I'm looking to build a self-tipping seat such as what you'd find in a cinema or sports stadium. What kind of hinge system would it require? I can't seem to find anything online.
This can be done with a torsion spring, or several other types of spring depending upon your design. Or it can be done with a counter-balance.
The counter balance only requires you to set the pivot point of the hinge forward a bit with a weight on the bottom close to the back of the chair bottom. You can practice with a few designs in cardboard to see how adding a weight to a beam and placing the hinge higher or lower and forward and back until you get a feel for how it will behave.