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Interference into amp from LEDs

Discussion in 'LEDs and Optoelectronics' started by jabelone, Nov 6, 2013.

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  1. jabelone


    Apr 14, 2013
    I have created a speaker system with a cheap amplifier circuit that I soldered together. It is being powered by the VIN pin of an Arduino (9v straight from the AC wall adapter). It is also being grounded through the Arduino's ground. The circuit works great, with no interference. On the arduino I have code that flashes and fades the LEDs through various patterns. The LEDs are being powered by 3 2N2222 transistors because they draw to much current. The LEDs are also grounded and powered through arduino's VIN and Ground.

    While the arduino is running the code but there are no LEDs plugged in I get no interference. As soon as I plug the LEDs in I get a high pitched humming sound. The humming sound gets louder and softer with the LEDs as they fade.

    I need to figure out to remove the interference if possible. I am pretty sure I should put a capacitor somewhere but have no idea where, or what size. I have many different capacitor sizes and almost all the standard resistor sizes as well as a few 2N2222 transistors.

    Thanks heaps!!
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Add an LC-filter between the output of the wall wart and the arduino's Vcc and a second LC filter from the wall wart to the amplifier's Vcc. Decoupling these voltages will remove the interference that is caused by the high current pulses through the LEDs (when you write "fade" I think you're using PWM for controlling the brightness) from the amplifier's supply.
    GND should be connected without filter. However, you should connect GND from the arduino to the wall wart and from the amplifier to the wall wart. Do not connect the amplifier's GND through the arduino as in that case any high frequency current from the arduino will generate interference on the amplifier's GND.

    For more information read e.g. this article.
  3. jabelone


    Apr 14, 2013
    Hi, thanks. I don't want to have to cut the end off the ac adapter other wise I can't easily remove it from the arduino. Am I able to solder the amplifier GND and + connections to the barrel jack of the arduino? (where the ac adapter is connected) Also yes, I am using PWM to control them when they fade - all other times it's just on or off and it still gets interference.

    So if I run the amplifier straight from the wall wart and the arduino straight from the wall wart then I will have much less interference? I don't really have the time to wait at the moment for an LC-Filter, or the parts to make one.

    BTW super cheap LC-Filter 2 of them shipped for $9.97

  4. jabelone


    Apr 14, 2013
    I just did some testing and if I wire the positive and negative of both the amplifier and the transistors (LEDs) directly to the barrel jack the buzzing goes away completely. Thanks heaps!
  5. jabelone


    Apr 14, 2013
    Sorry, I just realised it wasn't connected properly when I did the testing so obviously it wasn't going to be interfering!! Does anyone have any suggestions on how to make one? (even if it doesn't do a very good job, just to minimise it) Also, do you have any idea on a better way I can design the circuit so that it minimizes it?
  6. BobK


    Jan 5, 2010
    It might be simpler to just run the ampliifer off a different power supply. A 9V battery for instance.

  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I'd suggest that you bring the power supply leads in at one point, connect a low ESR electrolytic (around 1000 µF) across those points, and from there, use separate two-wire connections to the amp and the LED circuit.

    This is a good starting point but will not necessarily eliminate the noise; you may want to add some L-C filtering in line with the positive and negative wires to the amplifier. The board you linked to looks suitable.
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