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I’ve been searching around in a lot of Arduino communities, but have not found a comprehensive multimeter with leads much like a DMM. I am trying to make an ohmmeter that can safely measure the resistance of a finger. Can someone point me in the right direction for this or better yet walk me through the process? Thanks in advance

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You have two options:

  1. Provide a constant current and measure the voltage drop across it
  2. Include the finger in a voltage divider.

The second option is the simplest since it just requires a resistor.

According to this site "A calloused, dry hand may have more than 100,000 Ω because of a thick outer layer of dead cells in the stratum corneum."

So a "high" resistor in that sort of order of magnitude with the finger as the "low" resistor, and measure the voltage at the junction between them.

schematic

simulate this circuit – Schematic created using CircuitLab

Depending on what you are trying to do the attaching of the probes to the finger may be the tricky part.

DO NOT use sharp probes that may pierce the skin. The internal resistance of the human body, owing to all the wet squishiness inside, can be around 300 ohms. Excess current can cause subcutaneous burns and even death if applied to the wrong place.

Instead you need to create some electrodes that lie across the skin. The greater the surface area the better the electrical contact and the more accurate your results.

Metal foil placed on the finger and held in place with hypoallergenic ("MicroPore") tape is the closest to proper dermal electrodes that you can easily make. This gives good contact without any risk of penetration.

Don't be tempted to use self-adhesive metal foil tapes, for two reasons:

  1. While the tape may be conductive, the adhesive often isn't, and creates an insulating barrier between the tape and the skin.
  2. The adhesive is not designed for prolonged skin contact and may cause an allergic reaction in some subjects.
  • could I use an INPUT_PULLUP pin instead of +5V+R1 in the above? – dandavis Jan 7 '18 at 20:48
  • Not really, no. The internal pullup is "somewhere in the region of" 30k to 50k (ish). You need to know that resistance to calculate the finger resistance, and you don't know that resistance. – Majenko Jan 7 '18 at 20:49
  • I hoped to calibrate w/a 50k instead of a finger, but if that pullup is dynamic it obviously won't work. thanks. – dandavis Jan 7 '18 at 21:01
  • Is there no chance of electrocution with metal foil as long as the first resistor is high enough? And would the same configuration be sufficient if I were calculating the resistance of a palm instead of a finger? – rafferino Jan 7 '18 at 21:14
  • No chance, and yes. – Majenko Jan 7 '18 at 21:15

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