When the zombies come for you, odds are the power will be out. Whether that's the case, or if you just want to know when a friend tries to sneak into your backyard, this alarm can be made quickly, easily, and cheaply to address all of your security concerns.
Step 1: Mousetrap
This design is based around a normal mousetrap. I had a few lying around the house, but they had all been cannibalized at one point or another for various projects, so the best I had was this one without most of its triggering mechanism. This is not terribly important; a simple method for setting it off will be shown later. More significant is that the piece which snaps around ends up off of the wooden base. This means it will not set off caps without some modification, but that too can be remedied easily.
Step 2: Mounting Your Alarm
I wanted to sharped the end of a stirring stick to make mounting this alarm in the ground easier. In keeping with this project's post-apocalyptic sort of vibe, I tried to use just what I could find quickly, which meant scoring the lines I wanted cut then attempting to snap the stick along those lines.
This snapped the entire stick, so I had to settle for a short ruler. I taped it onto the same side with all the moving parts, so the metal loop would actually hit something when the trap was triggered. I just used some duct tape; pretty much anything else would be better, but it's what I had on hand. As long as it holds securely, how everything is attached is not critical.
Step 3: Caps
Next, a binder clip can be taped (or otherwise attached) to the ruler/paint stick. This can be used to hold some roll caps in such a position that the mousetrap will hit them when triggered, resulting in a loud bang.
Roll caps are cheap; you can get hundreds for one or two dollars. So don't be afraid to experiment a little and find the best orientation.
Step 4: Tripwire
What good is an alarm that won't go off? Triggering the alarm is perhaps the most important step. If your mousetrap hasn't been torn apart like mine, a thin string or wire (fishing line works especially well) can just be tied on to the pad which normally sets it off. If you set the trap and tie the other end of this line to a stationary object, you're set.
I had to rig a different mechanism, with a small metal rod attached to the line, and running through a loop on the base of the mousetrap to hold everything closed. It works, but requires slightly more force to activate, which may not be the best for a tripwire.
Regardless, if you make anything inspired by this, I'd love to see it. And as always, be safe with this and have fun.