Need help to check my schema - Heltec wireless shell board - Page 1
Forum section: electronics / beginners

author: MercadoNegro
author_rank: Newbie
timestamp: July 01, 2021, 02:32:27 am
content: Hello! My name is Gustavo, I`m not a technician or engineer, just somebody that had some time to spend at home during the pandemic.For that reason I started to play with ESP32, first with dev modules, then I bought something that caught my attention, a Wireless Shell by Heltec.Then I bought one to play with it and a Wireless Stick Lite (a dev style module) to work on it before starting to "design" a board myself.As my experience with electronics is non-existent then I decided to ask the forum about what I have done. Of course I expect all kind of critics, and most of them will be founded, but at the end I want to learn what I have done wrong and what have done not that wrong. [ Specified attachment is not available ]The initial idea:- A board to connect I2C sensors and have GPIO available to connect other kind of peripherals or even a board that I can attach to the connectors.- All external connections will work on +5V.- The board should have reverse polarity protection and short circuit protection on the IO.- Input voltage should be between 9v and 24v- Should have Micro SD card to log data.- Initially it should have an RTC, but I removed it to save cost.Schematics:At the beginning I created the schematics based on the Wireless Lite dev module, but then I started to replace parts to lower cost and add features, then I created the schematics mostly based on datasheets. The datasheet availability and information on it was one of the main factors to decide which chip to use, just because a complete datasheet have the entire information needed when somebody like me without years of experience on electronics want to use it.The board is powered by a buck converter AOZ1280CI, a 1.2A IC with 1.2mhz switching frequency to lower the voltage to 5V. I selected it because it was available on JLCPCB, it uses a small inductor, have enough power to run the sensors and the Heltec module. This was not my first option, my initial selection was a TI TPS54202DDCR, but I think I don`t need 2A and it`s expensive compared with the AOZ1280. I didn`t put the UVLO part of the circuit as I still don`t fully understand how it works.The next step was to use an LDO to power the module and the few 3.3v devices. My first option was the AMS1117, but I switched to NCP114BSN330T1G to reduce cost and space on the board, and I think 300ma is more than enough power.To make it 5V compatible I used 2x TI TXS0108EPWR 8 bit logic level converters. The Heltec module have 12 GPIO, then 16 bit was enough to use for I2C and GPIO, but I found the are not fast enough for I2C, then I used 3 of the available pins on the logic level converters for 3 of the input-only pins on the Heltec module, and selected a TI PCA9306DCTR level shifter for the I2C bus.At the end I added an eFuse TPS25921A, which is configured to shutdown at 1A on the 5V rail.This is a 4 layer board, then as I have 3.3V and 5V I decided to divide this layer in two copper areas with 2mm separation between them. This could be one of the mistakes, I`m not sure if this is the way to do it. Top layer is component and traces, 2nd layer is GND plane, 3rd layer is power plane, botton layer have as few traces as possible.I added an additional button to enable/disable WiFi and configure it when is needed and that`s all.This is the board:And this is how I divided the power plane in two parts, the bigger one for 3.3V and the smaller one with the L shape for 5V:Again, consider this the work from somebody that have very low knowledge of electronics, just doing it for fun, trying to build something while staying at home, then all comments, critics and ideas are welcome, for that reason this post is in the beginners section!Datasheets used:Heltec Wireless Shell:https://heltec-automation-docs.readthedocs.io/en/latest/esp32/wireless_shell/index.htmlAOZ1280CI - Buck converter:http://www.aosmd.com/res/data_sheets/aoz1280ci.pdfNCP114BSN330T1G - LDO:https://www.onsemi.com/pdf/datasheet/ncp114-d.pdfTPS25921ADR - eFuse:https://www.ti.com/lit/ds/symlink/tps25921a.pdfPCA9306DCTR - I2C level shifter:https://www.ti.com/lit/ds/symlink/pca9306.pdfTXS0108EPWR - 8bit logic level converter:https://www.ti.com/lit/ds/symlink/txs0108e.pdfThanks!Gustavo