This is a very simple trap that can be made quickly and easily. It makes a great first project.
The basic principle is that the lid and front of the trap are pivoted and suspended above the trapping chamber. A length of cord holds them up, runs over the raised back of the trap and is tied to a short spindle.
The weight of the lid creates tension on the cord, which in turn holds the spindle against a pair of pegs. One of these pegs is fixed, the other is a long rod which passes through a loose hole into the body of the trap and has bait attached.