So, just my luck, but the HPMA was brand new but defective. I RMA’d it with Honeywell.
However, I had a PMS5003 lying around which is very similar in design but has a slightly different pinout. Other than that, it also communicates over UART and needs 5V input for the fan. I changed the wiring to match the PMS5003, didn’t touch the code, and it works. The values match the ones reported by a Temtop M2000 station as well as the ones by the local AQI measurement station.
rx_buf: 66 77 0 28 0 7 0 9 0 9 0 7 0 9 0 9 0 0 0 0 0 0 0 0 0 0 0 0 151 8 1
0000082554 [app] INFO: {"pm25":9,"pm10":9}
rx_buf: 66 77 0 28 0 109 0 219 0 250 0 72 0 145 0 166 0 0 0 0 0 0 0 0 0 0 0 0 151 8 5
0000102570 [app] INFO: {"pm25":219,"pm10":250}
The second measurement is from directly next to my soldering station. Eww.
One down.
Next was the CCS811 which came off easily with the hot air soldering gun. I have a spare CCS811 here which I could desolder from its breakout board, but first I want to make sure the rest of the board is working.
The Si7021 is consistently reporting relative humidity of 119.00% and a temperature of 128.87°C. Which seemed odd until I looked at the code that is used for reading the data. The raw values reported by the sensor are just 65535 (0xFFFF) for both temperature and humidity.
And that was also the case for CCS811 before.
So I hooked up my crappy Aliexpress scope and while I’m no electrical engineer and I’m not great at reading the output, it looks to me as if SDA is just permanently stuck on HIGH.
So there is definitely more broken on that board than just the CCS811.
