For my monthly maker column in Dutch newspaper "De Volkskrant", I made a coolbox in which you can control the temperature in three separate compartments to keep craft beer at just the right temperature.
The contraption I build uses three Peltier-elements to cool, an old desktop computers power source, an LCD to show both the measured and the set temperature and finally an Arduino to tie it all together.
Step 1: Materials and Tools
- a coolbox. Mine was 40 x 15 x 30 cm
The active cooling part:
- three Peltier elements. I used 40 x 40 mm ones from Conrad: http://www.conrad.com/ce/en/product/189115/TEC1-12...
- three large heat sinks, including fans
- three small heat sinks, no larger than 40 x 40 mm.
- three thick pieces of thermally well conducting metal. I use aluminium. as thick as the lid of the coolbox and no larger than the Peltier elements.
- three power MOSFET's: IRF3205 (http://www.irf.com/product-info/datasheets/data/ir...
- a power source that can dish out 12V (or 15V, even better), 16A. I use an old desktop computers power source and will assume you do as well for the rest of this instructable. If you choose a different power source, I assume you know enough about volts, amps and watts to calculate if your source of choice will work.
The micro-electronics to control it all:
- three temperature sensors. I used the often used tmp36 and this instructable assumes you do as well. However, if you have calibrated thermistors, I suggest you use those. Arduino's and multiple tmp36's often cause problems with stable temperature measurements that you do not experience when only a single sensor is used on the Arduino.
- an 16 x 8 LCD screen. If you have the Arduino starter-kit: it's the one supplied in that kit. If you have never used the LCD screen with an Arduino before, either use the examples in the Arduino starter kit, or this excellent instructable: https://www.instructables.com/id/Connecting-an-LCD-...
- 4 10 kOhm potentiometers. (higher resistance no problem)
- 10 kOhm resistor
- breadboard and assorted wires
- soldering iron
- hot glue gun
- Dremel or other method to cut out make squares in to of coolbox
Step 2: Coolbox
- Use Peltier elements to draw squares on the lid of the coolbox as guidelines to cut out.
- Use Dremel to cut out squares: all the way through.
- use some carton or paper to make a mold for the styrofoam form that seperate the compartments.
- Cut out the forms from the styrofoam. Use a styro-cutter if you have one, I used a knife, also works. Make the forms slightly (1 mm) larger than they need to be, so you have to press them in firmly.
- hot glue the styrofoam forms into the coolbox. Make sure there are no air leaks between the compartments
Step 3: 12 V Side (Peltier & MOSFETs)
The Cooling Block
- Test which side of the Peltiers are hot and cold by connecting them to a low voltage power source, such as 5V from an Arduino, or 3V from 2 AA batteries. Feel which side gets hot, mark with an H. Check that the other side is cool, mark with a C. If both sides are hot, you have the + and - (VCC and GND) mixed up and are destroying the Peltier. Quickly change.
- Build the cooling stack: apply arctic silver between every layer. Start with the smaller heatsink (fins down). then the piece of alluminium, than the peltier, with the C side down. Finally the bigger heat sink, (fins up). Use a tie wrap (cable tie? in US English?) to tie the stack together. Arctic Silver is sticky, but it isn't glue and the cooling stack will be hanging from the top heat sink, so the tie wrap is really needed.
- Test if stack fits in the wholes made in the lid. The peltier, alluminium and lower heat sink should disappear into the lid, leaving only the bigger heat sink on top. You may need to cut away a bit more from the lid to make it all fit. Do not glue the cooling block into place at this moment
- Solder thick wires to the source and drain of the MOSFETs. Solder a smaller wire to the gate. Solder half of a breadboard jumper wire to the small wire. Cover all solder points in heat shrink tubing. The thick wires will need to be able to have 12V, 5A going through them.
- Bolt the MOSFETs on a small piece of wood. Do not glue or mount them directly onto the coolbox, they can get a bit hot.
- Connect the 12V part of the electronics using the schematic above. As you can see in the photo, I use wire connectors a lot, but you can also solder everything. The wire to and from the power source need to be able to have the total used current (more than 18 A, probably in the range of 20A) going through them.
- The fans are not in the schematic: the are connected parallel to the Peltiers (make sure you have 12V fans!)
- When using a desktop computer power source, connect the 12V and GND. Use a voltmeter to test which of the wires is the 12V. (Convention: 12V is yellow). Most desktop computer power sources require that a single wire in the main connector connects to ground for the power source to function. On mine this was the green wire of the main connector.
Step 4: 5V Side
- First connect the LCD (see photo) using the breadboard to the arduino. Upload the "hello world" example from the Arduino Liquid Crystal library (file -> examples -> LiquidCrystal -> HelloWorld) to test if the LCD is connected correctly.
- Solder wires to the tmp36's. Use colours to remember which wire is connected to which pin. Use heat shrink tubing on all pins and a final big piece of heat shrink tubing on the entire connection. These sensors will be in a moist environment so no metal should be left visible. Make sure wires are long enough to reach the breadboard from inside the coolbox (running through the holes). Connect half jumper wires to the end of the wires for easy connection on the breadboard.
- [optional] build a nice front for the three remaining 10kOhm potentiometers. Solder wires to the potentiometers. If you don't want to do this, you can just stick them into the breadboard as well.
- connect everything according to the schedule provided above.
- Upload the code provided in the .ino file to the Arduino. (If this is your first time working with an Arduino, I strongly suggest to browse the Arduino instructable channel for some tutorials first)
- Stick eveything on a small piece of wood that would fit between the heat sinks after assembly. Screw on the arduino and use the sticker on the back of the breadboard.
Step 5: Assemble
- place the cooling stacks in the holes
- hot glue the MOSFET board and the Arduino board in place.
- Lift the cooling stacks and lower the temperature sensors into the holes such that they dangle a few centimeters below the hole.
- Put the cooling stack back in place. [optional] Hot glue them.
- Connect any wires between the 12V and 5V side that you hadn't already connected.
Turn on! Use the potentiometers to set a target temperature. Store craft beer in compartments. Wait for measured temperature to stabilise at the target temperature. Enjoy beer at just the right temperature!
Note that cooling to the target temperature might take a long time. You may not even reach your target temperature. This can have two reasons:
- the Peltier used is not powerfull enough. Either get a more powerfull one, or try to get a power source as close to the maximum allowed as possible (15V on the once I use).
- Heat leaks to the cold side of the Peltier. Try to issolate the whole as well as possible. Fair warning: do not use polyurethane foam to isolate. I did and it makes it worse: the air bubbles do issolate, but the material itself conducts to much energy.
Good luck and drink responsibly!