this post was submitted on 06 Jan 2025
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bypass controller on a cheap LED strip (lemmy.ml)
submitted 2 days ago by dingdongitsabear@lemmy.ml to c/askelectronics@discuss.tchncs.de
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hiya!

I got a cheap LED strip with PSU, controller, and IR remote. I didn't look at it too much, figured it would be easy to stick it under my kitchen cabinets.

however, this thing blinks and fades and whatnot and I'm supposed to switch it over to constant light by repeatedly pressing the remote, which a) works shitty and also b) don't wanna do that. I just want to plug it into power and it lights up and that's the end of our interaction.

so, I opened up the PSU/controller and I'd like to locate the spots that give me +12V and GND and I can bypass the whole blinky fadey mess.

it's a single-sided PCB. the top three wires on the right are for the IR receiver, ignore 'em. the bottom 4 are R, G, B, 12 V, respectively. I'm shorting RGB as it's a white-only strip.

can you hazard a guess where I'm most likely to succeed?

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[–] dingdongitsabear@lemmy.ml 1 points 2 hours ago

well, I sorted it. just like @fuzzy_feeling@programming.dev and others guessed, that was the correct spot to get the power from. the issue with the fluctuating voltage was due to my shitty multimeter, checked it with another one and boom, 12 V. well, no boom, that's just an expression.

[–] CameronDev@programming.dev 3 points 1 day ago (1 children)

Bottom leg of the transistor to the left of the signal wires would be my guess for ground, do you have a multimeter to check?

Looks like a large ground plane to the left of the transistors, so you might be able to scratch away the coating and solder directly to it.

[–] dingdongitsabear@lemmy.ml 2 points 1 day ago* (last edited 1 day ago) (2 children)

I do, and when I check the 12 V wire, it fluctuates between 12 and 5 V (that's the blink/fade thing) so I need 12 V before it gets mangled. where am I most likely to get it from?

edit: how do I check if the plane is ground with a multimeter?

[–] VehicleTree@lemmy.ca 1 points 1 day ago

Are you checking between the 12V and one of these legs? It's typically connected to ground in these types of controllers. Here's the SOT23 pinoutfor reference if that is the same package.

[–] CameronDev@programming.dev 1 points 1 day ago (1 children)

You can't check for ground specifically, you just check for a 12v difference between two points, and call those +12 and GND.

Feels odd that they are pulsing the entire 12v line, but I'm guessing it was something cheap.

The bottom leg of the transistor definitely looks like GND to me, so I think you just want to probe around (carefully!!) to find the stable 12V. I would avoid the entire live side, and for safety, might be worth taping it up.

If you cant find a stable 12v, you may want to get a cheap 12v power supply? Old laptops are often 12v, power too chargers as well are often 12v.

[–] dingdongitsabear@lemmy.ml 2 points 1 day ago* (last edited 1 day ago) (1 children)

measuring difference between ground plane and the various points didn't give me a stable voltage. the black thingy leading to the 12 V line is a SS210 (search says that's a schottky diode) and on its output the mentioned fluctuation is happening. on its input there's some very low voltage happening that's also fluctuating, like sub 1V (got a shitty multimeter).

if I'm understanding this correctly, then this thing boosts the voltage but the fluctuation is happening somewhere else. in other words, there is no 12 V source on this board. or?

[–] CameronDev@programming.dev 2 points 1 day ago (1 children)

Maybe you can trace that low voltage fluctuating line back to one of the ICs?, and work put how to disable that IC?

Or, use the remote to output solid white, and find the voltage on that line, and then try find that voltage somewhere else and short the two?

I'm running out of ideas though, I suspect buying a cheapo power supply might be easier :/

[–] dingdongitsabear@lemmy.ml 2 points 1 day ago

that's a solid line, I'm gonna try that. hopefully won't burn nothing of value. thanks

[–] fuzzy_feeling@programming.dev 2 points 1 day ago (1 children)

i'd check those two contacts.
with your description, the lower should be 12v and the upper gnd.

[–] dingdongitsabear@lemmy.ml 2 points 1 day ago (1 children)

they are. and the voltage is fluctuating (for the fade in/out light effect) when measured there. so I need to find the fucker that's doing that and bypass it.

[–] fuzzy_feeling@programming.dev 2 points 1 day ago

is it fluctuating, because there is no load attached?

the fading as well as the color changing should be controlled by the three transistors next to the cables. the connections i highlighted are before those transistors, so there should be no fading at that point.

if it's still fluctuating, if you have a load attached, i'd just go with some cheap 12v psu and call it a day.

[–] litchralee@sh.itjust.works 1 points 1 day ago* (last edited 1 day ago) (1 children)

My assumption with this PCB is that it switches the GND, meaning 12v is always provided to the LEDs. So the trick is to find somewhere that has a permanent GND and then connect all the LED leads to that. But I don't see a large enough spot to land three new leads, except maybe where R6 is.

You'll have to verify if my assumption is accurate, although I do wonder if you could just get a different PSU outright. This sounds like a 12v LED strip, so any sufficiently sized 12v wall-wart would also suffice.

Can you also clarify: you want this strip to be always-on and all-white, but the strip uses RGB LEDs? While it does produce white, it might not have a very high CRI and thus may be unpleasant for certain lighting applications. There are dedicated white LED strips which will perform a bit better for color rendition, and that could potentially be an issue if food needs to look appetizing under these cabinets.

[–] dingdongitsabear@lemmy.ml 2 points 1 day ago* (last edited 1 day ago) (1 children)

it's a RGB strip but with white LEDs.

so when I bring 12 V to the 12 V lead and then GND to the R, G, or B contact, the respective LEDs light up. when I bring GND to one and then short them (R+G+B) all the LEDs light up.

sure, I tried it with a known good 12 V PSU and it works, but I'd like to use this one and just bypass the light-show circuitry.

[–] CameronDev@programming.dev 1 points 1 day ago (1 children)

When you say all the LEDs light up, do you mean they light up white?

Those LEDs are labelled R G B, so I would have assumed they were single colour LEDs?

[–] dingdongitsabear@lemmy.ml 2 points 1 day ago (1 children)

they are white LEDs i.e. they shine white. the R G B leads are used to trigger them individually, for the running lights and whatnot. so when I bring 12 V to the V lead and GND to e.g. R, all the LEDs marked R (image) light up. when I then short R with G, then all R and G light up, etc.

[–] CameronDev@programming.dev 1 points 1 day ago (1 children)

Interesting, never heard of that arrangement before.

[–] dingdongitsabear@lemmy.ml 4 points 1 day ago

the whole thing (5 meter strip, remote, PSU/controller) cost like $5 total so I'm guessing that's some ingenuity at play, like reusing strips meant for RGB lights and sumsuch