Many Tens units are designed to numb nerve endings. This is why they are used in pain management. They are a quick into to estim, but hardly designed for pleasure. Try a made for play box like Erostek 312/232, PES electro, or the many DIY solutions
Sorry waldowheres, that isn't true, quite the opposite in fact. TENS means Transcutaneous Electrical Nerve Stimulation and not numbing nerve endings. In use, TENS electrodes are rarely placed at nerve endings but rather between the site of pain and the brain. They are designed to excite the nerves, with stimulation which alters the signals from the site of the pain. They do not remove the cause of the pain though the effect lingers for undefined and variable periods of time. See this extract from Wikipedia:
Transcutaneous electrical nerve stimulation (TENS) is the use of electric current produced by a device to stimulate the nerves for therapeutic purposes. TENS by definition covers the complete range of transcutaneously applied currents used for nerve excitation although the term is often used with a more restrictive intent, namely to describe the kind of pulses produced by portable stimulators used to treat pain.
The typical stimulators marketed and used for sexual stimulation are little different than TENS machines except that they might have a greater range of waveforms, programmes etc; than the cheapest of the TENS machines. However a machine like mine, the Sanitas SEM40, for £20 (GBP) has 50 programmes, 30 of which are pre-programmed and 20 which can be individualised, infinitely variable output frequency from 1 to 120 Hz and infinitely variable pulse widths from 40 to 250. Additionally it is two channel with guaranteed channel separation, essential if you must cross the heart which must be bi-polar anyway to each side. I'd say, and my experience of it suggests this, that that is a pretty comprehensive specification. It even anticipates a pad or electrode dropping off which would normally give quite a jolt at the parting, by shutting down automatically.
I said above that the 'sex machines' are little different than a TENS machine. That is true as far as the electronics are concerned. Where the 'sex machines' score is in the appearance of the unit with very professional and maybe menacing looking knobs and switches. But doesn't my Sanitas SEM-40 look kinda good too, in my album via my Profile page? Where the TENS machines score is that they give the same effect for hundreds of £s, or $s less!