Ed, it is very simple, a length of wood 5/8" X 5" x 10" long set the hinge off centre on the length so that the longer is outside the loft if any weight heavier than the pigeon hits it,it drops down closing the hole. I cut the wire and fitted plastic round the edges,hinge screwed onto frame trial and error to get the correct balance so simple it's not true and the best bit only costs a hinge.