Introduction: $5 Mini USB Fridge!
Now that we're seeing those 12 volt camper coolers turning up at garage sales and thrift stores (I found one for $2.50), here's a neat little idea for turning it into a customizable mini-fridge powered by a USB port!
Step 1: Taking Out the Peltier/Heatsink Unit
You'll basically just need a phillips head screwdriver and thin socket or needlenose pliers to take apart the heatsink and fans, which will allow you to remove the unit from the cooler. Now, you might ask why you'd want to do this and not just use the cooler. The answer is because most of the time when you find one of these the case will be cracked and the cord is missing, but that's not a problem...
Step 2: The Peltier Unit and Clean Up
Sandwiched in between the heatsinks, you'll find the Peltier unit, which loks about the same size and thickness as a computer CPU. In between the top and bottom layer, you'll see the special substrates that have the unique properties that make the Peltier unit cool on one side while heating the other when an electrical current is applied. There will, in some cases, be spray foam insulation in between the two heatsinks, which is very easily broken away with just your fingers. You can safely remove the peltier unit from the other heatsink, as it will be held only by thermal compound. Once you've cleaned up the top and bottom heatsinks, place the peltier back in between the two heatsinks and retighten the bolts. If you have any thermal paste left over from putting together your own PC, you can optionally clean the old paste away and reapply new paste to each heatsink just as you would on a CPU heatsink/fan assembly before attaching it to a CPU.
Step 3: Attch a USB Cable
Peltier units are designed to operate at a voltage between 3-12 volts, and the 5 volts from your USB port work just fine. Although the amperage could ideally be higher, the 500 mw output is acceptable. Cut away the end of an old USB cable (or pick up a cheapy) and strip back a couple of inches of the plastic covering. Inside you will find 4 wires, usually within a braided or thin aluminum shield. The wire colors will be white, green, red, and black. Trim back the white and green wires, strip off a small bit of the black and red wires, and solder them to the red and black wires of the peltier unit. Wrap with electrical tape or use heatshrink tubing. If you need information on correct soldering procedure or the use of heatshrink tubing, there are many excellent Instructables that will gve you all the information you need, simply do a search!
Optionally, you can attach a 1K limiting resister in between the red and black wires, although you are quite safe at the voltage and mA to not really need one.
Now, plug in your USB cable to a USB port on your PC, and within about 30 seconds you will be able to feel one heatsink become very cool while the other becomes warm. Note which heatsink becomes cool, as that's what we'll want to encase inside the mini fridge.
Step 4: Building the Fridge!
I used foamboard because of its ease in cutting with an xacto knife and its insular properties, and basically built a box around the heatsink using a hot glue gun to attach the sides and top, and then ran a line of hot glue along the seams to ensure an airtight compartment. The bottom piece is cut into two halves, with a square section cut out in the center to make room for the peltier unit. I then glued the two halves to the underside of the cooling heatsink, then glued the left, back, and right sides of the fridge, and finally the top. See the diagram below:
Step 5: Final Assembly and Extra Touches
I used white plastic tape to hinge the door, although you could certainly use small hinges from a hardware store and simply glue them into place with the door fit into the front for proper opening and closing. I glued small lengths of foamboard inside the fridge assembly and then glued cut pieces of a flexible refridgerator magnet on both the inside of the door and the foamboard lengths to make a magnetic "catch" to hold the door closed. I also threw in a battery powered White LED and used a leaf switch to turn the light on when the door was open. I ran the wiring of the leaf switch along the inside and through a small hole in the back to attach to the AA battery holder glued to the outside back of the fridge, then used white plastic tape along the wire run, attaching it to the interior side.
For the handle I used a cheap hardware store drawer pull.
As you can see by the door, I wanted this to look like a store "cooler" that you see for drinks, so I cut a window and hot glued a section of plexiglass in the window.
Step 6: The Final Result
As you can see by the pictures, this cooler will hold a large bottled water, or a tall 20 oz plastic soda bottle, although my drink of choice is a Starbucks Vanilla Frappucino! The cooler will keep drinks at about 45-50 degrees and works best when your drink is already cold for obvious reasons. Optionally, you could use a 1 amp 7.5 volt DC adapter, which will lower the temperature significantly without making the lower heatsink too hot for placement on regular surfaces. In this instance, I would strongly recommend that you add the 1K limiting resistor in order to keep the DC adapter from becoming hot.
Step 7: Tricking It Out!
Now, add graphics from your favorite game or website and make your mini-fridge a one of a kind creation. I printed out on a sheet of inkjet transparency film to create the see through graphics you see here. Have fun, and watch the video for the nice ending... ;)
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