Init

.gitignore

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+build/
+sdkconfig

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.16)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(tempsensor)

README.md

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+
+ESP32C3 Temperature Sensor
+==========================
+
+This repo contains the hardware design, firmware, and HTTP backend for a small
+battery powered wifi temperature/humidity sensor.
+
+![](img/front.jpg)
+
+The firmware is written in C, using ESP-IDF (which is based on FreeRTOS).
+
+The backend is written in Python, using the native HTTP and sqlite3 support. It
+has no arbitrary limit on the number of sensors it can support, and uses gnuplot
+to render graphs and serve them over HTTPS:
+
+![](img/graph.png)
+
+The system supports over-the-air firmware updates, using two partitions with a
+fallback to the old firmware if the first POST is unsuccessful.
+
+TL;DR:
+
+* [Gerber](prod/v020-GERBER.zip), [BOM](prod/v020-BOM.csv), and [CPL](prod/v020-CPL.csv)
+* [Schematic](schematic.pdf), [PCB Layout](layout.pdf)
+* [Firmware](main/main.c)
+* [Backend](backend/daemon.py)
+
+Design
+------
+
+The board is designed around the ESP32-C3, a low power single core RISCV SoC
+with wifi and bluetooth support. The sensor is an unnecessarily fancy part with
+0.01degC/0.01%RH resolution, just for fun.
+
+The board has two 3.3V power supplies: an LDO and a switcher. The switcher is
+much more efficient than the LDO, but has a high quiescent current draw (~1mA),
+so it is enabled via a GPIO only when the wifi is turned on.
+
+I more or less followed the [ESP32-C3 design guidelines](https://www.espressif.com/sites/default/files/documentation/esp32-c3_hardware_design_guidelines_en.pdf)
+when laying out the board, with the exception of RF impedence matching. The RF
+trace and my antennas are all 50 Ohm, which, with the 35 Ohm LNA output, results
+in a VSWR of ~1.4:1. The loss (versus a matched line) is less than an S-unit, so
+it wasn't worth the money.
+
+I used small laptop syle wifi antennas, which are easy to find in bulk for cheap
+nowadays if they aren't compatible with 5GHz. Because they log signal strength,
+the sensors will be useful for future experimentation with PCB antenna designs.
+
+To avoid the "thundering herd" problem, each sensor has a unique "send delay"
+assigned by the backend, so the wifi connections and POSTs can be splayed while
+still performing the actual measurement at the exact same time.
+
+The sensors also support queueing measurments in non-volatile storage, which can
+dramatically extend the battery lifetime (at the cost of delayed data).
+
+The embedded flash is rated for 100K erase cycles. The default config allocates
+four 4K sectors for NVS, so we get 400K 4K cycles total. Each 4K sector can hold
+126 64-bit entries, so we can queue a total of 50,400,000 measurements before
+worrying about wearing out the flash. At one measurement per minute, that would
+take 95 years! I think we'll be okay...
+
+The board is programmed via pogo pads. I spaced the pads to match a pogo clip I
+ordered online, but it turned out the spacing was different than advertised...
+so I had to kludge something together myself to match the boards I already had.
+
+![](img/pogo.jpg)
+
+In retrospect, I could've made my life easier by avoiding routing in-between the
+pads so they could be bigger. Even as is, holding the pogo pins on with my
+thumb wasn't a problem at all.
+
+![](img/prog.jpg)
+
+Datasheets
+----------
+
+* MCU [ESP32-C3FH4](https://www.espressif.com/sites/default/files/documentation/esp32-c3_datasheet_en.pdf)
+* Sensor [HDC1080MBR](https://www.ti.com/lit/ds/symlink/hdc1080.pdf)
+* Switcher [MT2492](https://datasheet.lcsc.com/lcsc/1810262207_XI-AN-Aerosemi-Tech-MT2492_C89358.pdf)
+* LDO [SPX3819M5-L-3-3/TR](https://datasheet.lcsc.com/lcsc/1810181735_MaxLinear-SPX3819M5-L-3-3-TR_C9055.pdf)
+* 40MHz Crystal [Q22FA12800332](https://datasheet.lcsc.com/lcsc/1810171117_Seiko-Epson-Q22FA12800332_C255899.pdf)
+* RTC Crystal [M332768PWNAC](https://datasheet.lcsc.com/lcsc/2202131930_JYJE-M332768PWNAC_C2838414.pdf)
+* LED [HL-PC-3216S9AC](https://datasheet.lcsc.com/lcsc/2009091206_HONGLITRONIC-Hongli-Zhihui--HONGLITRONIC--HL-PC-3216S9AC_C499470.pdf)
+
+Battery Life
+------------
+
+I tested 22 sensors connecting every minute in my house: the room temperature
+sensors lasted 17-18 days, a sensor placed outdoors which saw daily temps
+30F - 55F lasted 10 days, a sensor in my 40F refrigerator lasted 12 days, and a
+sensor in my -5F freezer lasted 6 days.
+
+With the new code that stores readings in NVS and turns the wifi on much less
+frequently, the battery life will be much longer: hopefully it will scale more
+or less linearly, I'm currently testing 5-minute reporting intervals.
+
+TODO
+----
+
+Things I'm planning to experiment with, in no particular order:
+
+* Report early if temp/humidity delta exceeds a threshold
+* PCB wifi antennas
+* Add hardware to measure battery usage
+* Bluetooth support, especially mesh networking
+* Log collection over the network
+* Signed binaries ("secure boot")
+
+Cost
+----
+
+I had 30 rev 2.0 PCBs manufactured by JLCPCB in November 2022: the total cost
+including shipping was $13.80/each (4-layer, ENIG, two-sided assembly, DHL).
+
+![](img/lot.jpg)
+
+Using the pogo pads, I was able to flash all 30 in under half an hour!

backend/.gitignore

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+data.db
+__pycache__/