PWM controlled RGB LED Strip makes my speakers buzz

Other Hardware LEDs and Multiplexing
1

Hi, mates.

I've not much idea about electronics, but I did some little projects (always guided). Now I'd like to place some RGB LEDs in my desktop PC windowed case. For that, I bought a 5050 SMD strip with 300 RGB LED's (5 meters). Now I want to control it with an Arduino Uno.

I followed these two tutorials:

Jerome Bernard: RGB Led Strip controlled by an Arduino (notice there is a misplaced resistor on red color)
Overview | RGB LED Strips | Adafruit Learning System

I'm using IRFZ44N MOSFETs, as shown in the first tutorial. The LED strip is being powered with 12v from an ATX PSU. I want to power (and send serial data to) the Arduino board by using an USB cable. There's some confusion, because the Arduino is meant to operate optimally with 7v-12v, despite the USB port provides 5v.

Everything is working, but my problem is that I get noise into my speakers (monitor built-in) when:

  1. PSU is powering the LEDs.
  2. PWM values are varying (color fading).
  3. The USB cable is connecting the Arduino and the computer.
  4. Audio cable is connecting the monitor and the computer (or my project's GND).
  5. Speakers (monitor) are on.

I saw similar cases here: http://www.instructables.com/community/Noise-in-Amp-from-LED-Strip-servo/
And here: http://forum.arduino.cc/index.php/topic,122074.0.html
But I can't get what and where should I be adding to my project.

I'll really appreciate your help.

Edit: my project is being powered by an external PSU, not by my computer's PSU.

2

Hi Scratz

... I get noise into my speakers (monitor built-in) when:

  1. PSU is powering the LEDs.
  2. PWM values are varying (color fading).

Do you still get the noise with the PSU (only) disconnected from the LEDs?

And have you tested with a fixed PWM value?

  1. Speakers (monitor) are on.

Are these active speakers with their own power supply?

There's some confusion, because the Arduino is meant to operate optimally with 7v-12v, despite the USB port provides 5v.

The Uno processor etc runs at 5V. This can come directly from the USB port, or from a higher voltage external power supply fed into the jack socket - there is an onboard regulator to convert this down to 5V. So you could also test with an external battery (4 x AA, for example) or power supply.

Regards

Ray

3

Scratz:
I'm using IRFZ44N MOSFETs, as shown in the first tutorial.

Let's hope not...because they're not logic-level and won't work properly.

You need something that starts with "IRL", not "IRF".

4

I think the problem might be that the sound card might use the 12V for the amplifier. Have you tried supplying power to the leds from a separate power source , like a 9V wal wart ?

5

Hackscribble:
Do you still get the noise with the PSU (only) disconnected from the LEDs?

No, the noise disappears when the project is not being powered (Arduino is still running).

Hackscribble:
And have you tested with a fixed PWM value?

Yes, and the noise stops. Actually, the noise changes subtly as the PWM value is increased.

Hackscribble:
Are these active speakers with their own power supply?

Yes, they are integrated into a monitor. If I power off the monitor, they don't buzz.

Hackscribble:
The Uno processor etc runs at 5V. This can come directly from the USB port, or from a higher voltage external power supply fed into the jack socket - there is an onboard regulator to convert this down to 5V. So you could also test with an external battery (4 x AA, for example) or power supply.

Thank you very much!

fungus:
Let's hope not...because they're not logic-level and won't work properly.

You need something that starts with "IRL", not "IRF".

Nope, the first tutorial uses IRF. It worked for me even following the Adafruit tutorial, which recommends a STP16NF06 or a TIP120 transistor.

raschemmel:
I think the problem might be that the sound card might use the 12V for the amplifier. Have you tried supplying power to the leds from a separate power source , like a 9V wal wart ?

Actually, the PSU powering the LEDs is different from my computers PSU. I tried with another PSU and the problem persists...

Ops! I've just tried with an universal AC adapter at 9V and 12V and the noise has gone. But I still need to power it from an ATX PSU.

I hope it gives you some clues...

Thank you all.

6

Scratz:

fungus:
Let's hope not...because they're not logic-level and won't work properly.

You need something that starts with "IRL", not "IRF".

Nope, the first tutorial uses IRF. It worked for me even following the Adafruit tutorial, which recommends a STP16NF06 or a TIP120 transistor.

Conclusion: The tutorial is wrong.

Here's the datasheet: http://www.irf.com/product-info/datasheets/data/irfz44n.pdf

Check out the value for "RDSon" (third value, page 2).

It says "VGS= 10V", ie. You need 10V to fully switch it on.