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Thread: Blinking LEDs interfering with MQS output on Teensy 4.1

  1. #1
    Junior Member
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    Mar 2018
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    Blinking LEDs interfering with MQS output on Teensy 4.1

    Hi,

    I'm playing with some Sine waves through the MQS output that I set up using the Audio System Design Tool and also blinking 2 LEDs.

    When the LEDs are connected, I can hear a click/pop when they blink.

    I'm using pins 2 and 3 for the LEDs and I'm using elapsedMillis rather than a delay fo blink them.

    Is there some way to mitigate this?

    Thanks

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Nov 2012
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    A very first step is to try to test whether the problem is actually with the MQS signals coming out of the pins, or if the problem is something like a ground loop causing an amplifier to hear a change which isn't really part of the original signal.

    I'd start by just removing the LEDs or whatever resistor is connected in series, maybe even while it's running. If the problem goes away when the exact same thing is running but no current flows through the LEDs, odds are very likely it's a ground loop problem.

  3. #3
    Junior Member
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    Mar 2018
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    Thanks!

    I had the 3.5mm output jack and the LEDs grounded to the same pin of the Teensy; I now have only the jack grounded to that pin and everything else digital grounded to a different one and it works perfectly.

  4. #4
    Senior Member
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    Jul 2020
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    The problem was the LED current was sharing the same ground wire/connections as the audio signal. Wires have resistance
    (a few milliohms per inch or so, but depends on the gauge of course), connectors often have much more (10 to 100milliohm is
    possible).
    The resistance converts the LED current into a voltage superimposed on the audio.

    Audio is particularly troublesome as the ear is very sensitive to unwanted noises, even 60dB down from the wanted signal.

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