As a bit of a weird test for my new 555 IC, I have been thinking of stepping down a 9V PP3 battery to five or so volts and creating a square wave @ ~100Hz and shocking my finger.

I understand that a possible breakdown voltage of dry skin is fairly high, in respect to five volts, so my question is: Would the sensation of shock of a non-DC current be across the surface layers of the skin? Would I need to put an "easy" path to ground partly up the finger for it to work at such a lower voltage? I assume related (in terms of contact), how would those silly prank shocking devices achieve this?

I will attempt to limit everything to the lowest currents, the main intent is just to "feel" wave forms and frequencies, likely not that different, an interest of mine however.

  • 4
    \$\begingroup\$ You can feel 5 V in your mouth. Not that I've run a sound card output into my lips to see what different frequencies feel like or anything... \$\endgroup\$
    – endolith
    Oct 22 '11 at 14:27
  • \$\begingroup\$ you can feel 12V on your skin if it's wet with salty water. \$\endgroup\$
    – JustJeff
    Oct 22 '11 at 20:16

WARNING - Note that used improperly the equipment described below could cause death (or just possibly worse). read ALL the safety warnings available before playing.

One man says:

01-18-2004, 09:35 PM

Be real careful here. I design medical grade TENS and MicroTENS systems as well as defibrillators and if you don't know what you are doing you can cause injury or death. Improper safeguards can cause burns, muscle damage, even cardiac arrest. I would be very leary of following any schematics found in magazines or the internet. If you do venture into the endeavor make damn sure you use a medical grade or UL544 type isolation transformer if you plan on running the device on mains power.

You are unlikely to achieve sensation at 5V without the use of conductive pads, such as are made for ECG use - or in the skin probes if you are VERY keen (and not too smart?) and maybe not even then.

What you describe is often done using a so called "TENS Unit".
"T.E.N.S." = Transcutanaceuos neural stimulation.
These units usually produce voltages in the 10s of volts to 100+ Volt range.

Here is a circuit of a 555 based TENS unit. Note that they use a 10:1 stepup transformer AND the way it is used it can produce voltages of MORE than 10 x 9V due to "flyback action.

enter image description here

The article is from Elektor mag. They say:

How hard can it be to make such a device ourselves? The simple circuit uses a CMOS 555 timer to produce a brief pulse which feeds a 1:10 miniature transformer. Together with a 4.7 nF capacitor the transformer makes a parallel resonant circuit: the resonance leads to a considerable increase in the output voltage. The pulse width can be adjusted using a potentiometer, here shown combined with the on-off switch. Wider pulses produce higher output voltages. Since a peak voltage of up to 200 V can be produced, the transformer must have adequate insulation: Conrad Electronics type 516260-62 is suitable. A low-cost phono socket at the output gives reliable connection to the electrode cable.

The adhesive electrodes shown in the photograph (disposable and permanent types are available) can be obtained from pharmacies and medical suppliers. They generally have connectors compatible with 2 mm banana plugs, and so it is possible to make up the necessary cable yourself. To treat responsive parts of the body, such as the arm, the potentiometer need not be turned up far to obtain the necessary sensation. Less sensitive parts, such as the knee or foot, need a rather higher voltage and hence a correspondingly higher potentiometer setting.

Author: Klaus Rohwer - Copyright: Elektor Electronics Magazine


Several circuits not marvellous.

This circuit is from the above page and is notable in using a single winding flyback inductor rather than using a transformer.

TENS article scanned pages

Good discussion page - same as the warning one in first paragraph.

There is ongoing debate about the genuine effectiveness of TENSS units despite a vast user experience base and many peer reviewed scientific papers demonstrating that they do and don't produce statistically significant results. If they don't work for you you could try using the unit as a pipe descaler or pulsed battery charger ;-).

  • \$\begingroup\$ +1 just for showing people that, at least in some cases, you can use a battery without a regulator. \$\endgroup\$
    – JustJeff
    Oct 22 '11 at 20:17
  • \$\begingroup\$ This is a step in the right direction if I were to take this as a project. I really am interested in seeing/trying/making a simple TENS unit one of these days. \$\endgroup\$
    – Hobbyist
    Oct 23 '11 at 6:18

It's going to be hard to make something you can feel with a finger at 5V. As you say, it's all about skin resistance. Once inside the body, the current sees much less resistance.

I think what you're asking is whether you can use high frequency to capacitively couple thru the skin to get inside your finger where you can feel something. I don't know, I've never tried that deliberately. I have, however, touched many many 5V circuits with all kinds of waveforms active and never felt anything. I suspect the little current you can get from capacitive coupling is too small to feel. At really high frequencies, the skin effect dominates so the current will stay on the surface of your body anyway.

If you want to feel something at 5V, try making your finger wet. Just make sure that the current entry point and return point are well localized to your hand or someplace the current can't flow across your heart or brain. That could be bad, even at 5V with the right coupling (wet and salty skin). It's ultimately current that hurts you, not voltage. That's why it takes less voltage to cause trouble when the skin is wet or it is otherwise well coupled into your body.

If you just want to see what a mild shock feels like, touch the 9V battery to your lip briefly. That's usually moist enough to allow enough current to feel. Touching it to your tongue will definitely let you feel it. That's a quick old timer's trick to test a 9V battery. The feel of a good 9V battery on the tongue isn't pleasant, but I've never heard of it doing damage.


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