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Thread: Grounding issue?

  1. #1

    Grounding issue?

    Hi all,

    I think I have a ground problem. I have a Teensy 3.2 and audio adapter set-up as seen in the image. The touch pads work when plugged into USB power or if i touch a ground connection or some parts of the circuit (the 1/4 jack socket, the side of the screen etc).

    I did read in another post Paul asked what happened when you touch a wire (gently) from the ground on the teensy to the ground on the audio output (in this case the mono audio amp out to the speaker) and this make the circuit crackle and die.

    Is there any way to make this work without adding a ground wire that you have to touch as well as the pad or doing capacitive sensing with 2 pins instead of the touch pins?

    What am I missing?

    Thanks,

    Asha

    Click image for larger version. 

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  2. #2
    Senior Member+ Theremingenieur's Avatar
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    One path could be operating the circuit not from batteries but from a wall plug adapter which provides loose but enough capacitive grounding for stable operation.
    The RF based capacitive touch engine of the Teensy works similar as a 1940s heterodyning radio which also needed an extra ground connection for good reception.
    If you can‘t provide extra ground and you must stay with batteries, the state of the art solution would be to work with split electrodes in a form that you easily touch always both halves. The second half of all touch electrodes could be connected together and linked to the Teensy GND via a 4.7nF capacitor.

  3. #3
    Thanks for the reply Theremingenieur and sorry for the delayed response!

    Unfortunately I need the battery powered option and it is not an option for me to touch both halves (unless I could attach two wires with one to ground via a 4/7nF cap to the copper pads I am using on the underside?). The end result has to be a stand-alone box that only requires the touching of the pad to trigger a note.

    I have seen this video that uses the Adafruit 12-Key Capacitive Touch Sensor Breakout - MPR121 and I am wondering if I should switch to that module as the device created in the video is battery powered and requires touching of the pad.

  4. #4
    After some fiddling of the numbers I got it to work as it was above. The code I based it on was from this tutorial. In the code it has values that it checks to trigger the pads with the Teensy touch.Read. I had to tune these to my specific pads size and material (it does say to do this in the code but I am sometimes a bit blind to plain instructions it would seem). I made a bit of calibration code to print out all of the touch reads to the lcd screen and the serial port to see the differences in values when connected to the laptop vs just batteries and changed the values in the code accordingly. Anyways, it seems to work ok for now!

  5. #5
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    I have always used touch without connected to a specific ground. Battery powered isolated on a desktop, teensy 3.2, all 12 working, touch pads 5 mm dia, touch any pad and it works using code like ..... touchread padpin > trigval means touched, and touchread padpin < trigval means not touched.

  6. #6
    Quote Originally Posted by Teenfor3 View Post
    I have always used touch without connected to a specific ground. Battery powered isolated on a desktop, teensy 3.2, all 12 working, touch pads 5 mm dia, touch any pad and it works using code like ..... touchread padpin > trigval means touched, and touchread padpin < trigval means not touched.
    That is what I got going now, and I am using 2 AA batteries which seem to be doing okay also! It was all about calibrating the pads trigger values, my pads are 50mm square pieces of copper embedded in wood connected via jack socket which then runs to the teensy 3.2 pins. I have 8 pads at the mo.

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