As per the circuit description,  voltage across Capacitor C15 will never go above VBE(sat)  i.e. Base to Emitter saturation voltage ( 1.0V) of transistors T7 & T8 (when Base to Emitter  diode will get forward bias) in case of positive  DC offset voltage more than  0.55 V ( for T8) or a negative DC offset greater than 0.85 V (for T7) at the output and almost entire offset voltage (less  1.0V VBE(sat)  of T7/T8) will appear across R23 (15kΩ) and current through R23 will be limited to approx. 3.67 mA { i.e. (56-1.0)/15000} in case of DC offset voltage swing to even up to +V or –V supply rail. Capacitor C15 is already shown as non-polar in the circuit schematic. Mal  

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Very nice! My Version 1 is tested and is working very well. Sure, you can use it with any MCU, maybe with a small PCB instead of the prototyping PCB I used.

The PCB should be available (very) shortly from here: https://www.elektor.com/usb-naar-dmx-bare-pcb-160493-1.

The second version plugs on the Arduino extension connectors only: VIN, GND, D2 & D8. No soldering on the Arduino board required. The dotted lines only showed where they were connected in the first version. And indeed you can plug the output of the circuit into the barrel jack (and D2 & D8) but then you need a plug. I don't see why you would have to short-circuit the Arduino diode in that case (unless the input voltage is too low). I changed the drawing a bit (attached) to make it more explicit. Actually you can use this circuit with any microcontroller that has two pins to spare. The sense pin must be adapted to the input voltage, so I added a 3V3 Zener for protection. This should work with most controllers running at 3V3 and 5V. Also I added some filtering and a pull-down to the gate of Q2 to prevent the circuit from switching on when D8 is floating (maybe when the MCU is unpowered). With a few connectors added you get a nice universal module. K2 & K3 are screw terminal types, K4 can be a pinsocket or a pinheader. The V3 schematic is attached. P.S. Please note that I didn't try or test any of this.