Hi, everyone.
I'm working on a small and battery powered CO2 Sensor. As I'm new to making PCBs and have no experience I'd highly appreciate it if some of you could take a look at what I have done so far and maybe answer a few questions.
I've included everything in the attachments - even the zipped KiCad Project if anyone wants to take a more detailed look than just from the included images.
The project consists of a MCU for low power application, a 2.8V LDO (to be able to run from 3V battery), the CO2 Sensor and an E-Paper Display (The v2 of the Waveshare 1.54'' Display).
The CO2 Sensor uses I2C to communicate with the MCU and the E-Paper uses Half-Duplex SPI. (There also is a temperature Sensor included in the CO2 Sensor which I will also use for the E-Paper).
There is a green, yellow and red LED which I will turn on dependent on the level of CO2 measured (Those will be connected through wires to the PCB).
For some parts like the decoupling of the LDO the datasheets told me really precisely what values to use.
For the E-Paper there is a reference circuit which I included in the attachments and tried to reproduce in the schematic / PCB. If I understood correctly it is a charge pump to produce the voltage required to update the display.
Though I'm not sure if I read the reference correctly. Their labeling confused me bit (like the 3.3V on VDDIO. Is it just a notice or should I connect it?) and there are no notes regarding the layout on the PCB.
The diodes they used weren't available anymore so I replaced them with the closest available part I found.
So they first questions:
- Is my schematic correct? (I mainly asking this for the E-Paper part but of course I'm happy with any kind of feedback I can get)
- What is the inductor for? (I think it is used for blocking noise from the supply rail but I don't really know)
- Should I place the decoupling capacitors or the charge pump closer to the connector on the PCB?
- Do I even use the right connector?
- Do I have to somehow shield the charge pump so the Sensor doesn't pick up noise?
- Will the charge pump introduce noise to the rest of the supply rail?
For the Half-Duplex SPI I found official documents which told me to use a resistor (to eliminate the risk of shorts) between the MOSI-Pin of the MCU and the Data-Pin from the device to talk to. Though I found no information for the value I should use for the resistor. I picked a 1k so I don't risk a too large voltage drop across it. How do I estimate the value needed?
Next to the main questions above there are a few more things that I'd like to ask if anyone has the time for answering:
- I used a lot of vias in the Pads of the components. I heard others saying one should "fill" those. Do I have to care about this? How would I do that in KiCad?
- Under the MCU there is an almost isolated GND-Fill which I had connected with a trace to the main GND-Fill. Will this impact the performance of the PCB? Is there a better way to solve this?
- I included connectors for SPI, I2C and all unused Pins of the MCU to be able to connect more things if I have to later on. Is this okay or does it occupy to much space one the PCB?
- The datasheet of the CO2 Sensor states that while soldering it should not be heated above 235 degree celsius. As I will have to solder it with hot air: Will I be better of preheating (a few minutes long?) to about 150 degree and then increase to 235 degree for soldering or should I increase to like 400 degree and therefore solder it in a shorter amount of time?
I know some of the questions are probably a bit too specific to ask or are a bit wrong for this part of the forum but I don't know where else I should post this.
Thank you all for taking the time to read this!
This is the schematic:
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This the PCB:
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And this the reference circuit for the E-Paper:
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