As there’s already the possibility to control radio-relays via MQTT, the next move was to replace the connection from the button to board with a circuit integrating the relay HM-LC-Sw1-FM. https://de.elv.com/homematic-unterputzschalter-1fach-hm-lc-sw1-fm-fuer-smart-home-hausautomation-076793
But there was one problem. The relay works with an input and output voltage of 230V. That’s too much to control the board on the garage opener. So I had to integrate another relay that could switch an arbitrary input voltage but works with a rated Control Supply Voltage of 230V. I finally came up with an relay by Entrelec.
Now to the fun part. Connecting all parts together according to the wiring on the left. The Entrelec relay is directly connected to the garage opener board. The control supply is provided by the Homematic relay. The switch input of the Homematic relay is provided by the hardware button and finally some 230V to feed the relay. That’s it.
In order to test it, there’s no faster prototyping than to use Node-Red. It enables visually wiring up a control that either automates a workflow or provides a rudimentary GUI. https://nodered.org In this case, it’s a simple UI consisting of one toggle labeled „Garage“ sending out a message to the MQTT broker to fire up the Homematic relay.