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I've got a problem I haven't been able to sort out experimentally and am looking for help from someone who knows more than me. I'm making a handheld prop with audio and lighting effects. In the past I've used a commercial handheld amp (Aker VoiceBooster) as a base, tapping switched battery power from its circuitboard. That tapped power will run the controls, audio generator, LEDs, etc., and I'll use the amplifier on the Aker for the audio output. But this time I have a problem with an LED circuit generating noise on the power feed that is messing up the audio on the Aker. The problem seems to be that I'm using an ATTiny85 and pulse width modulation to drive a number of LEDs (with an N-Channel MOSFET because they need more power than the ATTiny can source). Simple circuit show in pic, LEDs on the right, noise apparently running back to the Aker's battery on the left. enter image description here How can I fix this? I think the answer should be bypass capacitors, but breadboarding a number of different sizes across the 5V lines near the LEDs doesn't seem to change anything. (Tried 0.1microF - 10microF MLCC.) I don't have a scope to put on this, so I don't really know the nature of the noise (beyond its audible effects). I may well be missing something basic, this is just a hobby thing and I'm a software guy. Can anyone clue me in?

EDIT: The PWM frequency is the default for ATTiny85, which seems to be 500 or 1000 Hz. I was trying to avoid having to figure out how to change it, but that might be the best approach. There are actually 6 LEDs (I only drew 3), pulling 20mA each. The resistors are 100 Ohm. I've tried caps across the battery, and across Q1 source to the hot side of the LEDs, without much luck. But I'm not certain of the PWM frequency, or exactly what values/types of cap is likely to be effective.

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    \$\begingroup\$ Can you hear that the noise is at the switching frequency of the PWM? If not, you have other problems. \$\endgroup\$ – Reinderien Aug 9 '17 at 1:48
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    \$\begingroup\$ P.S. What is your switching frequency? If it's below 20kHz, boost it up out of audio range. \$\endgroup\$ – Reinderien Aug 9 '17 at 1:49
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    \$\begingroup\$ how much power? Did you use separate lines for ATtiny so that LED current does not induce ground shift or supply dip.... input and output decoupling caps? what is your real problem? What R values? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 9 '17 at 1:54
  • \$\begingroup\$ could be battery impedance try fitting te capacitor across the battery. \$\endgroup\$ – Jasen Aug 9 '17 at 2:12
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    \$\begingroup\$ Noise can either be supply or ground. \$\endgroup\$ – Jason Han Aug 14 '17 at 9:51
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Filtering only occurs with a voltage-divider behavior.

Thus use the advice of 'laptop2d' and throw a ferrite between the LED prop and the amp. Then add that bypasscap across the LED prop.

Where to tie the RTN/GND end of the bypasscap? At the source pin of the FET switch.

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  • \$\begingroup\$ I'll try the ferrite if I can come up with one. Thoughts about value/type of bypass cap? I have tried electrolytics and MLCCs (because that's what I've got handy). \$\endgroup\$ – Eisen Aug 10 '17 at 16:41
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Throw a ferrite or wrap the wire around an RF choke on the wire between the LED prop and the Amp (if they are sharing the same power supply. Ferrites block high frequency noise.

Another thing you might have to do is shield the amplifier from the board with the ATtiny85 and LED's as it could be putting out radio frequency interference (RFI).

Last thing would be to change the LED frequency.

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  • \$\begingroup\$ Thanks. I tried shielding for RFI, but didn't see any change, so I think that's not the answer. I'll see if I can come up with a ferrite and give that a try. Also trying to sort out changing the PWM frequency on the ATTiny. \$\endgroup\$ – Eisen Aug 10 '17 at 16:39
  • \$\begingroup\$ I'd bet its getting through on the supply. If you shield, it needs to be continuous. \$\endgroup\$ – Voltage Spike Aug 10 '17 at 17:00
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Huh, could be resistance in the ground wires, the way you've drawn the circuit. Try a star ground, different return paths for led's and audio.

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