SEN66 environmental sensor

Summary¶

The Sensirion SEN66 is a multi-parameter environmental sensor measuring particulate matter (PM₁, PM₂.₅, PM₄, PM₁₀), CO₂, VOC index, NOx index, temperature, and humidity in a single I2C module. On libdrone, it mounts on the sensor mast above the propeller plane, where it samples uncontaminated ambient air rather than the drone's own downwash. Each measurement principle is physically distinct: Mie scattering for particles, NDIR absorption for CO₂, metal-oxide resistance shift for VOC/NOx. Understanding these principles tells the operator what the sensor can and cannot detect, and what environmental conditions limit its accuracy.

Concept¶

Particulate matter — Mie scattering¶

The SEN66 measures aerosol particle concentration by passing a laser beam through a sample chamber and measuring the light scattered by particles crossing the beam. This is Mie scattering — light scattering by particles whose diameter is comparable to the light's wavelength. The scattered light intensity and its angular distribution encode both particle size and concentration.

The sensor counts particles in four size bins: PM₁ (< 1 µm diameter), PM₂.₅ (< 2.5 µm), PM₄ (< 4 µm), and PM₁₀ (< 10 µm). Fine particles (PM₁, PM₂.₅) are the public health concern — they penetrate deep into the lungs. Coarse particles (PM₄, PM₁₀) are road dust, pollen, sea salt. The ratio between size fractions carries information about the particle source.

Limitation: the Mie scattering signal requires clean optics. Dust or condensate on the laser or detector window degrades readings. The SEN66 has an automatic cleaning cycle (heats the measurement chamber to evaporate condensate) that activates after each power cycle.

CO₂ — NDIR absorption¶

The SEN66 measures CO₂ concentration using Non-Dispersive Infrared (NDIR) absorption. CO₂ molecules absorb infrared light at a specific wavelength band centred on 4.26 µm — a consequence of the molecule's vibrational modes. The sensor shines an IR source through a sealed measurement cell; a detector on the other side measures how much light was absorbed. More CO₂ in the cell → more absorption → lower detector signal → higher reported concentration.

NDIR is the gold standard for CO₂ measurement: it is direct, stable over time, and not confused by other gases (each gas absorbs at different wavelengths). The SEN66's range is 400–30,000 ppm. Ambient outdoor CO₂ is approximately 420 ppm; a sealed room with people can reach 2,000+ ppm; combustion nearby can produce short spikes to thousands of ppm.

Cross-sensitivity: water vapour absorbs in nearby wavelength bands and can elevate CO₂ readings in high-humidity conditions. The SEN66 corrects for this using its onboard temperature and humidity sensor.

VOC and NOx — metal-oxide resistance¶

Volatile Organic Compounds (VOCs) and nitrogen oxides (NOx) are measured by a metal-oxide (MOx) semiconductor sensor. The MOx sensor consists of a heated metal-oxide crystal whose electrical resistance changes in the presence of reducing gases (VOC) or oxidising gases (NOx). Higher temperature (~350°C, maintained by a microscopic heating element) accelerates the surface reactions, increasing sensitivity.

The SEN66 outputs VOC Index and NOx Index (both 1–500 scale) rather than raw concentration values. The index is relative to a rolling baseline: index 100 is the recent background; values above 100 indicate elevated VOC or NOx; values below 100 indicate unusually clean air. This design makes the sensor practical in varying environments — the baseline adapts over hours rather than requiring factory calibration against a reference gas.

Limitation: MOx sensors are broad-spectrum — they cannot distinguish which specific VOC or NOx compound is present. They detect "something is elevated" but not "specifically benzene" or "specifically NO₂."

Why the mast height matters¶

The propellers create a toroidal recirculation zone: a doughnut-shaped ring of disturbed air approximately one rotor radius above the propeller plane, extending 80–120mm above the rotor disk at libdrone's typical hover thrust. Air inside this zone has passed through the propellers — it is turbulent, heated, and contaminated with whatever the motors and ESC are outgassing.

Mounting the SEN66 inside this zone measures the drone's own emissions, not the ambient atmosphere. The sensor mast height (85mm above the prop plane on the V2.4.6 reference design) places the sensor above the recirculation zone boundary with margin. See → induced-velocity for the fluid dynamics derivation. Every millimetre of mast above the minimum height increases the pendulum arm and slightly affects PID tuning; the mast height is set to the minimum that clears the recirculation zone.

Reference¶

I2C connection on H7A3-SLIM: I2C1 (SDA1/SCL1 pads, or via GX12 Connector A pin 5/6). Address 0x6B. No address configuration required — SEN66 has a fixed address.

Betaflight / ArduPilot note: The SEN66 is not natively supported by Betaflight or ArduPilot. It is read by the LCM-1 (Pi Zero 2W) companion computer or ESP32-S3 payload module via I2C, which handles the Sensirion I2C protocol and logs data to SD card or forwards via WiFi.

Procedure¶

Verify SEN66 on bench before first flight¶

1. Connect SEN66 to I2C1 on H7A3-SLIM or to the LCM-1 I2C bus.
2. Power on. Wait 30 seconds for warm-up.
3. Issue a 0x36 0x03 (measure start) command at address 0x6B and read back the 48-byte measurement frame.
4. Confirm CO₂ reading is in the range 400–600 ppm (normal room air).
5. Breathe near the sensor inlet — CO₂ should rise to 1000+ ppm within 5 seconds, then return to ambient within 30 seconds.
6. If CO₂ reads 0 or > 10,000 ppm at rest, check I2C connection and confirm 3.3V supply.

Rationale¶

The SEN66 was selected over single-purpose sensors (CO₂-only, PM-only) because the combination of four physical measurement principles in one module reduces the payload mass and GX12 wiring complexity. A separate CO₂ sensor, PM sensor, VOC sensor, and temperature/humidity sensor would require a custom circuit board; the SEN66 provides all eight outputs on a single I2C address. The trade-off is reduced specificity on VOC/NOx — if the application required identifying specific compounds rather than detecting elevated levels, a more specialised sensor would be needed.

Connections¶

requires: - payload-architecture - gx12-connector-standard - induced-velocity related: - supplemental-sensors - payload-sdk - payload-electrical-interface - gps-antenna-placement leads_to: - payload-sdk - payload-integration