High pitched noise from color-changing LEDs...

[robmay]

Hey guys, so weird question about power and LEDs. I got a bunch of color-rotating LEDs, thinking they would be awesome in a light plate on a pedal. They absolutely are, but there's an issue. I get a weird high-pitched whine, but only when the LED change happens and includes blue. I've tried putting a big cap across the power rails, and while a 100u cap made a significant cut to the whine, I couldn't get it to completely go away, even with a 1000u cap. I'm not sure what else I can try here, and I'd really love to have this work for future designs.

Here's a link to a video showing the problem: https://youtu.be/b77Z4F5qR5o

Some notes: Pedal is a simple dual op-amp OD/fuzz. Input is grounded in the video. The noise is the same with a guitar plugged in, though. The video is with a 100u cap across the power rails. There is a pulldown resistor on the input, and a 330 ohm resistor from the LED cathode to ground. There was no change by putting the resistor between 9V and anode.

Here's a link to the LEDs I used (not much technical info there): http://amazon.com/microtivity-IL603-5mm-Slow-Rotating-Pack/dp/B007RO9WWE
The thing that came with them said the voltage is 3.2-3.6V, and other packs of them come with 200 ohm resistors.

Let me know if anyone has any ideas, even if there's some sort of transistor or diode trick that would help isolate the LED from the rest of the circuit, I'm willing to up parts counts on a few things for something so cool. Even if anyone could explain power theory or why this might be happening, I'd appreciate that too. I'd always rather give up with the knowledge of *why* something is impossible.

Thanks!

[vigilante397]

I used some similar LEDs for a while and had the same noise problem on high-gain (distortion and fuzz) circuits. Someone in a thread (that I can't seem to find now) mentioned a filtering fix for this problem, but I had already given up on these LEDs and just switched to regular boring single color LEDs

[robmay]

Glad I'm not the only one with the problem! I'll start trying some different searches, but hopefully the person who posted that is still around and active....

[slacker]

Try putting a big cap directly across the LED, maybe with a 100nF cap in parallel with the big one. Also take the ground wire for the LED directly back to the ground connection on the DC jack, so on your breadboard put the LED as close to where your power come in as possible. This helps stop the noise from the LED getting into the ground of the rest of the circuit which might make it quieter.

[ElectricDruid]

Yep, sounds like PWM noise from the LED leaking into the power supply. I agree with Slacker. Isolate the LED's ground and power from the rest of the circuit as far as possible.

If you get really desperate, run the LEDs off a separate 78L05 taken from the main supply, again with plenty of capacitance (100uF electrolytic plus 100nF ceramic). That helps too.

HTH,
Tom

[PRR]

It whines because it has a computer in it.

You may have to go this far to get the whine down.



Sealed metal box around "LED". Good metal-metal contact to the pedal enclosure. Hole for light to get out, two holes for 100r resistor bodies. Yes, have the resistors half-in half-out of the box. 10uFd bulk-cap and 0.1uFd for supersonics. Keep all internal wiring very short.

[Jdansti]

Many years ago I had a similar problem with a blinking LED that I tried out in a wah. The blinking circuit was built into the LED similar to your multicolored job. If I recall correctly, it clicked rather than whined. Anyway, as PRR mentioned, these auto-LEDs have additional circuitry and they're not designed to be quiet. They're mostly used in toys and novelty knick knacks. Just the other day I saw a plastic ring made for kids with a multicolored LED in it. We have a couple of tumblers with multicolored LEDs in the bottom so your drink lights up. No worry about noise in these applications.

[btschock]

Not sure if anyone is still intrested but I fpund a way to queit these to almost no whine with the color change and no wierd musical songs from the fliker candel leds as well. Minimal parts I could use.  (Edit) the last resistor that was left unmarked was a 470r.

[moosapotamus]

Here is what has worked for me...

Attach resistor (clr) to +9V. From the other end of the resistor, put a 10uF cap in parallel with the LED to ground. If multiple LEDs, give each it's own resistor and cap.

I asked this same question many moons ago and, IIRC, this solution was suggested by Jack Orman.