In our appartment, we have used a Venta LW45 air humidifier during the winter to increase air humidity to acceptable levels. These humidifiers follow a very simple principle: A ventilator blows air through the device, while several rotating discs are placed in the water and provide enough water to the airflow through them. The humidifier has three operating levels (1-3) and signals an error if the water reaches a low level.

Now, since I have ZigBee temperatur and humidity sensors in every room, I wanted to automate the humidifier and write rules in OpenHAB that would turn on the humidifier whenever the humidity falls below a threshold and also turn it off again once an acceptable level is reached.

Previous versions of the Venta humidifier (LW14, LW24, LW44) are simply using a 220V motor with a manual switch that changes between different resistor values to control the speed of the motor. With their current generation of humidifiers, Venta controls the motor through a fully electronic control board with two UP + DOWN buttons.

Opening the device shows the main PCB, which is connected to a control panel (with the switches and LEDs visible at the upper side of the humidifier) through a 10-pin IDC flat band cable. The power supply is a 24V external power supply that is plugged into the control panel. The schematics of the control panel is as follows:

As a first step,