I'm not using the LM2596 (yet), I power the RP Pi (5V) and Reef-PI_HAT separately with 12V. I've bought components to integrate the LM2596 on the Reef-PI_HAT, but first I have to understand why the Reef-PI_PWM module is not working as expected. With the Reef-PI software I got very strange behavior in the user interface, when I started to control the PWM module directly (Python) it did seem to work however (think it's related to the used NPN transisors). Have ordered an oscilloscope to measure what comes out of the PCA9683 chip. Also 'recycled' an old oscilloscope, but not sure if that one still works (>30 years old).
J8 is an 'I2C' bus with 5 and 12V to connect to other boards. That's were I'm having some problems now, the I2C bus communicates with the PWM chip, but I don't get the expected output voltages (0-10V) yet.
Ahh ok, that makes sense. I have not read a schematic since High school lol so I am a bit rusty.
