So it's not really a dollar because of tax....but it's close. It also will convert a 9V battery to 5V. Haven't tried going lower. Buy a few extra in case you damage one. Good for up to 5 volt (5V) 1 amp (1A) output...maybe more

Small too: 15mm x 30mm

Step 1: Quick Disassembly

What you need: $1 (plus tax), soldering iron, solder, and jumper wires (or header pin connectors).

Needed 5 volts for a microcontroller and didn't want to pay much or wait for the thing to be shipped to me. Then I remembered that the USB car chargers convert 12V to 5V to charge phones. I went to the local electronics store and saw the cheapest one was $8. Maybe I could wait. On my way home I drove past a dollar store (the one where everything inside is $1). Went in, and sure enough, they had a 1 amp (1A) charger. Bought 4 of them just in case.

Once home, disassembly was pretty straightforward. Unscrew the tip that plugs into the lighter, and then pop apart the casing. A spring, fuse, and metal cap fell out. These things are cheap though. Out of the 4 I bought, three had different circuit board configurations. Not to worry though, the 12V and 5V connectors were always in the same place. The 12V is the end opposite the USB end, and the 5V is the USB end.

If you need a USB end, then you are good to go after you de-solder the metal piece on the 12V input side. Solder a wire, header pin, or whatever you choose to either one of the holes. If you don't need the USB end (because whatever you're powering doesn't have a USB cable), then de-solder the four connections on the bottom of the board. You may have to bend some metal tabs up that also hold it on. What you're left with are 5 holes. The common for the 12V passed through the spring clips in the body housing that was removed. The outside pins on the USB connection are the 5V and common. Conveniently an addition connection exists next to the USB ground for the 12V ground. If you want to be sure it works, then get out your multimeter and connect it to a 12V source. You should get a 5V output that supports up to 1A - maybe even more since we removed the fuse.