here pictures of the dehumidifier:

firmware source code

here pictures of sensor modules (not mine, but also not an original):

the sensor modules kept failing. my guess for the reason is that the IC and passives are right next to the cold and humid metal radiator, and the plastic separator isn't enough to protect it.

the sensor module uses an unidirectional pulse-width modulation protocol (similar to Infra-Red protocols used by remote controls):

• the data line is pulled up by the receiver, and the sensor module pulls it low to transmit bits
• 0 bits are transmitted using a 500 µs low, 500 µs high pulse
• 1 bits are transmitted using a 500 µs low, 1500 µs high pulse
• 4 bytes are transmitted
• this is followed by a break condition using a 15 ms low pulse
• the first byte encodes the temperature, in °C
• the second byte encodes the relative humidity, in %
• the third byte is the first negated, to detect transmission errors
• the fourth byte is the second negated, to detect transmission errors

the replacement modules I designed implements this protocol in an STM8S003 micro-controller, and gets the humidity measurements from an AHT20 sensor over I²C. the parts are placed on the front side of the PCB, less exposed to the cold humid metal plate.

here is the schematic, fabrication output, hardware source files, firmware source code, firmware binary