As is readily obvious, this system could use some improvements. In my limited spare time, I'll be trying to develop them. I'll be updating this section as I go, with my prototypes, and thoughts on improvements. I'll do my best, but I make no guarantees that my "here's where to go next" suggestions will actually work. If you want to pass me in development, be prepared to roll up your sleeves and do some (a lot) of testing before you trust it. Not that you should totally trust what I've done, either. Caveat emptor.



Overvoltage protection



First off, you'd want an overvoltage protection circuit to stop your power supply from overloading the capacitors (which will kill them). I developed two possibilities (see the first picture); the first using readily available parts I had knocking around, and the second using an uncommon IC chip (the TC54) which would do a better job.



The first uses a zener diode (you can also use regular diodes- which cut on around 0.7V- in series to create a poor man's zener; I used two in my first prototype, to get a turn-on at around 1.4V) and a resistor to selectively activate a transistor, which discharges the capacitor. Since the voltage on the gate of the transistor is basically going to be the voltage on the cap. minus the threshhold voltage of the zener, the transistor will turn on quite slow- not so good.



The second uses a specialized chip designed to shut off circuits when the voltage droops too low (typically because the battery is dying). BEAM robotics people use these frequently, along with it's cousin the 1381. This guy is better than the zener, since it applies a much higher voltage to the gate of the transistor (Cap voltage-0.3 V), which makes it sharper.



In both cases, you want the sum of your trigger voltage (either the zener or the TC54) plus the turn-on voltage of the transistor (for a typical silicon transistor, this should be about 0.7 V) to be just below the max voltage of the capacitor- I'd test this out before relying on it. You also need a transistor rated (and properly heat-sunk) to handle the amount of power you'll be seeing. You also want a fairly high hfe rating (a.k.a. current gain); I used a 2n3055 (hfe around 50) which was OK, especially since I was using normal diodes as the voltage trigger; however, zeners and TC54's have much lower current ratings, so a higher hfe transistor is probably much better- I did a bit of poking around on digikey, and it looks like the TIP140 (hfe =1000) will do much nicer.



Consistent, tunable, non-caveman switching



While fun, the "bashing two rocks together" switching technique has many obvious drawbacks. The thing to do here is probably to rig up a 555 timer as a "one-shot" with a potentiometer to change the pulse duration. You can then use that fixed pulse to trigger a bank of mosfets (which are happily parallel-izable for higher current handling capability) to discharge through your electrodes. A possible circuit diagram is the second image on this page. You'd want to pick R3 to vary around whatever point you want- I'd say from 200k to about 2M ohms (giving a pulse ranging from about 0.3 to 3 seconds). I've got some big honking mosfets (FDA032N08 which can hopefully handle around 200A each) on their way; I'll be able to give this a try soon.

