashed the bike because of the involuntary muscle contractions in there arms.

Sparky • August 16, 2007 9:13 AM

@NotMe: I’m afraid I can’t. I haven’t seen the actual circuit in years, after having moved a couple of times, although I guess it’s somewhere is a box, waiting to be found in the distant future.
We never even draw a diagram, we just used a few standard circuits and fiddled with it until it worked.

It’s very basic, really. A wave rectifier is just 4 diodes, and is found in most power supplies. The flyback circuit is rather simple and easy to find using google. The output of this was again rectified and fed into the flash capacitor (this is important: they can handle very fast discharges without breaking down, and very high voltages). The only slightly novel part in the circuit was the trigger mechanism: we used a neon bulb, which has a striking voltage of about 100 volts, and an adjustable voltage divider network to control at what charge it would trigger the thyristor that would discharge the capacitor into the primary coil of the ignition coil.
Initially, we used a zener diode to control the trigger, but this circuit didn’t source enough current to quickly turn on the thyristor.