A low power FireBeetle ESP32-E based thermometer with an ePaper display

See docs/wiring.md for wiring information.

Power consumption

Measured with a Nordic PPK2; full traces and methodology in docs/notes.md.

Setup	Deep-sleep floor	Sensor wake	Display refresh
ESP32-E + BMP390L + DESPI-C02 ePaper (FDN340P power gate)	~18 µA	ULP bit-bang I2C every 5 s, avg ≈0	—
XIAO ESP32-C6 + BMP581 + GDEH0576T81 ePaper	~14 µA	LP core I2C every 60 s: ~1 mA × 3 ms	~93 mC (3.2 s × 29 mA avg, 322 mA peak)

The main CPU only wakes on a ≥0.1 °C delta or a safety-net tick, so a display refresh is the dominant event on a typical day. Long-term average stays in the 15–40 µA band, which gives a load-only runtime of roughly 1–3 years on a 400 mAh LiPo depending on how often the display refreshes. At this current level LiPo self-discharge (a few %/month ≈ 15–25 µA equivalent) is comparable to the load itself, so expected runtime is on the order of a year — to be confirmed against real long-run measurements. A 2600 mAh 18650 sits near the self-discharge floor and is expected to age out before the load meaningfully drains it.

Two gotchas worth surfacing:

• DESPI-C02 ePaper adapter leaks ~534 µA from its boost converter even with the panel hibernated. A P-channel MOSFET (FDN340P) on its 3.3 V line eliminates it.
• XIAO C6 USB Serial/JTAG stays on in deep sleep by default (~20 mA), masking all savings. Disabled via ARDUINO_USB_CDC_ON_BOOT=0 plus matching build_unflags.

For context: the original 2021 prototype (wake + refresh every 60 s, no ULP) ran a 2600 mAh cell flat in 8.5 days at ~12.6 mA average — the ULP/LP-core redesign is ~700× more efficient.

Hardware

Controller boards

• Firebeetle ESP32-E https://wiki.dfrobot.com/FireBeetle_Board_ESP32_E_SKU_DFR0654

  • ESP32 datasheet https://www.espressif.com/sites/default/files/documentation/esp32_datasheet_en.pdf
  • ESP32-WROOM-32E datasheet https://www.espressif.com/sites/default/files/documentation/esp32-wroom-32e_esp32-wroom-32ue_datasheet_en.pdf
  • TP4056 battery charger IC (onboard) https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdf https://www.best-microcontroller-projects.com/tp4056.html

• Seeed Studio XIAO ESP32-C6 https://wiki.seeedstudio.com/xiao_esp32c6_getting_started/

• ESP32 getting started https://docs.espressif.com/projects/esp-idf/en/latest/esp32/get-started/index.html

• ESP32 power consumption https://diyi0t.com/reduce-the-esp32-power-consumption/

Temperature sensor (multiple options)

• DS18B20-PAR OneWire interface
  • datasheet https://datasheets.maximintegrated.com/en/ds/DS18B20-PAR.pdf
• BMP390L I2C interface:
  • datasheet https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bmp390-ds002.pdf
  • breakout board https://wiki.dfrobot.com/Fermion_BMP390L_Digital_Barometric_Pressure_Sensor_SKU_SEN0423
• BMP581 / BMP585 I2C interface (Bosch next-gen pressure sensors, ~0.5 µA deep-standby)
  • Fermion breakouts: SEN0667 (BMP581), SEN0666 (BMP585)

Display (multiple options)

• Adafruit 1.54" Tri-Color eInk / ePaper 200x200 Display with SRAM - SSD1681 Driver https://www.adafruit.com/product/4868 https://learn.adafruit.com/adafruit-1-54-eink-display-breakouts?view=all
  • uses Good Display GDEH0154Z90 https://www.good-display.com/product/285.html
• Good Display GDEW0154M09 black and white, 200x200, 1.54" fast full refresh https://www.good-display.com/product/206.html
• Good Display GDEW0213M21 black and white, 212x104, 2.13" DES screen https://www.good-display.com/product/354.html
• Good Display GDEW029I6FD black and white flex, 296x128, 2.9" (UC8151D, partial updates) https://www.good-display.com/product/209.html
• Good Display GDEM0154I61 black and white flex, 200x200, 1.54" (driven via GDEY0154D67 SSD1681 driver) https://www.good-display.com/product/535.html
• Good Display GDEH0576T81 black and white, 920x680, 5.76" HD (SSD2677, partial updates) https://www.good-display.com/product/702.html

Good Display panels are driven through a DESPI-C02 adapter. Its boost converter leaks ~534 µA in deep sleep, which an FDN340P P-channel MOSFET on the adapter's 3.3 V line eliminates — see docs/wiring.md.

Other components

• FDN340P P-channel MOSFET (SOT-23) — power-gates DESPI-C02 3.3 V during deep sleep
• AO3400A N-channel MOSFET (SOT-23) — switches the XIAO C6 battery voltage divider so it doesn't draw ~10 µA continuously

Battery

• Current builds: PR502535 single-cell LiPo pouch, 400 mAh https://www.gotronic.fr/art-accu-lipo-3-7-vcc-400-mah-pr502535-5812.htm
• Original prototype: RS Pro 18650 26H Li-ion, 2600 mAh https://fr.rs-online.com/web/p/batteries-taille-speciale/1449406/