Chinese 7-segment RGB LED clock problem - Page 1
Forum section: electronics / general

author: musicant
author_rank: Newbie
timestamp: March 28, 2019, 04:27:31 pm
content: After watching Big Clive's build of a clock a month or three ago, I ordered the fancier version from AliXP (SMT, more matrix display modules), as well as a 7-seg version using discrete RGB 5050 LEDs to light the segments.https://www.aliexpress.com/item/Large-Inch-Rainbow-Color-Digital-Tube-DS3231-Clock-DIY-kit-with-customizable-colors-Electronic-kit-Gift/32916987930.htmlThe matrix kit arrived first, and I had no problem getting it going. The RGB kit I just completed powered up, but no blue LEDs were turning on.After redoing a few suspect µC joints, no joy, so I dug deeper. I discovered the 3 LEDs in the 6-pin package are all facing the same way (anodess on one side, cathodes, the other), but the PCB is laid out such that two of the LEDs are in parallel (anodes connected together, ditto cathodes)!!!This would allow some sort of Charlieplex I assume...IF those two LEDs were facing opposite directions, and my assumption was that the vendor had supplied the wrong LEDs. However, I can't find such "alternating" LEDs anywhere, Ali, Digikey, ...ok...that's all I checked.Any opinions? Do such LEDs exist (I thought they did)? Is this just another unscrupulous vendor disposing of defective kit?

author: tooki
author_rank: Super Contributor
timestamp: March 31, 2019, 09:33:11 am
content: Did you figure it out? That kit looks neat, I wouldn’t mind getting one, assuming it’s not crap.How did you determine that the LEDs are wired in parallel? I find it hard to believe that this was deliberate, since it couldn’t work properly. Is it possible that they got bridged during soldering?Do you have the schematic? Could you share it, if so?

author: Psi
author_rank: Super Contributor
timestamp: March 31, 2019, 09:43:54 am
content: If i had to make up a story to fit this, it would go something like.The kit was designed to use up a large quantity of reject LEDs that someone got for free. They were rejects because the blue die was bonded around the wrong way.The kit manufacturer has now run out of these reject LEDs and so now must use LEDs that don't have this fault.They saw this coming and designed a new PCB to support the non-reject LEDs but somehow your kit got mixed up and the wrong combination of PCB version vs LED type got put together into a kit.Or, it could be someone else in china copied the kit design and wasn't aware it was designed to use reject LEDs.

author: Psi
author_rank: Super Contributor
timestamp: April 02, 2019, 03:28:36 am
content: China has a massive manufacturing industry and this means lots of rejects parts. They're created when people make mistakes or when production machinery is being setup/tuned and test-runs are being done.There's a big industry in china making things out of reject parts because they're so cheap and therefor the profit is high.

author: tooki
author_rank: Super Contributor
timestamp: April 02, 2019, 07:55:43 pm
content: Quote from: musicant on April 01, 2019, 06:12:19 pmYou mean this one?http://files.banggood.com/2018/07/1323917.doc(It took 2 mins of googling to find it.)Quote from: musicant on April 01, 2019, 06:12:19 pmI think it's a lot more likely that there are solder bridges somewhere, connecting some pins together that shouldn't be, making the PCB appear to be wrong.Quote from: musicant on April 01, 2019, 06:12:19 pmI don't think there'd be any compelling reason to manufacture SMD LEDs with a strange pinout, since one can achieve the same effect in the PCB layout, and making an alternate pinout would cost more (since it would forgo economies of scale). And I doubt one would end up with such a pinout by mistake.

author: Psi
author_rank: Super Contributor
timestamp: April 14, 2019, 12:04:42 am
content: i've seen many LEDs sold cheap that have the wrong pinout to what they should.5mm leds with the long leg negative or SMT leds where the datasheet says green dot is + but it's actually -Also seen COB leds where the LED was soldered onto the Aluminium backing pcb around the wrong way, so silkscreen says + but its actually -China makes so many led so fast that on occasions someone hits the 'go' button on 10000 leds without double checking the polarity on the work order.