I was planning to release this way earlier than October, but work and private life got in the way 🙁
Small introduction into the problem:
Two distinct signs:
- Slow to respond to particle changes.
- Display only shows 001
The sensor used in the Xiaomi Air Purifiers is the: PMS9003M by Plantpower
This sensor uses a pin named SET. The function of that pin is to change between two modes:
In Active Mode: The Sensor will send data every measurement (2.3 Seconds when there are no big changes detected, or 200-800ms if large changes are detected)The small fan is turned on all the time.
In Sleep Mode: The Sensor will not measure automatically (only if a measure command is send) and the fan will only spin up if there is a measurement.
The hanging of the Display and the slow responses are cause by the the following things
1. My Air Purifiers contain a software bug where if you turn off the display (button on the backside of the Purifier) the Plantpower Sensor won’t go back in active mode.
2. Sometimes the sleep mode gets activated without clear reason.
Highly Likely assumptions:
1. Xiaomi averages the sensor data quite aggressively, this is a nice feature in location where the Air quality is very bad (China, etc)
It would not surprise me if they would change the averaging algorithm based on your location.
2. Xiaomi uses sleep mode to extend the life of the small fan. Depending on the manufacturer of the 20mm fan, the rated life time will be around 25k hours (approx. 2.8 years running 24/7)
3. The air is clean, I live in the Netherlands and for the air purifier even getting above 10-20 is rare. If I solder or do other dirty work the sensor measures way higher values. My baseline in normal use is actually 1.
The new Solution:
I made a small PCBA + cable that goes between the sensor and the Plantpower sensor. This small board will pull the SET pin to always high. This forces the sensor to keep sending updates.
How to install:
Remove the back cover of the sensor:
Thanks Nat Julian Belza for pointing me to the above picture.
Remove the two screws holding the Plantpower sensor.
Now remove the cable to the Plantpower sensor. (you need to pinch the connector)
Then connect the colourful cable to the plant power sensor.
Connect the Xiaomi Air purifier cable (The black one that you removed in step 2) to the adapter board.
Push the cable into the housing. (I pushed mine to the right of the Sensor)
Screw the sensor back in to its original place.
You’re done! easy as 1-2-3.
You can buy one here:
Never expiring Xiaomi air filters: https://flamingo-tech.nl/2021/06/20/xiaomi-air-purifier-3h-modchip-is-here/