[HN Gopher] Show HN: 32V TENS device from built from scratch und...
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Show HN: 32V TENS device from built from scratch under $100
Author : autonomydriver
Score : 61 points
Date : 2025-11-17 15:06 UTC (4 days ago)
(HTM) web link (littlemountainman.github.io)
(TXT) w3m dump (littlemountainman.github.io)
| dmitrygr wrote:
| > down to ~3.0V (discharged).
|
| That is NOT how one uses lithium batteries one foe snot want to
| go boom. Consider 3.6V as empty. Discharging them down to 3.0 can
| cause them to go boom when recharged...
| cenamus wrote:
| Of course you can go down there? That's literally the lower
| limit of design voltage?
|
| Below 2,5V is usually when you don't wanna use them anymore
| dmitrygr wrote:
| only if you want them to lose capacity fast and become spicy
| pillows in your lifetime
| clumsysmurf wrote:
| 2.5V is the absolute minimum typically mentioned in spec
| sheets, 2.8V and above is perfectly fine.
| kees99 wrote:
| Most cell OEMs will specify safe discharge (low threshold)
| voltage in a datasheet. 2.75V is quite common [1].
|
| That being said, system designer might choose higher cut-off
| point, since:
|
| 1) charge/discharge curve is S-shaped. There is very little
| energy in that last few millivolts;
|
| 2) battery (protection) circuit, and/or battery itself probably
| have some small leakage current. However minuscule, over
| months/years on a shelf, even some nano-amps of leakage will
| add up. If you want device to survive that, you have to factor
| this in, so that rest cell voltage still stays above safety
| threshold even _after_ storage.
|
| Also, "Li-ion" is quite a wide category. Don't use arbitrary
| voltage as a fast rule. Look up datasheet, or characterize
| actual cell you use. For some[2], disconnecting at 3.6V would
| mean leaving 50% of capacity unused. For other[3], that would
| be a reasonable, if somewhat conservative threshold.
|
| [1] https://docs.rs-online.com/080b/A700000007848112.pdf
|
| [2]
| https://www.murata.com/-/media/webrenewal/products/batteries...
|
| [3]
| https://ntrs.nasa.gov/api/citations/20140005830/downloads/20...
| (page 4)
| Kirby64 wrote:
| 3.6V is considered the nominal voltage, certainly not the low
| end cut off.
|
| 3.0V is considered basically the highest voltage. Most
| chemistries suggest even lower, 2.8, or even 2.5 in some
| situations assuming you can control the cutoff carefully.
| Perfectly safe to do so. You only start to have issues when
| you're south of 2.5 without a load.
|
| Most advanced battery usages let the cells drop even below that
| during heavy load.
| shadowpho wrote:
| >3.6V is considered the nominal voltage, certainly not the
| low end cut off.
|
| This is not right (3.6v certainly is and can be cut off
| depending on device and battery).
|
| One thing you are not considering is discharge after the cut
| off. Fuel gauge, protection circuitry, the cut off circuitry
| and battery itself has some discharge.
|
| So you don't want to have the cut off being too low because
| then the battery is permanently dead after not using it for X
| period of time.
|
| You want to leave some margin there.
|
| Depending on product, battery chemistry and design I have
| seen cut-off at 3.0-3.6v.
| Kirby64 wrote:
| Anyone setting cut-off at 3.6V either is using it in some
| insanely industrial, ludicrous application where you need
| to handle cases like multiple years in storage... or
| doesn't know how to properly design their protection
| circuitry.
|
| The margin is already there at 3.0V. You can still recharge
| batteries discharged below 3.0V. It just becomes dicey
| below ~2.5V.
| shadowpho wrote:
| >Anyone setting cut-off at 3.6V either is using it in
| some insanely industrial, ludicrous application where you
| need to handle cases like multiple years in storage... or
| doesn't know how to properly design their protection
| circuitry.
|
| It really depends on application, battery size and
| leakage. In consumer world of electronics for example
| there's an often requirement to make sure device turns on
| after being on a shelf for 1/2 - 2 years.
|
| Then when you do the math it ends up needing to set the
| limit to 3-3.6v.
|
| >The margin is already there at 3.0V. You can still
| recharge batteries discharged below 3.0V. It just becomes
| dicey below ~2.5V.
|
| The margin isn't big enough for some products.
| Furthermore some of the more leading edge batteries (in
| terms of energy density) have higher leakage which
| requires having more margin.
| dmitrygr wrote:
| ^^ this
| lambdaone wrote:
| Monophasic waveforms are generally considered less safe than
| biphasic waveforms. That's why many TENS units have an output
| stage based around a pulse transformer, so they can deliver two
| pulses, one in each direction, shortly after each other. Leaving
| this out seems to me to be a false economy when you've gone to
| all the effort of building the rest of the system.
| clnhlzmn wrote:
| I see
|
| > Each electrode channel is tied to a finger pad
|
| but the layout shows the finger pads are all tied together. What
| am I missing?
| sowbug wrote:
| My trusty TENS device has gotten me through a few bouts of
| whatever it is I have had for the last year (pudendal neuralgia,
| sciatica, pelvic floor issue, not sure). But it really is a pain
| in the ass (ha) to apply the sticky electrode pads.
|
| Has anyone ever seen something like a TENS shirt or shorts that
| has the pads built-in?
| ricardobeat wrote:
| That final paragraph smells of LLM. I might be becoming a bit too
| paranoid, but without any pictures or description of how well it
| works, can't tell if this is real or not.
| zhouyisu wrote:
| I tried to build some TENS device, but lost in "safety maze".
|
| I reverse engineered some existing device, there are TONS of
| safety measures.
|
| At least: current limiting resistor, transformer, voltage/current
| feedback, GND isolation, MCU protection.
|
| After replicate all of these, I still not brave enough to try it
| myself, I just find it too dangerous.
|
| PS: TENS device is fun. article seems like a bait.
| NoSalt wrote:
| This is cool, and all, but you can buy a TENS unit for $20.00 to
| $40.00 on Amazon.
| cheschire wrote:
| Tangentially related homebrew effort by Basically Homeless to
| built an aimbot using similar tech.
|
| https://youtube.com/watch?v=9alJwQG-Wbk
| adolph wrote:
| I wonder if this is being used irl. Seems like the next thing
| would be to attach it to a digital scope like the below.
| Connecting the low latency of digital compute to the durable
| self-powered biologic actuators of a human seems like a natural
| evolution.
|
| https://www.youtube.com/watch?v=DCp8-tc0dfY
| bn-l wrote:
| What do people use this for?
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(page generated 2025-11-21 23:02 UTC)