To be able to open and close the shutter when the fan is active or not, the signal of the stock ventilation control bus (pluggit IconVent 160) needs to be analyzed.

Using an oscilloscope reveals a pulse width modulated (PWM) signal with a 5V amplitude and a frequency of 3.915kHz.

When the control unit generates a 50% duty cycle PWM signal, the connected fans will stop operation. A duty cycle above 50% will produce inward air flow, below 50% will prodcue outward airflow.

With this information, the firmware to analyse and replicate the PWM fan control signal can be developed.