Desk CO₂ Monitor: ESP32-C3, SCD41 and E-Paper

A CO₂ meter that sits on the desk, wakes every few minutes, takes one reading and goes back to sleep. No backlight, no fan, no cloud account: an e-paper panel holds the last number without drawing anything, and the battery is meant to last long enough that charging it is a rare event rather than a habit.

This page is a test record for the site build — the structure is real, the build is not finished. Every number below is either quoted from a datasheet, with the source linked at the end, or marked as still to be measured.

What it does

Measures CO₂, temperature and relative humidity with a single Sensirion SCD41.
Shows the last reading on a 2.9″ e-paper panel, which keeps the image with the power off.
Sleeps between readings. The ESP32-C3 wakes on the RTC timer, takes one single-shot measurement and goes back down.
Runs from one 18650 cell through a protection board. USB-C for charging and for flashing.
Needs no network. Wi-Fi is on the chip and stays off unless you want the readings logged somewhere.

Parts

Qty	Part	Why this one
1	ESP32-C3 module board with USB-C	Single RISC-V core is plenty for one sensor and one panel, and the chip has native USB — no serial adapter on the board.
1	Sensirion SCD41	True NDIR CO₂, not a VOC sensor guessing at it. Single-shot mode is what makes the sleep cycle possible.
1	2.9″ e-paper panel, SPI, 296 × 128	Holds the last frame with no power. A reading that updates every few minutes does not need anything faster.
1	18650 cell + protection board	Cheap, replaceable, and standard enough that anyone repeating this already has one.
2	4.7 kΩ resistors	I²C pull-ups, if the sensor breakout does not carry its own.
1	Printed enclosure, FDM	Two parts, no supports. Vents on the back so the sensor sees room air and not its own board.

The whole bill of materials. Nothing here is hard to source.

How it goes together

Wire the SCD41 to the I²C pins and the panel to the SPI pins. Keep the sensor away from the regulator — its own heat lands in the temperature reading.
Check the sensor answers on the bus before anything else. If the scan finds nothing, it is the pull-ups or the wiring, never the firmware.
Flash the firmware and confirm one full cycle on USB power: wake, read, draw, sleep.
Only then move to the cell. A build that misbehaves on battery is much harder to read than one that misbehaves on a bench supply.
Print the enclosure, fit the panel behind its window, and leave the vents clear.

Firmware

The whole loop is four steps, and the interesting part is that there is no loop at all: deep sleep resets the chip, so every wake starts at the top.

// Pins and timing, in one place so the rest of the sketch never hardcodes them.
#define SDA_PIN        6
#define SCL_PIN        7
#define EPD_CS_PIN     10
#define EPD_DC_PIN     3
#define SLEEP_SECONDS  300

void setup() {
  Wire.begin(SDA_PIN, SCL_PIN);

  // Single shot: the sensor is powered up, measured once, and left alone.
  scd4x.measureSingleShot();
  scd4x.readMeasurement(co2, temperature, humidity);

  drawPanel(co2, temperature, humidity);

  esp_sleep_enable_timer_wakeup(SLEEP_SECONDS * 1000000ULL);
  esp_deep_sleep_start();
}

void loop() {
  // Never reached: deep sleep restarts the chip at setup().
}

Why the panel keeps a shadow of the previous reading

E-paper does not repaint a pixel, it moves particles, and a partial refresh only moves the ones it has to. Do that a few dozen times and the digits that stopped changing leave a faint outline behind. The fix is a full refresh — slower, and it flashes the whole panel black and white — every so many partial ones. How often is a judgement call: too rare and the ghosting stays, too often and the display blinks at you all day.

Numbers

Figure	Value	Where it comes from
CO₂ measurement range	400–5000 ppm	SCD4x datasheet
CO₂ accuracy	±75 ppm at 400–1000 ppm	SCD4x datasheet
CPU	32-bit RISC-V, single core, up to 160 MHz	ESP32-C3 datasheet
SRAM	400 KB, plus 8 KB RTC FAST retained in deep sleep	ESP32-C3 datasheet
Programmable GPIOs	22	ESP32-C3 datasheet
Deep-sleep current, chip only	5 µA typical, RTC timer and RTC memory on	ESP32-C3 datasheet
Deep-sleep current, whole board	to be measured	depends on the module’s regulator and USB bridge
Battery life on one 18650	to be measured	follows from the board current above
Full panel refresh time	to be measured	panel and driver dependent

Datasheet figures are quoted; everything that depends on this particular build is still open.

The chip datasheet tells you what the silicon does in deep sleep. It says nothing about the regulator, the USB bridge and the status LED your board vendor put next to it — and on a build like this, those three are the entire battery life.

Sources

Sensirion SCD4x datasheet — measurement range, accuracy, single-shot mode.
Espressif ESP32-C3 series datasheet — core, memory, GPIO count, deep-sleep current.