Single Dotstar Odd Behavior with Level Shifter

EL Wire/Tape/Panels, LEDs, pixels and strips, LCDs and TFTs, etc products from Adafruit

Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.
Locked
User avatar
Cardanis
 
Posts: 2
Joined: Sun May 22, 2022 4:55 pm

Single Dotstar Odd Behavior with Level Shifter

Post by Cardanis »

So I've got both a strand of 60 dotstars running and powered off of a single Raspberry Pi Zero. I'm powering the rpi with micro usb, and then pulling power for everything else off the 5v pin. I also have a single dotstar soldered onto a breakout board, and it works when swapped into the same breadboard as the strand is running from. Even though I seem to be driving the LEDs fine, I wanted to try out the recommendation to use a level shifter to bump the 3.3v to the 5v the dotstars actually want, and this is where I ran into issues. With the 74AHCT125 inline, I can still run the 60 strip fine (I think it's even a little brighter? maybe?) but when plugging the single breakout dotstar in,it only displays a blinding white with no changes possible. So in short:

No Shifter, Strip: Works fine
No Shifter, Single: Works fine
Shifter, Strip: Works fine
Shifter, Single: Blinding white, no control

I'm not even sure where to start trying to diagnose this, and was wondering if anyone had any thoughts.
Dotstar Wiring.png
Dotstar Wiring.png (324.57 KiB) Viewed 97 times

User avatar
diaphone
 
Posts: 34
Joined: Thu Apr 21, 2022 8:38 pm

Re: Single Dotstar Odd Behavior with Level Shifter

Post by diaphone »

What do you have available for equipment to diagnose with? My first inclination, from recent experience with these level shift chips on breadboards with wires, would be to try adding series resistance to the 125's outputs that go to DI and CI. Instead of just wires from pins 3 and 6 right to the dotstar, try adding a 100-ohm resistor in series with each of these wires. My thought here is that these CMOS-output chips switch very fast (high slew rate) and can set things ringing when there is unchecked inductance from wires, capacitance from what's being driven, or both.

User avatar
Cardanis
 
Posts: 2
Joined: Sun May 22, 2022 4:55 pm

Re: Single Dotstar Odd Behavior with Level Shifter

Post by Cardanis »

Well I gave that a try and it behaved exactly the same. The only tool I have on hand is a cheap, old Amazon multimeter, not sure how much that would help here. The only other possible explanation I can think of is that the shifters aren't actually what they're labelled as (grabbed them cheap off Amazon after the package with my Dotstars arrived and I realized I'd forgotten them, didn't want to pay shipping again for something so small). My project is very small-scale so I'm probably safe to just keep driving with the 3.3V directly from the rpi, was just hoping to do it the "proper" way from the start. Thanks for the advice!

User avatar
diaphone
 
Posts: 34
Joined: Thu Apr 21, 2022 8:38 pm

Re: Single Dotstar Odd Behavior with Level Shifter

Post by diaphone »

Basic multimeters are not very useful for debugging digital logic, beyond just checking if things have power to them or not. As soon as you're working with something that's not either constant DC or a constant-amplitude, constant-frequency sine wave, you really need an oscilloscope to see what's going on. How many single neopixels have you tried? What about chains inbetween your extremes of 1 and 60? It also might help to include a good high-res photo of your setup. We're assuming that it matches the illustration you included, but sometimes the problem is that the real-world parts configuration doesn't exactly match the diagram, and an extra set of eyes might spot something like a chip installed backwards, something not connected to the right pin, or maybe you meant to buy a 74AHCT125, but they sent you a 74ACT125, 74AC125 , 74LVC125, or 74HCT125 instead. It's very easy to order the wrong logic chip, especially the more letters and numbers there are. I don't think a wrong part is your issue, I'm just using this example to illustrate why a photo of the real setup can be so helpful.

Locked
Please be positive and constructive with your questions and comments.

Return to “Glowy things (LCD, LED, TFT, EL) purchased at Adafruit”