Display-o-tron HAT - 3 of 6 background LEDs not working


#1

In September 2018, I bought a new “display-o-tron HAT”, but not at pimoroni shop. In February 2019, I tried it out the first time, and noticed that LED 0, 3 and 4 of the backlight (all three channels R, G and B resp.) are dark. Even the R channel of LED 5 seems to have a slack joint, sometimes it’s even dark too. Despite this, the display works as expected. All pins on the back look soldered well. What can be the reason of that? Does the backlight consists of a typical shift register RGB like WS2812? I have good soldering equipment and a lot of experience, should I desolder the display in order to have a look at the SMD-LEDs behind it? Is there any schematic available?

Cheers!


#2

Do you know if you’re running “dot3k” or “Dot HAT” examples? Half of the backlight not working is more often than not a symptom of running Display-o-Tron 3000 code on a Display-o-Tron HAT.

The LEDs are attached to an SN3218 18-channel driver IC and the configuration of LEDs between Dot3k and DotHAT are difference since DotHAT moves the bargraph to the captouch chip and uses the freed up channels for more backlight zones.


#3

I am runing the examples of the “dothat” folder.

And yes, I can confirm that running backlight.py from the “dot3k” folder on a “display-o-tron HAT” only ignites LED 0,1 and 2. That’s what I did yesterday, but corrected it and used the “dothat” folder subsequently. But because LED 0 is dark anyway (as mentioned in my initial post), even with the dot3k script a can’t get LED 0 on.
I just downloaded the datasheet of SN3218 meanwhile and noticed that each led is wired individually, no multiplex or shift registers, and even without limiting resistors. So I suppose that either the affected RGB-LEDs are not populated (is there a final inspection at manufacturer side?) or they are not soldered correctly, or the print has some damage. I think dismounting the display is a good idea.


#4

There is, indeed, a final inspection- and we manufacture these in-house. It’s possible either we overlooked the problem, or it was a poor connection that failed after inspection. Sorry about that!

Did you order direct from us?


#5

Unfortunately not, I bought it on ebay/Germany.

The first display-o-tron that was sent to me had a broken display, so I got a replacement which looked good, and I didn’t have to send the first item back. That is I own two HATs now, maybe I can swap parts or the whole display.


#6

I’m told it’s quite possible that- during shipping- the perspex diffuser moved around enough to dislodge the LEDs from the PCB.

It might be possible to swap the display, but it’s quite challenging to do so without damaging it. If all else fails I’m sure we can ship you a replacement directly and - hopefully - third time lucky.

We also stock the displays separately- https://shop.pimoroni.com/products/display-o-tron-lcd


#7

Ah, that’s good to know!

Maybe I remove the pins of the first (broken) HAT using a side cutter, cleaning the pin holes and mounting a brandnew LCD from the shop, that will be cheaper than buying another display-o-tron, just because of the backlight problem. Three of the RGB-LEDs from the second (bad backlight) HAT still work, and it’s still readable in dark environment.