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https://www.airgradient.com/diy/

Founder is very supportive, good product, easy to build

The sensor will display PM2.5, CO2, Temperature and Humidity on the display and can optionally send the data to any server for data logging (e.g. the AirGradient platform or any other cloud backend).

Parts: Wemos D1 Mini USD 2.24 Wemos OLED display USD 2.47 Plantower PMS5003 PM Sensor USD 13.89 Senseair S8 CO2 Sensor USD 28.00 SHT30 or SHT31 Temperature and Humidity Sensor Module USD 2.55

Details: https://www.jefftk.com/p/testing-air-purifiers

I'm using a Temtop M2000 because it was the cheapest option I found with per-minute data export: https://temtopus.com/collections/temtop-co2-monitor/products...

It’s not the cheapest but this is what I use https://www.airthings.com/view-plus

We run one of these (https://www.adafruit.com/product/4632) off of an Arduino Uno with a small LCD hat (w/ micro SD for logging as well). I think that's the same sensor as in the outdoor units that Purple Air sells. ~$150 for the whole setup, but could definitely run that off a smaller / cheaper board, display only. Going strong for 2+ years at this point.

I use a Nova SDS011 sensor connected a raspberry pi, then feed the data into HomeAssistant with a python script and MQTT.