What I came up with is not perfect, but it will do for some basic tinkering. All of the critical components are cheap and available at RadioShack. However, before you even _start_ gathering materials, you should know that:
I am not an electrical engineer, and my knowledge comes only from _LIMITED_ practical experience. You will be working with things that could, if you misuse them, hurt you. If you decide to build this, TEST EVERYTHING before you start using it, and make sure that nothing is shorting before you plug it in. Also, USE A SURGE PROTECTOR! As far as I know surge protectors will shut off if they detect a short, so that might help you out.
I'm pretty sure that in the event of a short you're just going to fry your power supply and leave your house and wiring untouched, but one never knows. In any case, you've been warned.
OK, so materials:
-7805 voltage regulator
-LED (5v LED's are nice, 3v will work, but you'll need a resistor)
-Two wire terminals
-Radio shack 1.5v-12v power supply
-Veroboard/perf board/prototyping board/ whatever kind of board they call it
-A bread board
-A case for the electronics
You'll also need:
-Pliers (wire cutters)
-Wire strippers (Optional, real men use the scissors on a Swiss army knife)
-Hot glue gun and hot glue (also optional, but they help secure things)
-Soldering iron and solder
The glasses and desk fan are to keep you from inhaling solder fumes and to keep you from going blind. I've had a piece of solder shoot up and hit my glasses before. If I weren't wearing them I might be blind right now, and that wouldn't be much fun. It's a pain, but wear them.
Step 1: Step 1
I tested mine to see if it would work, and which wires were which. I also felt the wires and brick to make sure that they weren't heating up at an abnormal rate, indicating a short. I'm a little paranoid about shorts, it's one of my things.
Step 2: Build the Circuit
Yes, I know that the LED doesn't come on under 1.5v, and that it's dim at 3v and 4.5v, but I rarely use below 3v, and I know that if I don't have a light to remind me I'm going to leave it on. I could have just used a variable regulator for this project, and then used the 5v regulator solely to power the LED, but in Montreal all of the RadioShacks have been bought out by this dumb store called "La Source", which appears to have higher prices and no electronic components. Anyway, that's just my opinion (La Source, please don't sue me). I bought these components last time I was in Plattsburgh, before I though of this project.
If anyone knows of an electronic components store in Montreal, please drop me a line in the comments. I appreciate it.
Step 3: Solder It Up
I left the switch and LED disconnected. Why? So I could mod them into a nifty case, of course (and because the switch doesn't have pins).
Step 4: Snip It and Seal It Up
You can leave it out if you like, but I like things in cases. I also don't like contacts capable of delivering 12v just lying on my desk, so I modded a case for mine. Now, being a diabetic, I use insulin pump sites, and mine come in these small plastic cases that were otherwise worthless... until now.
For those of you who have diabetes and use Insets, you can remove the white plastic inserter from it's package by grabbing the part that's connected to the case and ripping it out (be sure to bend the needle back first so you don't stab yourself). After you do that you need something to close up the bottom. I took the plastic from a package for one of those Ascensia disks, cut that out, and hot glued it to the bottom. It's pretty sweet.
I was planning on using an Altoids tin, but the switch didn't really fit, and I made the hole too big for it, so I decided to abandon that plan, and I'm glad I did because my new case works so much better... and I have so many more of them. (Non-diabetics: I go through one of these every 2-3 days, because I have to change my site that often. Therefore I have a lot of them.)
You'll also need to solder the wires for the switch and LED, and solder them to the circuit. You did leave the ground disconnected from your 7805 and your second terminal, right?
Step 5: Plug In, Enjoy.
I'd try it at low voltages first. Your LED should come on at 3v. However, if the LED doesn't come on, unplug it immediately, and check your circuitry. This means one of two things, either your LED is dead or backwards, or you have a short, and you are rapidly killing your power brick. Examine everything with a multimeter, make sure the LED is going the right way, and then try again. Electrical fires are not fun, so make sure you don't start one.
Another note, I realized that the 7805 has a heat sink for a reason, and that my case was pretty much sealed, so I drilled a few vent holes just to cool things down a bit. I doubt I'll need them, but oh well.
I hope you enjoyed this, and please let me know what you think or ask me questions. Again, if you life in Montreal and know of a place to buy electronic components it would be awesome if you could let me know.