Sensors

Supported by the Altruist Urban firmware (ESP32-C3 and ESP32-C6). Each driver is hardware independent and publishes typed rohi-hal observations; see Sensor HAL (rohi-hal) for lifecycle, timing and events.

BMP280 / BME280

Forced mode: temperature ×2, pressure ×4, humidity ×1, IIR filter off, one conversion (≈19 ms) per sample; the sensor sleeps in between.

Address. The driver probes 0x76 first, then 0x77, and uses the first address whose chip ID register reads a BMP280 (0x56..=0x58) or a BME280 (0x60); devices with another ID are skipped. Either strapping works without a rebuild. If valid sensors answer at both addresses, 0x76 wins (two BMx280 are not supported). Probing is repeated on every re-init, so a sensor moved to the other address is found again. If nothing is found, the failure is a bus error, an unsupported chip ID or no device, in that order of precedence; the last two keep the sensor NotDetected.

SCD40 / SCD41

Probing stops any running measurement, reads and validates the serial number, then starts periodic measurement (a new value every 5 s). Each sample waits on the data-ready flag (poll every 500 ms, at most 20 polls) and validates every word with CRC-8. The first valid value arrives about 5 s after start, until then the sensor is WarmingUp.

SDS011

The command flow and wire protocol (sensors/sds011/protocol.rs) are derived from the community sds011-rs driver (MIT OR Apache-2.0). The sensor is kept in query mode and asleep between cycles, so the fan and laser run only while measuring:

wake -> warm-up (30 s) -> sample 1..N -> filter / aggregate -> publish PM2.5 / PM10 -> sleep (90 s)

Constants (sensors/sds011/mod.rs), also available as sds011::Config:

These values are a starting point and are to be confirmed on hardware against the legacy firmware behaviour.

• Valid samples only. Malformed, truncated and checksum-invalid frames and values above 999.9 µg/m³ are dropped. Sampling stops when SAMPLE_COUNT valid samples exist or the window is over.
• Aggregation (sds011/aggregate.rs, fixed arrays, no heap): rounded integer mean per channel; with at least 5 samples the lowest and highest are discarded first, independently for PM2.5 and PM10. Example, PM2.5 10 12 11 80 9: drop 9 and 80, result 11.
• Failure. Too few valid samples give Error::InsufficientSamples and nothing is published. A silent sensor fails the cycle at once (NoResponse). Either way the sensor is told to sleep and the runner retries after its recovery backoff.
• Robustness. Every UART read is bounded to 5 s, stale bytes (e.g. frames of a sensor still in active mode) are discarded, unexpected frames are skipped and the wake-up is retried up to 3 times.
• The first measurement starts right after boot; the cycle’s own warm-up is the only wait.

Noise

Metric: sound pressure level in dB SPL (re 20 µPa), unweighted. It is not dBA and no A-weighting is applied.

Levels include the microphone’s frequency response and the device’s own noise (e.g. the SDS011 fan). They compare between devices with the same microphone model, not with a calibrated, A-weighted sound level meter. This is the v1 metric the telemetry schema can rely on; an A-weighted level would be added as a separate, tested metric under its own name.

Raw audio is never stored: a 256-sample stack chunk plus the 8 KiB DMA ring buffer (≈128 ms) are the only buffers.