A low power ESP32 based thermometer with an ePaper display
| Prototype | Custom board |
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See docs/wiring.md for prototype wiring information and hardware/thermometer-c6/ for the custom board. More pictures lower down.
Measured with a Nordic PPK2 on pure ESP-IDF firmware; full traces, dates and methodology in docs/notes.md. Figures are config-specific — panel, sensor and board all matter.
Read the node tag on every figure. @3V3 rail is the load alone, with the
board's input tree bypassed; @4V2 bat is the whole deployment path, regulator
and charger leakage included. They are not interchangeable, and the battery
figure is not simply the larger one.
| Setup | Deep-sleep floor | Sensor wake | Wake + refresh |
|---|---|---|---|
| Firebeetle ESP32-E + BMP390L + GDEH0154Z90 via DESPI-C02 (FDN340P gate) | 19–20 µA @3V3 rail |
ULP bit-bang I2C every 5 s, avg ≈0 | ~112 mC @3V3 rail (was ~600 mC before light sleep — the Z90's ~21 s refresh used to spin-wait) |
| XIAO ESP32-C6 + BMP581 + GDEH0576T81 via DESPI-C02 (FDN340P gate) | 15.5–16 µA @3V3 rail |
LP core I2C every 60 s: ~1 mA × 3 ms | ~45 mC @3V3 rail — superseded, re-measure (see below) |
| XIAO ESP32-C6 + BMP581 + GDEW029I6FD via Seeed ePaper Driver Board (no gate) | ~25 µA @3V3 rail21.7 µA @4V2 bat |
LP core I2C every 60 s: ~1 mA × 3 ms | 12.2 mC @3V3 rail10.05 mC @4V2 bat |
| thermometer-c6 rev A + BMP581 + GDEM0154I61 (on-board gated booster) | 18.3 µA @4V2 bat |
LP core I2C every 60 s | ~24.3 mC @4V2 bat |
| thermometer-c6 rev A + BMP581 + GDEH0576T81 (on-board gated booster) | 19.05 µA @4V2 bat |
LP core I2C every 60 s | 36.77 mC @4V2 bat |
The XIAO's GDEH0576T81 figure is superseded: it was taken while the panel ran its coldest waveform LUT, which cost 48% of panel busy time. The last row is the deployment configuration on the corrected waveform — daily budget and derivation in docs/notes.md.
A buck draws less current at 4.2 V than its load does at 3.3 V, which is why ~25 µA at the rail reads 21.7 µA at the battery (~90 % efficient). An LDO runs the other way, ~1.15× the charge for the same work — so the custom board beating the XIAO by 3 µA at the same 4.2 V is a real win, not a topology artefact. Its wake figures include LED blinks, left on for observability. Blank cells are unmeasured, not zero; both are on the PPK2 backlog.
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. At one refresh per hour that puts long-term averages in the ~16–51 µA band depending on rig and refresh cadence — load-only runtime of roughly 1–3 years on a 400 mAh LiPo, with the deployment configuration measured at the bottom of that band (~2.3–3.8 C/day, ~380–620 days; derivation in docs/notes.md). But at this current level LiPo self-discharge (a few %/month ≈ 15–25 µA equivalent) rivals the load itself, so expected runtime is on the order of a year, and a 2600 mAh 18650 will age out before the load meaningfully drains it. Not yet confirmed against a full-length run.
Two gotchas worth surfacing (the adapter-leakage ones are under Hardware below):
- XIAO ESP32-C6 battery operation has a hard floor at ~3.6 V: its 3.3 V rail is a pure buck (SGM6029C), which below VOUT + ~245 mV starves its bootstrap and lets the rail sag ~a VTH below VBAT — deep sleep survives, but wakes collapse into 0.5–0.9 A brownout-restart storms. Firmware shuts down at 3.7 V (~12–15 % of Li-ion capacity abandoned); full regime map in docs/notes.md. LiFePO₄ cells (3.2 V nominal) are unusable on this board. The custom board uses an LDO and shuts down at 3.5 V instead.
- Don't use the Seeed ePaper Driver Board's JST battery connector: its ETA9740 charger/boost idles at ~330–500 µA — ~20× the whole system's sleep floor, flattening a 400 mAh cell in ~7 weeks doing nothing — and it double-converts BAT→5 V→3.3 V. Solder the battery to the XIAO's underside BAT pads instead, JST empty, switch off.
The Arduino-era ~20 mA deep-sleep USB CDC gotcha is gone: the pure-IDF USB-Serial-JTAG console doesn't hold the port active.
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.
Looking to replicate this? Easy to start with options are tagged [start with this] below.
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thermometer-c6 — the custom board this project builds towards, replacing the dev-board rigs below: ESP32-C6-MINI-1 + BMP581, a universal 24-pin FPC panel interface with the Good Display booster on-board and P-FET-gated, USB-C charging, an LDO 3.3 V rail, a GPIO-switched battery divider and a 32.768 kHz crystal. Schematic, PCB and rationale: hardware/thermometer-c6/
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Seeed Studio XIAO ESP32-C6 https://wiki.seeedstudio.com/xiao_esp32c6_getting_started/ [start with this]
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(Early 2021 prototype) 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
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ESP32 getting started https://docs.espressif.com/projects/esp-idf/en/latest/esp32/get-started/index.html
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ESP32 power consumption https://diyi0t.com/reduce-the-esp32-power-consumption/
- BMP390L I2C interface:
- BMP581 / BMP585 I2C interface (Bosch next-gen pressure sensors, ~0.5 µA deep-standby)
- BMP581 https://www.bosch-sensortec.com/products/pressure-sensors/bmp581/
- BMP585 https://www.bosch-sensortec.com/products/pressure-sensors/bmp585/
- Fermion breakouts:
- SEN0667 (BMP581) https://wiki.dfrobot.com/sen0667/ [start with this]
- SEN0666 (BMP585) https://wiki.dfrobot.com/sen0666/
- (Early 2021 prototype) DS18B20-PAR OneWire interface
- 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 [start with this]
- (Early 2021 prototype) 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 panels are driven through one of two adapters (same Good Display reference booster on both — panel compatibility notes in docs/wiring.md):
- Good Display DESPI-C02 (switchable RESE 0.47/3 Ω) https://www.good-display.com/product/516.html
- Seeed ePaper Driver Board for XIAO (fixed RESE 0.47 Ω, ETA9740 charger — see gotcha above) https://www.seeedstudio.com/ePaper-breakout-Board-for-XIAO-V2-p-6374.html — wiki: https://wiki.seeedstudio.com/xiao_eink_expansion_board_v2/ [start with this]
The DESPI-C02's boost converter leaks ~534 µA in deep sleep, which an FDN340P P-channel MOSFET on the adapter's 3.3 V line eliminates; the Seeed board's ungated standby measured only ~9 µA, so it runs without a gate.
- 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
- Current builds: PR502535 single-cell LiPo pouch, 400 mAh https://www.gotronic.fr/art-accu-lipo-3-7-vcc-400-mah-pr502535-5812.htm [start with this or equivalent]
- Original prototype: RS Pro 18650 26H Li-ion, 2600 mAh https://fr.rs-online.com/web/p/batteries-taille-speciale/1449406/
Custom rev A board (with BMP581), GDEM0154I61 & 400mAh battery
Custom rev A boards (with BMP581), GDEH0576T81 & 400mAh battery
Seed Xiao C6, BMP581, GDEH0576T81 & 400mAh battery
Seeed Xiao C6, e-Paper Driver Board, BMP581, GDEW029I6FD & 400mAh battery
Firebeetle ESP32-E, BMP390L, GDEH0154Z90 & 400mAh battery
Firebeetle ESP32-E, DS18B20, GDEW0213M21 & 2600mAh battery
