I'm based in HK so prices are definitely lower since I can order directly from taobao. The purifier was around 350 usd, it's the Mijia Air Purifier MAX Enhanced

I'm based in HK so prices are definitely lower since I can order directly from taobao. The purifier was around 350 usd, it's the Mijia Air Purifier MAX Enhanced Edition. For the sensor, I used an ESP32 with a small eink screen (from lilygo) with a sensirion SPS30 sensor for detecting particles (see this very good teardown https://www.mistywest.com/posts/teardown-sensirion-particle-matter-sensor/ https://www.mistywest.com/posts/teardown-sensirion-particle-...), a sensirion SGP30 sensor for detecting VOCs and a BME280 to measure temperature, humidity and air pressure. I think total cost is roughly 60 usd. ESPHome makes it relatively easy, see this guide for a similar setup https://neon.ninja/2021/11/breathe-better-with-this-indoor-air-quality-sensor/ https://neon.ninja/2021/11/breathe-better-with-this-indoor-a... Hope that helps, it's a fun project to figure things out (it was my first project with ESP32).