In theory, could I put electricity (low level, of course) through my skin? How about GND?
This might seem like an XY problem…
That is what those body fat scales do: they measure resistance.
But at what level would it be safe? Could I for example blink an LED with the LED’s GND through a wire and 3.3v through my skin?
You can search Google for the term “skin conductivity”. There are many hits, mostly scientific papers. E.g.https://www.researchgate.net/publication/38084094_Conduction_of_Electrical_Current_to_and_Through_the_Human_Body_A_Review shows a table:
Table 1. Estimated effects of 60 Hz AC currents∗
1 mA Barely perceptible
16 mA Maximum current an average man can grasp and “let go”
20 mA Paralysis of respiratory muscles
100 mA Ventricular fibrillation threshold
2 A Cardiac standstill and internal organ damage
15/20 A Common fuse breaker opens circuit
Although you are not dealing with 60HZ AC currents, the paper claims that it is the current, not the voltage that is the problem. So I would not try to blink an LED with the current flowing through your skin, because you are already in the critical range of 20mA.
But maybe you can describe what you really want to do. Very often, there is a different solution that has the same effect. E.g. measure resistance and then switch current depending on what you measure (without the current path going through your body).
That’s why I said that it’s an XY problem…
I wanted to have for example a small speaker such as this one by Pimoroni. I wanted to have one wire running to some device (like a Raspberry Pi Pico) and the other going through me. So you can’t put enough power for the speaker through your skin safely?
IMHO, your better off trying something like this,
Adafruit 8-Key Capacitive Touch Sensor Breakout - I2C or SPI
The amount of signal you would need to actually get sound from the speaker would not be safe.
Have a look see at this.
Power-Over-Skin Makes Powering Wearables Easier | Hackaday
Hacking Haptics: The 19-Sensor Patch Bringing Touch To Life | Hackaday
Thanks for the link. Very impressive. Too bad it is in the µW domain.
BTW: for the RP2040, you don’t even need a capacitive touch sensor, since the pins can do that on their own. You just add a wire and touch it. No idea about MicroPython, but CircuitPython supports this. This has been disabled for the RP2350 because of the E9 bug though.
I am using this for some projects: a simple wire from the board to the enclosure, and I have a hidden “sensor button”. Pimoroni also sells adhesive copper-tape, this helps if you need to create a larger active surface.
The Hackaday link looks promising. How would I actually wire the device receiving the power though?
I don’t know anymore about it than you do. I frequent the Hackaday site, saw that stuff, and posted it here for perusal. I haven’t done anything even close to what I linked too. I just thought it was cool and relevant. ;)
This is all research, nothing you can buy. And it is all extremely low power. So nothing for your use cases. Nothing for us normal makers.
But couldn’t I make it with just like some metal pads attached to me and some resistors to control how much goes through or something like that? Like just blinking an LED?