Power OctoPi from printer I'm wondering if there is some trick to power my OctoPi with the power supply of my 3D printer. I'm using an Geeetech I3 Pro W.

The power supply itself should be able, but the output is as far as I'm aware of 3.3 volts. Not my desired 5&nbsp;V for USB, it would be a shame if I really would need to buy a new power supply when I have a strong one actually running. My current power supply causes a lot of "Under-voltage detected!" warnings.

After thinking a little about the specs, there are cigarette lighter adapter for cars they use 12&nbsp;V. Has anyone experience with using that on his printer?
What you are looking for is called a "buck converter" or a "step down module". These literally cost about half a buck/Euro a piece. These converters convert a high voltage into a low voltage, the better ones are able to draw 2 to 3 Amps, which is required for stable operation of the Raspberry Pi.

If you have an old computer power supply of a decent brand (probably not as you refer to a kit/assembled printer, but added for completeness), you can even use the standby 5&nbsp;V line out and switch the power supply on using a relay to short the green wire of the PSU to ground. This is how I use it on one of my printers.

Note to power the Raspberry Pi through the micro USB port, to not bypass safety features.