Custom PCB for a Split Flap Display - Page 1
Forum section: electronics / projects

author: ecreative
author_rank: Newbie
timestamp: September 16, 2025, 08:55:34 pm
content: Hey,My first post, which I've likely put in the wrong category, but I wasn't sure which one is most relevant to my project. I'm building this awesome Split Flap display project by Morgan Manleyhttps://www.instructables.com/Split-Flap-Display-3D-Printed-Modular-Compact-Encl/. It involves an ESP-32, connected to a PCF8575 Expander Board connected to a ULN2003 driving a small stepper motor - it's very cool to see it running.However, my current build is hand soldered together. Ever the tinkerer, I wanted to see if I could combine the two required boards. My first time making a schematic and PCB. I've only ever used breadboards with simple circuits before, so I wondered if we had any experts who might be able to look over my design and look for anything obviously wrong, as I couldn't find a schematic for the generic PCF8575 and even if my schematic is correct. I'm sure I'll make some people wince at my PCB design but I gave it a good go based on visuals and probing boards I have to hand..I've attached someone else's schematic for the ULN2003 - Unlike their schematic I went for a 0.1uf capacitor as that's what my boards have on them.An image of the PCF8575Two screenshots of my KiCad schematic and PCBFYI: The reason for the two lanes for PWR and GND is due to seeing a voltage drop when daisy chaining 7 JST PH cables across 8 modules. I'm hoping doubling up will solve the issue.Any help would be appreciated, even if it's a grilling

author: Benta
author_rank: Super Contributor
timestamp: September 16, 2025, 10:05:56 pm
content: First: very, very cool! Reminds me of Frankfurt Airport, the big arrival/departure signs always fascinate me.Second: very good first schematic. You didn't succumb to the sad fashion of drawing little boxes with one or two parts in each.I need a little time to look at the actual circuit.

author: thm_w
author_rank: Super Contributor
timestamp: September 17, 2025, 12:26:33 am
content: This is the correct section.Quote from: ecreative on September 16, 2025, 08:55:34 pmHow much current is one stepper drawing? Might be a good idea to measure it.Does the stepper motor have to stay on or can the power be shut off once its flipped?In terms of the PCB, get rid of the ground traces and add a big ground fill on the bottom of the board.

author: ecreative
author_rank: Newbie
timestamp: September 17, 2025, 07:46:53 pm
content: Quote from: Benta on September 16, 2025, 10:05:56 pmYes I remember them at airports when I was younger and wanted to make my own. Thank you very much, I truly appreciate the help.Quote from: ecreative on September 16, 2025, 08:55:34 pmHow much current is one stepper drawing? Might be a good idea to measure it.Does the stepper motor have to stay on or can the power be shut off once its flipped?In terms of the PCB, get rid of the ground traces and add a big ground fill on the bottom of the board.It draws around 0.3-4amps while running.The stepper motor is de-engerised in the firmware after the flip.Good shout on the big GND pour. I wasn't sure what was the best approach.

author: ecreative
author_rank: Newbie
timestamp: September 22, 2025, 08:04:57 pm
content: As mentioned, I've placed the GND with a larger pour. Also thought this might help with heat dissipation from U2.If anyone is able to check my schematic, I'd really appreciate it

author: thm_w
author_rank: Super Contributor
timestamp: September 22, 2025, 09:03:08 pm
content: I can't comment on if schematic is all good, but to make it more readable I would add more Ground symbols instead of having wiring going across the page.New PCB layout definitely looks better.

author: ecreative
author_rank: Newbie
timestamp: September 22, 2025, 09:22:17 pm
content: Quote from: thm_w on September 22, 2025, 09:03:08 pmThank you for the input. Is something like this a better approach?

author: thm_w
author_rank: Super Contributor
timestamp: September 22, 2025, 09:25:08 pm
content: Looks way better

author: xvr
author_rank: Frequent Contributor
timestamp: September 23, 2025, 11:58:52 am
content: The RN2 connection looks a bit mysterious. If you untangle the wires around it, you'll find that these are pull-up resistors for both sides of the SCL and SDA lines, while the corresponding power lines for the pull-ups have been forgotten.

author: ecreative
author_rank: Newbie
timestamp: September 23, 2025, 01:20:15 pm
content: Ok after speaking with a few people, I've made some changes to my schematic:Changes made:Removed the MosfetsAdded an AP2112K 3.3V regulator to drop VIN down to 3.3 V for the logic.Powered the PCF8575 at 3.3V so its I2C pins match what it says for the ESP32.Moved the SDA/SCL I2C pull-ups to the 3.3V rail.Left the ULN2003, stepper motors and previosuly incorectly named A3144 on the 5V rail.I'd still love someone to check over my AP2112K and capacitor/resistor layout to make sure I've not gone off the rails.

author: ecreative
author_rank: Newbie
timestamp: September 23, 2025, 03:18:39 pm
content: Forgot to include the updated PCB. See attached.

author: Benta
author_rank: Super Contributor
timestamp: September 24, 2025, 11:24:26 pm
content: I've looked at your schematic and I think you're going the wrong way.The first approach was OK, but the MOSFETs looked strange. I suppose they comprise some kind of level shifting between 3.3 V and 5 V?First, I'd stay with running your circuit on 5 V and leave it at that and remove the 3.3 V LDO.Second, I'd solve the 3.3/5 V incompatibility with bidirectional level shifters. The TI TXS0102 comes to mind, but there are probably others. It's a two-channel device, and that's all you need.Third, the A0, A1, A2 jumper circuit won't work, you can't leave any of those inputs open. You'll either have to add pullup/pulldown resistors, or use a two-way jumper setup. The PCF8575 data sheet is terrible on this point (and on others).Cheers.