I got this garage door automation circuit with which I want to replace the proprietary microcontroller with an Arduino, so I've been reverse engineering it and it seems to work in a weird way.
It is fed with 240VAC and a transformer creates steps it to around 25VAC. The sensors and radio receiver use this unrectified supply. Somehow, can't tell for sure how, there's also rectified 5VDC rail. Likely through an onboard buck converter, given there's a small transformer device there.
The sensor's output is normally low, sinking current through a resistor, a photo-couple's input and a normal LED. The LED and Photo-couple's LED are in parallel, sharing the same resistor.
At the Photo-couple's output, the emitter is going into a microcontroller pin and the collector is connected to a 4094 shift-register, at a parallel output pin. The collector also branches to a DIP-switch to optionally connect it to another MCU pin.
The Data and Clock lines of shift register go to other 2 pins in the MCU.
I find this arrangement strange. Does anyone have any experience on this?