This is a belt extruder that I made for my dual extruder printer and has a 1:4 reduction and is filament sensor compatible.
This extruder should be compatible with any printer and should be a direct fit in a any 20x20 extrusion profile.

I shared the step files so you can modify the parts easily.

Material list:
1x Nema17 stepper motor
1x Hobb-Goblin - 5mm ID Drive Gear
1x M5x40mm screw
1x Gt2_20T_6mm Pulley
1x Din557 nut
1x Filament sensor (https://www.amazon.com/Sunhokey-Filament-Detection-Detecting-Filament%EF%BC%88Pack/dp/B07N2GQHQT/ref=sr_1_5?dchild=1&keywords=filament+sensor&qid=1606337204&sr=8-5)
1x 3x30mm screw
1x 200-2GT-6 belt
1x Pneumatic union M10
1x Extruder prusa spring
2x 688z bearing                                                                                                                                                                                                                                       1x 625z bearing

Attention:
You will have to make a cut on the M5 screw to make the screw that locks the Grive Gear in place to not peek out of the Drive Gear, so that the Drive Gear can go inside the bearing.
Don't tighten the belt to much so the printed pulley don't bend (this problem will be solved in the future updates).

This is a work in progress, so expect updates in the near future. I would like you to share, in the comments, your opinions and any problems you find.

I would love if you shared fotos of my extruder in your printers.