Page BottomI had a bad fall ~1985, into a building excavation, and then the wood heater fell onto me. This resulted in an immeadiate huge bruise, and back and spine damage. Some doctor somewhere sent me to physical therapy, and they used a transcutaneous electrical nerve stimulation (TENS unit) device (wikipedia link 
), which did help a little. They cost over $1000 each to buy (and rental was also pricey), and seeing i was out of work and my job had retroactively terminated my health insurance, there was no way to take one home. So i set about making one. And then another. And with no internet in existance at the time!
The first one i made is on the right. It's the basic low end model, in the $500 bracket, with single set of paddles, variable frequency and intensity, three rechargeable batteries. It worked fine, but the problem is the body seems to remember, and even anticipate an uninterrupted constant pulse train, so it's effectiveness diminishes over time. This occured with all TENS of this basic type.
The second one i made is on the left. It fires two sets of paddles, in a very long pseudo-asynchronous pattern. It also uses a full H-bridge driver on each paddle so i could reduce the number of rechargeable batteries to two. Each set of paddles has a freqency and intensity knob, and a switch to go from constant to the random(ish) intermittancy. This one would have been a $1000+ unit if it was medically certified and had a doctor prescription on it.
Just an assortment of pics. I was storing these in a dirty space, made no attempt to clean them up. Excuse the rats nest wiring, i was broke, disabled, and in a lot of constant pain. I am not going to spend time now cleaning these for show on the internet, because they are 23 years old, i have no paddles for them any more, and i have other things to do. Chances are, i'll be pulling the knobs off and tossing the rest.
You can see the size relative to the "9 volt" rechargeable battery. Every part there, case, batteries, knobs, electronics, all came from Radio Shack for under $25 total each unit. The paddles were a lot more money. I had two rechargers, one to plug into the car lighter socket, one for 120vac wall outlet.
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