Before this thing becomes so technologically advanced it can't be put together without a built-in microcomputer chip and a couple of servos, let me remind folks that these things are supposed to be simple.
Mine was built using a 6.3 vac filament transformer. Back in the day they were readily available just about anywhere really cheap. Now that vacuum tubes are no longer in common use it might be necessary to ask somebody at Radio Shack. Personally, I'd not go any higher than this in voltage. Remember these things are a short-circuit sort of device and using a lower voltage tends to hold the current down some. I put a simple push switch in so I could better time the application of current. An argument could be made that this is not strictly necessary but it also keeps the thing from shorting out when you lay the probe across the end of your screwdriver. The probe is made from an old set of tweezers broken apart and separated by a wood block of some sort. It ain't fancy. And it certainly ain't complicated!
You don't really need any kind of indicator that current is flowing. You can kind of feel and hear the thing in operation.
They are not, nor were they ever intended to be, a cure-all device. Nor are they the magical fix for the Dreaded Verdigris. They do one thing, and that only: they heat the center pin and dry out flange felt. In the process they tend to shrink and "iron" the felt some. Used correctly the effect is frequently long term.
Like most tools it takes some getting used to. Yes, it is possible to burn the wood in the flange fork. But, using a 6.3 vas transformer, it does take some doing. I do remember encountering a few actions in which using the zapper did loosen the pin in the fork but these pins were already on the loose side and--though this is a guess on my part--I suspected the wood was a bit on the soft side. Not every action builder used hard maple, you know.
I stopped using it, not because it didn't work, but because I rarely, these days, work on actions with original action centers. If I come across an action with a bunch of sluggish action centers that needed a quick and cheap fix I'll probably start hollering at Ryan to get the thing back.