I was inspired by http://thereifixedit.com. I feel a kinship with the innovators whose work is featured there. After all, I just expanded my universal remote, and they built one from scratch. I decided to document my own attempts at problem solving. Hopefully the engineering is better than theirs, but I feel confident my problems had less reason to be solved in the first place.
The Problem
Sometimes I forget to close my garage door. I've left it open all day and all night. This is a perfect example of a problem that barely needed to be solved in the first place. Right in my wheelhouse.
Solving It
What I need is an unmissable indicator of my forgetfulness. I'm thinking of a light that comes on when the garage door is open. Ideally, one that I can see while watching TV or laying in bed.
Design
The concept is simple:
There's an LED, a power source, a switch, and a couple resistors.
Parts
The switch is a magnetic reed switch. It's normally open, but if a magnet cozies up to it, it gets all closed. Perfect. I don't know the model number, because I just found it in my garage, left over from when they installed the security system. You could get one for about $3, and you could go wireless for $50.
The power source is a couple of AAA batteries. I happened to have a battery holder that size lying around, so that's what I went with. I think you could get a new one for $2.
The LED is a panel mount model I bought just for this. It is green, and snaps into a 1/4" hole.
According to my math, I need a minimum of 40 Ω of resistance to avoid burning up my LED @ 3V. All I had were a couple of 100 Ω resistors, so I went with that. In parallel, that gives me 50 Ω plus the resistance of the wire & switch, so I won't burn the LED up.
Then there's some miscellaneous wire, solder and heat-shrink tubing to keep everything neat. Oh, and a magnet. I happened to have one of those too.
Assembly
Most of the stuff sits inside the house. I picked out a nice location in a closet and drilled a hole for the LED. I soldered up the battery pack, resistors, LED, and the leads to the switch and fed them through into the garage.
At the other end, I just had to stick the switch in place and mount the magnet to the garage door. And by "mount," I mean "stick it to the garage door because it's a magnet."
Victory is Mine
After just a little tweaking to get the position of the magnet just right, I have a working light. It has already reminded me to close the garage door once, and I expect it will have a long life saving me from myself.