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I would like to switch the output of a tens unit (one of those things that uses electricity for pain relief) to 8+ different outputs. I want to create an electro tactile device. The idea is to switch the output of each lead from the tens device to 8 different channels. The channels would output to points held against the skin. Much like an lcd screen, each pair of outputs from the two sets of 8 channels would address an individual electrotactile pixel. I would probably want to be switching at ~1000hz. I've been looking at analogue multiplexers and switches, but they all seem to be limited to vcc from the microcontroller side. The tens device has a max voltage output of 7.3Vrms and a max current of 13.6mArms. Max frequency output is 150hz.

Perhaps at the low settings I want to be using the tens unit I don't have to worry about the vcc limit (5v)? If I take this approach how do I connect the two devices togethor, is it enough to connect the grounds?

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  • \$\begingroup\$ TENS units typically have output in 30-60V range. They are powered by 9V batteries but typically have a step-up transformer at the output. Anything less would not cause enough current to activate nerves through the skin. \$\endgroup\$
    – 42-
    Commented Apr 7, 2023 at 0:00

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I would like to switch the output of a tens unit to 8+ different outputs.

"Max frequency output is 150 Hz."

"I would probably want to be switching at ~1000 Hz."

Something is not right here. It seems to me that you think you can use one TENS machine to do the job of eight+ by switching fast enough. I can see a few problems:

  • You will no longer be applying a 150 Hz signal to the skin. You will be applying a 1 kHz signal modulated at 150 Hz.
  • The average current will fall to 1/8 of the current in correct use (if everything else goes well).
  • The TENS unit is probably running a constant current generator circuit. Rapid switching of the load will probably cause havoc with the internal regulation.

Note: SI Units named after a person are capitalised when abreviated and lower-case when spelled out. "V" for volt, "Hz" for hertz, etc.

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  • \$\begingroup\$ The current will be 64* less, since there are 8*8 skin contact points. But this shouldn't be an issue because the contact points are much smaller, you need much less current to illicit a sensation. I hadn't realised that, yup, I will be putting out a 1khz superimposed with a 150hz signal. But I'm not sure this will be a problem, I bet that it will still cause a sensation. I am however slightly concerned about your last point and current limiting. If the switching causes it to fail it might be possible to cause nerve damage. Do you have any thoughts on how to approach the problem differently? \$\endgroup\$
    – Marcus
    Commented Apr 21, 2016 at 6:54
  • \$\begingroup\$ Sorry, I don't know. I did read a question here some months ago about, I think, a tactile glove or trying to sense human fingers closing but I can't remember enough to find it. \$\endgroup\$
    – Transistor
    Commented Apr 21, 2016 at 17:18
  • \$\begingroup\$ Ok, thank you for the time. You have made me realise this needs more thought and a more considered approach. I just wanted a quick proof of principle prototype before investing more time, but I see now that it's unwise. \$\endgroup\$
    – Marcus
    Commented Apr 28, 2016 at 7:14
  • \$\begingroup\$ Thank you for taking the time to update. You've had the 1% inspiration and now you're into the 99% perspiration stage. :^) \$\endgroup\$
    – Transistor
    Commented Apr 28, 2016 at 7:15

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