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Freeform ESP8266 OLED MQTT client

My second freeform creation, or third if we include The CyborgDuck is here. I wanted to do something simple in order to gather more experience and improve my skills.

When I was thinking what to build as a next, simple free-form circuit, my eyes fell on my RoomMonitor project (link is to a GitHub repository with its firmware). That’s one of my long-running projects. Basically, a bunch of sensors connected to ESP8266, battery powered, measures conditions in a room and sends data to some online service via MQTT. Nothing that much interesting. The challenge here is to make it last as long as possible on a battery. It might evolve into a weather station in the future. Right now, I am running the second prototype and planning the third one. Then, maybe, I might think about making some final form for that.

But since this is a battery-powered device and the micro-controller sleeps most of the time, there’s no display. I have always planned to make another device which would act as a remote display of sorts.

The idea is simple:

Use ESP8266 again, I have plenty of them and know them well enough.
Use a small OLED display and show only one value (temperature).
Powered through a USB connector, since I have plenty of USB power sources. But that also means I need to step-down the voltage from 5V to ~3.3V
Include LED which would indicate low battery voltage on the RoomMonitor device.
Have some touch-like input which would allow to cycle values on a display or similar.
Have a way how to program the ESP8266. For the firs firmware upload and for emergencies, when over the air update fails. Normally OTA should be used since it’s easier than connecting FTDI every time.
Since I am very new to free-form, I wanted to have the construction as simple as possible. Therefore, I decided to:

try to do most of the circuit in flat layers which would be easier to solder together,
use simple shape - cube or cuboid,
use PCB breakouts for some things - OLED display module, USB connector, and LDO.”

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