Make your own TENS

Transcutaneous electrical nerve stimulation or TENS is a technique in which electric pulses are applied through the skin in zones of chronic pain, such as injured joints or muscles. It provides a fast relieve with little secondary effects. It is usually recommended for low back pain, producing amazing results. The pulse generators are fairly cheap, with several models under 100 bucks. You can also make your own at a much lower cost. I found on the internet a schematic by Klaus Rohwer in extreme circuits:

The 555 is used to produce a pulse of variable width, that is controlled by a potentiometer, at a frequency of about 100Hz. This pulse is fed in a transformer. I could not find or build the suggested transformer, that is 2.8 henryes, so I picked a transformer from 220V-12V converter, and used it inverted. Used like that it is a 1:20 transformer, but it makes the job. At the output a capacitor is used to create an LC oscillator of about 2KHz. this is important because the applied signal must be AC to guarantee recombination of possible electrolysis products. I tested the signal on myself and found it to be a bit asymmetric, i.e., one of the electrodes itched more than the other. This improved by replacing the 4n7 output capacitor by a 10n, that diminishes the peak voltage. For equalized outputs a transformer with 2 identical secondaries is required (working on it). For the PNP transistor I used a 2N2907 instead of the of the BC556 and for the diode the 1N4007, and added a 1K resistance in series with a led in parallel to the primary of the transformer to observe a light of bright proportional to the pulse width. I made my own electrodes with copper foil (best results with thick foil), but I recommend to buy them.