In this project, I'm designing a WiFi-connected PCB that adds remote control capabilities to an old-school Venta LW 45 air humidifier.
The control panel - connected to the main PCB via a 10-pin ribbon cable - of the humidifier provides five LEDs indicating power status, level and error and two buttons to turn the device on/off and to change levels. I insert my own custom PCB where the ribbon cable is plugged in, thus allowing me to read out the HIGH/LOW status of each cable pin as well as pulling down each cable pin emulating a button press.
As the Venta LW45 is powered through an external 24V power supply, I tap into that power source, but need to convert to 3.3V for the ESP8266 that is used to provide the WiFi control.
To read out the status of the LEDs, I use LM339 / LM393 dual / quad differential comparators and to emulate button presses, I'm using three 2N7000 FET transistors.