I had been wondering for some time just how fast a mouse trap takes to slam shut once it has been tripped. What follows is a description of a simple way to measure the closing speed, along with the results I obtained.
The trap I used was a classic Victor brand mouse trap. This is the kind that uses a spring loaded bar held back by a hook. The front of the hook rests on a small protrusion on a metal piece used as a bait holder. When the bait holder is moved even slightly, the hook releases, and the upward force on the bar from the spring pushes the hook out of the way and allows the bar to slam down on the front of the trap. Any mouse that is unlucky enough to have its neck in the way of the bar will be dead.
I’ve included timing results from both the mouse trap and the larger rat trap. I’ve also included a step showing how the closing speed can be estimated using a simplified model based on the physics of an ideal torsion spring.
Step 1: Trap modification and Equipment Setup
I used my oscilloscope to capture the timing of the process. Two events need to be captured: The release, when the hook separates from the bait holder, and the closing of the trap, when the bar contacts the wooden base. The release of the trap was used as the triggering event for the scope.
The different parts of the mousetrap were used to create two switches which allow the release and closing events to be represented on the scope. To make electrical connections to the various parts of the trap, I soldered wires to the U shaped tacks that are used to fasten them to the wooden base. A 9 volt battery was used as a voltage source to produce a signal that represents the state of the switches.
The trigger event for the scope (the release of the trap) is generated by using the bait holder and hook as the contacts of a switch. This is effectively a single pole, single throw normally closed switch. It is in the closed state when the trap is set, allowing the positive terminal of the battery to connect to the bait holder. When the trap is released, the battery will no longer be connected to the bait holder and so the voltage on it will drop to zero. The scope was set to trigger on the falling voltage seen on the bait holder.
The second switch generates the signal representing the closing of the trap. This switch is single pole, single throw normally open switch. It is in the open state prior to the bar impacting the front of the trap. This switch uses the spring loaded bar as one contact. The second contact is a strip of aluminum tape on the front edge of the trap. The battery voltage is applied to the bar, and the metal tape contact is connected to the second channel of the scope. When the trap is set, the voltage on the metal tape front contact will be zero. When the bar impacts the front contact, the voltage on it will rise to the battery voltage. By examining the timing between the voltage drop on the bait holder and the voltage rise on the front contact, the time from release to closure of the trap can be determined.
The diagram shows how the different parts of the trap are used to make the two switches, and how the switch states change from the setting of the trap to its final closure. The schematic also shows how the two switches, the battery, and the oscilloscope connect together. The nodes numbered in the schematic correspond to the parts of the trap numbered in the picture of the actual trap. Node 1 is the hook, node 2 is the bar, node 3 is the bait holder, and node 4 is the metal tape contact added to the front of the trap.