Hi!

How about automatic powering off your printer when printing is done?
This is a new, upgraded version of my project.
https://www.thingiverse.com/thing:3333899/

This one is low voltage, much safer for the user. I recommend to make this one.
I want to share my upgrade for 3D printer which gives Auto Power Off functionality.
Some printers, equipped with ATX PSU may do this by simple M81 command. Lucky owners!!!!
All others, or almost all others, know these timers to switch off the printer after printing is finished. We also know that expected finishing time may be different than calculated by slicer software. So we add one or two hours to not switch off the printer too early.
I started to be tired of this solution and invented my way of secure switching off the printer just after the printing. Tested with Ender 3. Should work with similar printers.
After finishing printer cools down and raises printhead frame to the maximum height. It triggers switch and disconnects power from printer.

What do you need to upgrade your printer?

MOST IMPORTANT IS TO KNOW HOW TO WORK WITH POTENTIALLY LETHAL LIVE VOLTAGE.
You will not need to take a risk, but mistake can always happen!!!
SO PLEASE, ASK PROFFESSIONAL TO HELP YOU WITH CABLING.

about 1/2 meter of two-wire cable. 2 x 1mm2 at least.
soldering iron
soldering skills (not advanced)
some insulating tape or themorshrinking tube
two screws for fixing to frame. I had hammer nuts (T-nuts) with 4x14 screws.
two screws with nuts to fix microswich. Mine are 2x12mm but it's not so important.
microswitch with lever, any current. Mine is 5A (size 20x10mm distance between the holes 9,5mm). lt's also a good idea to use small switches like used for XYZ stops (you may have one free after installing BLTouch or 3DTouch)
24V DC relay (12V if your printer is 12V) capable to take 20Amps or more. Current on the low voltage part is high 15-20Amps. It may be also electronic relay, quite common nowadays.
some time :-)
Assembly instruction:
Print V2 arm. PLA or any other material except of TPU. Fancy colour is highly recommended! 20% infill. Layer 0.2mm is OK. No supports. No rafts. No brim.
V1 arm is fine too, but there is a risk of collision with some models of head in very rare cases of high and wide prints
prepare all parts
power off the printer
UNPLUG POWER CABLE!
disconnect low voltage connector (2 wires)
unscrew PSU from the frame
remove plastic cover of PSU cables
make connections as shown on the schematic. Solder end of wires for better contact. Use thick wires, similar to original. Cable leading to micro switch may be much thinner - small current. Remember to connect NO (normally open pair from relay).

Secure all connections and relay by thermo tubes or insulating tape

Guide new cable through PSU cover hole.
Install PSU plastic cover back.
Mount PSU leading new cable through frame as seen on the photo.

Assemble printed arm to the frame. Fix it high as possible for a start.
Install microswitch.
Bend microswitch arm as seen on pictures. Do not bend whole arm from switch, bend arm in the middle, to trigger the switch earlier. It helps to securely stop Z motor and frame before hitting the switch body. PSU has big capacitors which extend the move of the fraction for a second after disconnecting the power.

Move frame up by hand and check if it push the switch lever with any part. Adjust arm if necessary. It may happen that your printer needs different shape of arm. Ask me for help or make remix. Up to you.
measure needed cable length to the microswitch
weld cables to pair of connectors which are normally closed (NC) IMPORTANT
secure connections with tubes or insulating tape.
IMPORTANT! Secure also third, not used pin of microswitch. It may have voltage when the printer is turned off by the switch

You are done with electro-mechanics. Now it's time for software and adjusting.
Add "special" commands to end sequence in your slicer (i.e. Cura--Settings--Printer--Manage Printers--Machine Settings--End G-Code window). Disable M84 command with ";"G4 S300 ; wait 5mins to cool nozzle
G0 Z250 ; go up to max height

You may adapt commands to your needs. Decrease Z250 part if your printer is smaller than Ender3 which has 250mm max height
5min pause is really needed. Too much heat from hot nozzle without cooling fan active may cause filament jamming in the head.

Print anything. Small one for sure.
Wait until frame goes up.
Use of terminal is also OK. Octoprint or any other one. Just send commands:
G28
G0 Z250
adjust printed arm with microswitch to trigger it. It should power off your printer!

You are done!!!
Congratulations. No more power waste. You are GREEN now!
Starting printer is simply by pushing the frame little down. Power is coming back.

In case of another microswitch you may adapt holes in the arm or ask me for help with it.
.stp files enclosed

Good luck and happy printing!

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Power consumption measured just now by curiosity (with Octopi powered from Ender's PSU):

system idle: 11W
nozzle heating: 56W
bed heating: 233W
all heating: 273W (does not sum up somehow. Possible power reduced to voltage drop on cables - high current OR PSU efficiency depends on load, which is qite normal).
PSU only, 24V socket disconnected: 0.0W (may be less than 0.1W)