xiaomi-humidifier-water-sensor-digispark

Xiaomi SmartMi Evaporative Humidifier water level sensor for digispark ATTiny85

Description

This project is modified from the original to work with a digispark clone.

Benefits

• Based on cheap clone
• Fits fully inside the case
• Calibration by shorting P3 to GND
• In-circuit programmable

Parts list

• Digispark clone, get the blue board with the microUSB socket from e.g. AliExpress.
• USB to microUSB cable for loading the software
• 1M resistor
• Wire

Notes

P1 is used for serial because it has a LED on it. So you can see if it is sending serial packets, and also it does not interfer with capacitance measurement.

Sensor response is not very linear below 0.5L, but because below 0.5L the drum can't really reach the water anyway, it does not make sense to linearize it:

Compiling

You need the CapacitiveSensor libary - just install it in Arduino IDE.
Also grab and install the SendOnlySoftwareSerial library here: https://github.com/nickgammon/SendOnlySoftwareSerial

Step by step guide

1. Disassemble the humidifer and remove the power board (located in the back where AC cord goes in)
2. Desolder the blue rectangular component on the power board, marked U03. Good idea to clear some of the lacquer off, and after that desoldering wick and some wiggling of the pins should do it.
3. Cut off 2 pins of the header that came with your digispark and solder them to GND and 5V pads.
4. Connect wires to the TP and TXD pads.
   
5. Program the digispark with the sketch.
6. Solder one end of 1M resistor to P0 and clip the lead.
7. Fit and solder in the digispark. Connect TXD to P1 and TP to P2.

• Angle the ATTiny85 a little towards the top of the PCB, so microUSB plug clears screw extrusion when connected.
• Leave longer leads on the through-hole wires on the digispark. On P1 to be able to attach a USB-TTL if you need to debug and on P2 to solder the resistor to the wire.
  

8. Connect the sensor and the main ground on the back of the PCB, because the ATTiny85 obviously does not have a separate analog ground.
   
9. Calibration:

• The humidifer should be empty, the sensor clean, and the bottom part should be fully assembled (incl. the drum). Do not try to calibrate outside of the circuit or with partial lower part, it will not yield a good result.
• Remove the top part and fill the water until it just touches the very bottom part of the sensor. This will be your "minimum level". Reinstall top part.
• Connect P3 to GND on the digispark board with a temporary jumper.
• Plug in the humidifer and immediately press the power button, so it turns on.
• If the calibration process started successfully, the humidifer will display 5 bars without flashing for 10 seconds. After 10 seconds it will display 1 bar. If the humidifer displays 3 bars, you have a short in the sensor or the sensor wiring. Fix it and try again.
• After humidifer displays 1 bar, start to fill the humidifer with water. The sensor will use default values to show you the level. Fill the water until 5 bars begin flashing. If the level does not change from 1 bar at all, and only at the end directly displays 5 flashing bars, then you probably have a problem with the circuit or sensor was not dry/clean when you started, so you will have to fix and try again.
• The calibration is complete and stored in the EEPROM, and will be used on startup.
• Unplug the humidifier, remove the jumper between P3 and GND and reassemble the humidifier.