Measuring Skin Response

[Johnny1010]

Hi everyone!

I am trying to measure the skin conductance, for that I found this circuit online.

So from it what I have understood so far:

- There is a low pass active filter at first stage with cut off of 1.6 Hz and variable gain.
- The second stage is also a filter with unity gain and is biased at half the voltage supply and the first stage is biased at the output of second.

What I don't understand:

- Why is the gain variable at first stage as the Rskin changes the gain would change?
- Isn't the first stage enough?
- Can't understand the biasing of the op-amps as well.

 

[chemelec]

Your Image Does NOT show.

 

[Johnny1010]

Here is the link to it
https://obrazki.elektroda.pl/1838370700_1446817605.png

 

[chemelec]

The Biasing looks correct for an LM358, Single Supply op-amp.
Yes, the pot will change the gain.

Skin conductivity will also change, Depending on your Skin type, Moisture Content and Size of the Contact Area.

Without actually building it, I can't say How good this circuit is!

 

[Tony Stewart]

This is a classic inverting gain design with negative DC feedback on the (+) input. so that the galvanic skin response is neutralized.

https://en.wikipedia.org/wiki/Electrodermal_activity

Capacitance measurements use ac signals to accomplish better accuracy of conductance and is also used in soil moisture meters.
DC methods have an inherent problem form electrode chemical voltage and motion artifacts that generate varying DC.

 

[Johnny1010]

Tony Stewart are you talking about the neutralization of the skin conductance level i.e the tonic EDA? Why not bias the first stage at say 1.5V ans use just the single stage.

 

[Johnny1010]

chemelec what could be the problems with making a simple voltage divider followed by a low pass filter like this one

 

[chemelec]

chemelec what could be the problems with making a simple voltage divider followed by a low pass filter like this one
View attachment 95344

When you say "skin conductance", Do you mean: Human Skin?

E1, Sine Wave?
What Frequency?
What is the device that has Pin 34 and Pin 40?

 

[Johnny1010]

chemelec Yes, I mean the electrodermal activity that is the change in the conductance of the human skin due to the presence of sweat for instance or any sort of emotional arousal.

Let's assume E1 to be DC, 3V for instance.

Pin 34 and 40 are nothing it just shows the points where I'll be taking the output.

 

[chemelec]

Why not just use an Ohm-meter.
But Regardless of what you use, you need a GOOD Contact Points.

CONTACT RESISTANCE is Your Greatest Problem.

If for example I hold my Ohm Meter Probes, one in each hand with DRY Skin, I get a reading in the High Meg-Ohms.
If I wet my Probe to get Good Contact, I get a Reading in the LOWER, Thousand of Ohms.

If the Skin has Sweat on it, It will have a High Salt Content, Resulting in a Very Low Surface Resistance.
The Resistance Measurement will be Mostly from that Moist Salt, Not the Actual Skin.

 

[Johnny1010]

Yes, you are right but that moist salt will be produced by the skin and in situation of an emotional arousal like it used in lie detectors. I want to make a wearable kind of a thing to measure thus response so ohmmeter is not a good choice.

 

[chemelec]

How Far Apart are your Electrodes?

 

[Johnny1010]

The location of the electrodes would be the ventral side of the distal forearms and the electrodes would be 1-2 inches apart

 

[chemelec]

OK, So What Method of Detection do you want?
A Meter, A Sound Change or ??

If this is actually for a LIE Detector, it is Not very reliable!