We use the mouse traps that don't kill the mice. With only capturing a mouse every few weeks we forget to check the trap and have had a few die because we forgot to check them which completely defeats using the humane trap in the first place.
Now we get a bright LED light in the garage near the fridge that lets us know when a mouse has triggered the trap.
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Step 1: Instrumenting the Trap
Using a normally open reed switch and a small magnet you can make the reed switch close the circuit when the gate on the trap closes.
I used a small nail in a drill to make two holes in the trap to mount the reed switch
I used a soldering iron on low heat to melt a pocket to mount the small magnet
After fitting the reed switch over the door and the magnet in the door we added glue to hold both in place.
The trap now acts as a switch that is open when armed and will close the circuit when an unlucky (but more lucky than the last two) mouse enters the trap and the door closes.
Step 2: Building My Beacon
At the current time I only wanted a noticeable light when the trap triggered.
I had an old wall-wart phone charger which would supply a tiny 250mA of 5V that had been kicking around for years.. It fit this project perfectly.
Generally you want to run about 10mA through an average LED.. Since there is a 0.7V drop across the LED the resistor calculation is (5.0-0.7)/0.010 = 430 Ohms.
I had some bigger LEDs so I actually used slightly smaller resistors..
I mounted the circuit to a board I had laying around and screwed it to the wall in a normally dark but very visible location
Step 3: Attaching the Trap-Switch
I had some handy experimenter single conductor sockets that fit nicely over the leads of the reed switch.
Alligator clips or some sort of connector should be used because you want to be able to disconnect and take the trap to a distant location to release the mouse.
Step 4: Hopefully the Magnet Will Not Spook the Mouse...
I haven't bated it yet but here is the armed trap sans bait.