This simple design was created to automatically power off a Creality Ender 3 3D printer. It is a mechanical switch and therefore it is not necessary to interfere with the electrical wiring of the printer.

It works as follows:
When the print is done, the print bed automatically moves forward (in almost all G-codes) to present the print. This motion is used to activate the pulley, which in turn activates the lever that moves over the power switch of the printer. So if the pulley is turned > the wire is pulled > the lever is turned > the power switch is activated by the lever.

For a short video: https://youtu.be/fVJdt2u7Oyc

Assembly:
When the 3 parts are printed, assemble them. As for the two shafts (green in image), I used 2mm (diameter) nails, which can be pressed in the printed holes. Next, a wire (red in image) can be tied to the lever, through the main body (rigid pulley) and to the pulley. The pulley has a hole where the end of the wire can be (super)glued into. I used a small piece of filament to press the wire into the hole/glue to further secure it. Make sure the wire is strong enough and is non-elastic.

The length of the wire is dependent on the location where your print bed activates the pulley.

The assembly can be pushed on the aluminium slotted beams, behind the LCD screen. Optionally, you can secure it here with a bolt (yellow in image). I designed the model for an M3 heat stake insert and M3 bolt at this position, but ended up not needing them.

To activate the pulley I use the standard clips that came with the printer and hold my print bed in place. You can also get creative and for example hotglue a M5 nut underneath your print bed (I did this first, but it fell off after about 10 prints).

Then there is one more thing to do:
In most G-codes, it is normal to first move the print bed to the "present print" position, and then cool the nozzle. However, if you turn off the power after presenting, active cooling won't be initiated. This could cause clogging as the nozzle heat can creap up and heat up you filament/bowden tube, causing problems. To prevent this you can alter the G-code a bit, so that it first cools, and presents the print afterwards.

In Cura (I have little experience with other slicers), go to: Manage printers > Machine settings and replace the "End G-code" with the following:

G91 ;Relative positioning
G1 E-2 F2700 ;Retract a bit
G1 E-2 Z0.2 F2400 ;Retract and raise Z
G1 X5 Y5 F3000 ;Wipe out
G1 Z10 ;Raise Z more
G90 ;Absolute positioning

M104 S0 ;Turn-off hotend
M140 S0 ;Turn-off bed
M109 R40; Wait for the hotend to cool to 40C
M106 S0 ;Turn-off fan
G1 X0 Y{machine_depth} ;Present print

M84 X Y E ;Disable all steppers but Z