I've been playing with LEDs for video work for a while. I made some LED panels using strips and off the shelf controllers which always worked nicely while on full brightness. Every time I dim them down, I start to get some weird "scanline" flickering. That happens to all sorts of LEDs and dimmers (Arduino included) I've ever played with.

Yesterday, a friend told me he got a modded 12-24V 8A Dimmer which extinguish this kind of problem. Once I got it in my hands, I was surprised it looked just like a 12-24 Dimmer I got from eBay. Googling around I found out it's a very popular circuit, with MANY different PCB layouts and different vendors.

[![enter image description here](https://i.stack.imgur.com/wC5s9.jpg)](https://i.stack.imgur.com/wC5s9.jpg)

I tried to find Flicker related mods online, but couldn't find anything. Obviously I opened his circuit and it's surprisingly similar to mine, apart from 3 differences I spotted:

[![My friends modded PCB](https://i.stack.imgur.com/EWwxP.jpg)](https://i.stack.imgur.com/EWwxP.jpg)

I tried to transpose these mods to my PCB. I think I got two of them right (Yellow and Green), but I just can't find where to do the mod which I circled in red (the track that has been interrupted).

[![My PCB and where I think I should place the mods](https://i.stack.imgur.com/TJBFX.jpg)](https://i.stack.imgur.com/TJBFX.jpg)

I tried to follow the tracks, but they go underneath the LM358 and the 3-pin connector. The other IC is a 555.

I have two questions regarding this situation, due to my limited electric engineering knowledge:

* Where I should interrupt the board on my PCB Layout?
* What exactly did this MOD do that eliminated the flickering on video?

Here are a few other pics of the boards:

[![Bottom of friends board](https://i.stack.imgur.com/TKWj2.jpg)](https://i.stack.imgur.com/TKWj2.jpg)

[![Bottom of my Board](https://i.stack.imgur.com/vTWcX.jpg)](https://i.stack.imgur.com/vTWcX.jpg)

Any help or tips are extremely appreciated! Docs and Papers regarding LEDs and Flickering are also welcome :D

Thanks in advance!

EDIT: I managed to unsolder the pot connector and the two ICs so I can get a better view of the board. I did destroy one pad on the U1, which apparently didn't connect anything, just the jumped pins.

There's also a solder blob between U1 and the middle pot pin. It's a jumper to rejoin a track I manually cut and repaired afterwards.

[![Overview](https://i.stack.imgur.com/E0LeD.jpg)](https://i.stack.imgur.com/E0LeD.jpg)

[![758 Side](https://i.stack.imgur.com/GpMS0.jpg)](https://i.stack.imgur.com/GpMS0.jpg)

[![555 Side](https://i.stack.imgur.com/7UoGa.jpg)](https://i.stack.imgur.com/7UoGa.jpg)

I'll be posting demos of both circuits working, so I can show you the visual difference too.

---

Adding more info I couldn't on my first post:

I edited my tests to show you. Different cameras, but same results. First one is my friend's circuit, 100% flicker free (I had to crop it because there was sensitive content in the frame):

[Flicker Free Circuit](https://vimeo.com/243966287/10ef662cc2)

The little blackouts are just me not being able to hold the supply wires, not circuit faults.

[My Flickery Circuit](https://vimeo.com/243966513/8889308df2)

As you can see, it only stays flicker-free on max power. As soon as you dim out, it starts flickering.

Both tests were powered by a regular 9V battery (not ideal, but it's what I had on hand).

Based on the link pointed out by jsotola, I could gather a few more pieces of info:

* Frequency is governed by the 555 astable. The capacitor between pins 2 and 6 of the 555 should be responsible for that frequency.
* This calculator helps to find the right cap for the frequency I want: [Calculator](http://www.ohmslawcalculator.com/555-astable-calculator)
* Assuming my R1, R2 and C values are respectively 1 kΩ, 10 kΩ and 100 nF, the output frequency should be around 680 Hz, which matches the linked video oscilloscope test. I will try to get one and verify my circuit.
* A comment by brokenmonkey on the same link gave some video related tips:

> 
> I know 6 kHz is a safe frequency for video but it can cause audio interference, and 20 kHz is safe for audio but maybe not all types of video.
> 
> 
> 

Following his logic, replacing C1 with a 11 nF capacitor should do the trick. Still, my friend's circuit has a couple of other things applied, as shown on the images.

My friend was also emphatic that the waveform should be square. I guess it comes from factory that way too...

Thanks again for all the help :D