A typical exploding coin bank works like this: drop in a coin, you hear a bang and the bank breaks apart into pieces. Exploding coin banks may be losing their appeal as a surprise object because you know what to expect the second you drop the coin, don’t you? Not with this time delay exploding box that I recently designed.

Mine will not explode at the drop of, in this case, a 3/8 ” dia. steel ball. It will do so only after a few seconds of delay, thanks to a delay mechanism hidden inside.

The Box

The box consists of the top and bottom (3/8 ” x 3″ x 4-1/2″ each), the left and right sides (1/4 x 3 x 2-1/2″ each), and the front and back (1/4 x 3 x 4-1/2″ each). Every piece, except the top, is very simple. The top has a number of smaller pieces attached to its underside, which make up the time delay device. Begin by cutting all the six main parts to final dimensions. Drill and chamfer a 3/8″ dia. hole, on the centerline of the top, but towards one end, then ease all the edges. Center the mousetrap on the bottom and screw it in place.

To hold the four sides together, magnets and nails are used. Mount two round magnets (1/8″ dia. x 1/8″ long) flush on the front and back. To get holes drilled precisely 1/8″ deep to house the magnets flush, first, find a piece of scrap that’s 1/8″ thick and at least 2″ x 3″.

Place the front or back section of the box on the drill press table and zero out the drill bit by lowering the drill bit until it touches the surface of the workpiece, and setting the drill’s depth lock.

Now slide the scrap underneath the workpiece, raising it 1/8″ to drill the hole at a perfect depth. Glue the magnets in place using CA glue or epoxy. I pre-drilled holes for the nails to avoid splitting, before driving the nails into the edges with the clamping force of a vise. Partially drive a 3/4″ screw into the left side, front and back. When the box is assembled these screws will catch the bow when the mousetrap is triggered, sending the parts flying.

The Time Delay Device

The time delay device includes a platform (1/4 x 2-1/2 x 4″), two strips (1/4 x 1/4 x 3″ each), a stop block (1/4 x 1/4 x 1/4″), a chute (3/8 x 1 x 1″), one thin angled block (about 1/2″ thick) and two thick angled blocks (about 3/4″ thick). The exact angles and sizes of the three angled blocks are not critical, though they do need to align with each other once the time delay device is assembled.

After cutting out all the pieces, drill and chamfer a 7/16″ dia. hole on the centerline of the platform, near one end. Glue the two strips, stop block and three angled blocks to the platform and drive some brads randomly on the platform, making sure the ball will be able to roll between all the brads. The brads slow the movement of the ball as it travels down the time delay device.

To make the chute drill a 7/16″ dia. Hole in the 1″ x 1″ block, then glue it to the underside of the platform, directly below the 7/16″ dia. hole. Make a few test runs of the steel ball and make adjustments as needed before screwing and gluing the time delay device centered to the underside of the top.

Finishing and Assembly

Apply a few coats of oil finish of your choice, if desired. Once dried, carefully set the mouse trap, assemble the box, place a steel ball by the box’s side … and wait for a curious mind that can’t resist the temptation!