I want to measure the electrical resistance of my fingertip. I'm building a thimble with a few sensors and one is this resistance probe. My multimeter tells me the resistance is around 2 MΩ. A simple voltage divider might not work. So I tried this circuit: [opamp] A voltage divider feeds 1.2 V to…

[image] Johan_Ha: And I get values around 800 when I touch X. You even can short-circuit X and get the same voltage as provided by your voltage divider at the + input of the opamp. Rule of thumb. the opamp tries to keep its inputs at the same voltage. [image] Johan_Ha: resistance is arou…

[image] Johan_Ha: Are the resistors in the MΩ magnitude too large for opamps? Which op amp are you using? [image] Using Operational Amplifiers in your Arduino project General Electronics 5: Choosing the right Op Amp for your application For simple applications you…

[image] johnerrington: Which op amp are you using? UA741CP datasheet Yes, I have noticed how the measured values vary a lot. That's why I have to focus on getting a good design on the thimble, which should fit perfectly on my finger. It squeezes gently around my ring finger, when I put it on…

[image] Johan_Ha: uA741CP Will not run from a single ended 5V supply.

[image] Johan_Ha: I want to measure the electrical resistance of my fingertip. A resistor/resistance needs two connections, so between your finger and what? There is more resistance from one hand to the other than between two fingers on the same hand. And yes, it WILL vary a LOT depending…

[image] johnerrington: Will not run from a single ended 5V supply. What do you mean? It already does run nicely. 5 V and GND is connected to the opamp, as well as + and - and out. [image] DVDdoug: between your finger and what? Between two spots, 10 mm apart from each other. They are t…

[image] You need to change the op amp to one that is suited to working single ended from a 5V supply. UA741 is not.

You mean an opamp which works with 5V at V+ and GND at V-?

[image] johnerrington: You need to change the op amp That's not a requirement but a meaningful modification of the circuit. Nowadays, 50 years after the 741, 5V rail-to-rail opamps are available and almost industry standard.

[image] Johan_Ha: What do you mean? It already does run nicely. 5 V and GND is connected to the opamp, as well as + and - and out. The 741 is not spec'd to run from 5V. It certainly won't swing its output rail to rail (or anything like it). A modern low voltage rail-to-rail opamp would be t…

Resistance at fingertip

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johnerrington April 15, 2022, 6:18am 3

Which op amp are you using?

Also the resistance you measure will depend on the size of the electrodes, the contact pressure, and how dry/damp your finger is.

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Resistance at fingertip

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