Wisp

Wisp

Wisp is an open-source, self-hosted ESP32 room climate monitor. No cloud — it publishes readings over MQTT into your own Home Assistant instance, which exposes them to Apple HomeKit.

It tracks:

• CO2 — SCD41
• VOC / NOx (air quality) — SGP41
• Temperature & humidity — SCD41, cross-checked against BME280
• Barometric pressure — BME280 (also compensates the SCD41's CO2 reading)
• Ambient light — BH1750

All hardware, firmware, and infrastructure needed to build and run one live in this single repo.

Repo structure

Why these sensors

The SGP41 replaces the more common BME680: the BME680's gas reading only becomes useful through Bosch's closed-source BSEC library, which doesn't sit well with an MIT-licensed firmware. Sensirion's Gas Index Algorithm (BSD-3-Clause) does the same job openly. Dropping the BME680 would also drop barometric pressure, so the BME280 sits alongside the SGP41 for that — and because the SCD41 needs ambient pressure to compensate its own CO2 reading, that pressure reading isn't a bonus, it feeds the primary measurement.

Temperature and humidity are read from the SCD41; the BME280's readings are kept as an independent cross-check rather than the source of truth, so firmware can notice a failing sensor.

Full rationale: docs/concept.md.

MQTT topic structure

home/climate/<room>/{co2,temperature,humidity,voc,nox,pressure,lux}

First room: living room.

Licensing

Each subfolder is licensed separately, matching its content type:

• /hardware → CERN-OHL-S v2
• /firmware → MIT
• /docs → CC-BY-4.0