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I want to do something similar to this question: PCB "Touch" Button

However there the PCB was not described that much. I have a design where each 'finger' is 1mm apart.

enter image description here

And I'm not sure should I make the finger contats larger to allow better contact with the skin, or rather smaller but more frequent to go over smaller distance of the skin.

My gut feeling the increased contact path with larger fingers will not overcome the disadvatange of covering longer length through the skin. So smaller more frequent fingers should be better?

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    \$\begingroup\$ Eventually dirt, grease, oxidation and whatever will add to the surface and it will stop functioning. That is probably why all cheap keypads you find in the world adds a mechanical barrier that adds some form of protection and consistent connection. \$\endgroup\$
    – Dejvid_no1
    Commented Sep 3, 2022 at 21:35
  • \$\begingroup\$ Mechanical barrier, you mean soft pad with carbon so then the finger is pressing the button from the outside, while inside has the carbon making the connection even easier? I'm willing to clean this button once a week, if the button will be reliable within one week then I'm fine with this flawed design. \$\endgroup\$
    – Anton Krug
    Commented Sep 3, 2022 at 22:26
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    \$\begingroup\$ No problem. But hey I found metal domes on Digi-Key! Search for 5133TR for example. Hope that helps. :) \$\endgroup\$
    – Dejvid_no1
    Commented Sep 4, 2022 at 16:04
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    \$\begingroup\$ Thanks, I have these stock in person, but wanted to practice this comb fingers style just to do it for sake of doing and in process educate myself on why exactly they are not videly used anymore. \$\endgroup\$
    – Anton Krug
    Commented Sep 7, 2022 at 12:03

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For resistance it depends on pressure, conductor and moisture of skin over the surface area. There may be a preference you have for pressure which affects contact area, so you may experiment with a DMM and record your range of values for dry and oil skin vs pressure for a fixed probe contact area. Don't use the sharp tips which will conduct thru the subcutaneous layers which have lower ionic resistance. These experiments may help. But essentially more trace width and area of contact will reduce resistance of skin, while closing the gap for insulation and contamination. The untinned copper will oxidize so you may choose to DIY etech or use organic gold plating or tinplate or hot air solder leveled HASL.

The reference circuit will work with a wide range of pressures and skin moisture levels. But for one person to use, less variability. enter image description here

There are endless options for patterns but you want the finger capacitance to be much large than the trace capacitance. A 100 pF is nominal value for a finger tip ( from the HBM spec) unless you push hard, then x10. Sensitivity is up to you to define. Impedance is also up to you when you choose the frequency.

If you make the circuit signal to idle ratio 4:1 minimum with 1:1 hysteresis then you may want to define 100 pF for the finger tip and idle capacitance of 25 pF between traces. Geometry makes a difference and length of path for total surface area of finger to conductors.

It makes perfect sense to experiment and measure with a relaxation oscillator different pad designs for input capacitance and use a 1M feedback resistor with a good solder mask for insulation.

Saturn PCB toolkit V8.21.exe can tell you this in pF /cm for any coplanar gap and track width with a ground plane or not with some imagination.

You can make it a round target or two coplanar spirals or interleaved forks.

For contamination control, you can use a solder mask over the whole surface.

Carbon keypad buttons use a resistance which degrades over time. Your finger capacitance depends slightly on moisture but more on pressure and is very sensitive at higher frequencies used across the pads but no RF is need)

I have seen some students use pads as a DJ sound control and use rotary finger patterns to control volume or linear control for crossover and not just ON/OFF.

So use your imagination to define a spec, simulate it, make it, test it and measure it. Then repeat until perfect. (Or copy someone else's recipe)

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    \$\begingroup\$ Surface contamination varies greatly while capacitance does not. If you can handle the power consumption of an oscillator running at say 20kHz, then use the resistive method with larger gaps, smaller tracks. All it takes is 2 inverters 2 large resistors (10M, 10k) and 1 small cap \$\endgroup\$
    – D.A.S.
    Commented Sep 3, 2022 at 22:41
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    \$\begingroup\$ on the other hand complexity is increased to compare targets for moist surfaces, so that's another form of contamination but high resistance until dust accumulates.. So it depends on how reliable you want your sensor. Like consumer goods with glass surfaces and wipe detect suppression software or any old switch for Wake up and auto sleep. A FET or CMOS will also need ESD protection. If you used stray hum and a Darlington , you could detect stray signals with touch or even a handwave on a fixed installation. \$\endgroup\$
    – D.A.S.
    Commented Sep 4, 2022 at 1:12
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    \$\begingroup\$ Your pattern could also be a checker board with pads in each square all fed by bottom trace vias.. With 2 detectors close together and swipe left or right and detect the sequence for two different functions \$\endgroup\$
    – D.A.S.
    Commented Sep 4, 2022 at 1:17
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    \$\begingroup\$ That will work fine and you can measure your range of dry to moist finger resistances with your comb like surface area to determine the Rgs value. But do add a series R and TVS protection or something for ESD. A cap also may reduce EMI if present. \$\endgroup\$
    – D.A.S.
    Commented Sep 4, 2022 at 12:14
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    \$\begingroup\$ 100 pF * 10k = 1 us, while up to 1 ms or 10k*100 nF would do more without switch delay being noticed \$\endgroup\$
    – D.A.S.
    Commented Sep 4, 2022 at 13:31

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