Subtle hints for those having trouble:
level 1: activate the door using a battery
level 2 (outside): find the battery (up left from the house) and activate the way to the lab
level 3 (lab): use the blueprints and papers to find a code
level 4 (dark forest): find a hidden battery in the trees to activate the keypad
level 5: (outside minigame): there are multiple ways to solve this one :)