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I designed a PCB that includes capacitive touch. The circuit works with 5V powered from USB.
When Laptop is connected to 220V line, the PIC can sense capacitance variation.
When Laptop is not connected to 220V line(self battery powered), the PIC can't sense any capacitance variation.
enter image description here

enter image description here

I only add 2 bypass capacitors to the input. I did not do the ground design of the PCB very carefully.

Is the source of the problem PCB ground design? Is it error in the 5V input circuit design? or what else?
What would you suggest me for solving the problem? Can I fix the problem by adding some components to the 5V line?

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    \$\begingroup\$ Try touching the USB connector shield with your other hand. \$\endgroup\$
    – Jeroen3
    Jan 30, 2021 at 13:47
  • \$\begingroup\$ I think touch can require sensitivity calibration and proper pcb layout (ground). I think you're not supposed to have ground plane on either sides of pcb where you touch it. Did you check the datasheet for the MCU or touch ic (if you have a dedicated touch ic)? \$\endgroup\$
    – Ilya
    Jan 30, 2021 at 13:55
  • \$\begingroup\$ There are ground plane only bottom side, not top Side. Absolutely you are right, but i need a tempotary solution maybe adding components or what else. \$\endgroup\$
    – Berker Işık
    Jan 30, 2021 at 14:12
  • \$\begingroup\$ Make it look like this —| ☝🏻|— \$\endgroup\$
    – Tony Stewart EE75
    Jan 30, 2021 at 21:02
  • \$\begingroup\$ @Jeroen3 I use TTP223 now instead of PIC itself. I faced the point which you say. When I contact PCB GND with my other hand, TTP223 can sense. Without touching PCB GND, it doesn't work. How can i solve this problem? Thanks. \$\endgroup\$
    – Berker Işık
    Feb 4, 2021 at 15:59

1 Answer 1

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For a touch sensor to work it must increase the capacitance significantly between signal and return.

For a localized pair this means the existing capacitance must be low. If the terminals are coplanar , you might be adding at least 50pf to 5pF with no ground plane under it, then when bridging the surface air gap, you don’t even have touch both contacts when coated with soldermask. If you have two spiral terminals closely spaced, you can touch anywhere inside the spiral.

For an isolated terminal, it relies on your return path to ground return.

Using a higher kHz frequency naturally lowers the impedance of all localized and finger capacitance equally, but makes it easier to shunt stray E-field noise with a smaller load resistor.

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