My total cost for the incubator in this Instructable was about $30. Your cost may be less if you can salvage or re-purpose parts you already have around the house.
Estimated assembly time once you have all the bits together: about 2 hours
You will need:
Serrated knife or hacksaw blade
Sharpie pen or other marker (to mark on the wire and cooler, not the eggs)
Pencil (to mark the eggs if desired)
1 styrofoam ice chest, the thicker-walled the better (I got my Omaha Steak shipping box free by asking on Craigslist.)
1 bottle lamp assembly (Home Depot, about $10 - I salvaged mine from a lamp from a thrift store for $6)
1 lower-element, single pole water heater thermostat ($7.57 at Home Depot)
1 25 W bulb
1 12 Volt transformer (from any defunct electronic device)
1 PC Core fan (scavenged or a dollar or two - I got mine from PC Recycle for $2)
1 plastic (preferred) or glass from an inexpensive picture frame ( I salvaged mine for $1 at a thrift)
1 shallow dish for humidity
Wire hardware cloth (about $9/roll if you don't have leftovers from another project)
1 indoor/outdoor probe type thermometer/hygrometer, about $12 at Home Depot or Walmart
1/2 C salt
1/4 C water
Large zip-close plastic bag
For candling: 1 Mag-Lite flashlight or other similar extremely bright light
A source of freshly laid, fertilized eggs (NEVER refrigerated)
To increase humidity rapidly: a spray bottle of water
Step 1: Put Window Into Top of Cooler
With your serrated knife or hacksaw blade, cut inside the marked line at least 1/2", so that the window overlaid on it has a solid area of support, all the way around.
Tape in place with duct tape. I use yellow because it's what I had in my garage.
Step 2: Put Lamp Assembly (heat Source) & Thermostat Into Cooler
Some lamp assemblies have a hole in the side of the base of the lamp, through which a wire can be run. Mine did not, so I had to drill a hole.
Split the two halves of the long wire. Cut the ribbed half off another 8" and reserve that piece. and strip all ends about 1/2".
Wire according to the diagram below. The wiring diagram has been borrowed and adapted from AK Michelle's 'Home Made Brooder' page at BackYardChickens.com.
EXERCISE ALL DUE CAUTION WHEN WORKING WITH THE LAMP ASSEMBLY. ELECTRICITY CAN AND WILL INJURE OR KILL YOU. LEAVE UNPLUGGED WHEN WORKING WITH BARE WIRES.
I will also give written instructions:
Run the wire from the outside, through the lamp base, through the wall of the cooler. You will now be working with the wire inside the cooler. Tie a half-hitch in the wire where it will sit in the cup-shaped base of the lamp assembly. Run the long, non-ribbed wire out through the side-hole in the cup base. Pull snug. Attach the stripped end of the non-ribbed wire to terminal #1 on the thermostat
Run the reserved piece of wire (I have used a piece of orange 12ga wire for this, so you can see where it's going) through the side-hole in the cup also. Attach the outside end to terminal #2 on the thermostat.
Attach the stripped end of the ribbed wire to the silver terminal on the lamp base.
Attach the stripped end of the 8" piece of wire (coming from terminal #2 on the thermostat - orange, in my pictures) to the brass terminal on the lamp base.
Put as little wire inside the cup base as possible, for snug fit when you assemble the lamp assembly. Press the rest of the lamp assembly together. Screw a 25W bulb into the lamp socket.
Put a piece of electrical tape over the terminals on the thermostat, for safety.
Plug in. If you've done it right, the light will turn on. If you've done it wrong you'll probably have a circuit breaker pop. If the breaker didn't pop and the light didn't come on, check the switch on the lamp assembly, which might be turned off. If both of those fail, try a new light bulb.
At this point it is useful to turn the thermostat as close to 99F as you can estimate, put the lid on the cooler, and leave it alone for a few hours. If all is well, the light will turn on and off as the temperature in the cooler reaches or drops below the desired temperature.
Step 3: Install PC Fan Into Cooler
I have used a hot glue gun to attach square legos to the back side of my fan, as spacers, so the fan isn't smack up against the wall of the cooler. You might use a bit of the leftover styrofoam from the lid, if you like, instead. For this fan, I have tried attaching it to the wall with bits of wire hanger. I find this allows a lot of vibration and thus more noise than necessary, but I have not yet found a tape that will stick to styrofoam, so there you go. You can experiment with how to best attach the fan to the wall. I like to attach mine so it blows across/above the light bulb, moving the warmed air.
Cut the jack off the end of the power cords on the fan. Poke a hole through the wall of the cooler, just large enough to run the wires through to the outside. Strip the ends of the red and black wires, about 1/2".
For power, you need a 12V transformer. Any old 12V 'wall wart' transformer will do: I have used one from a long-defunct baby monitor, and one from a phone charger. Cut the jack off and separate the two halves of the wire (You will end up with black and black with white stripe, or similar). Separate the two a few and strip the ends about 1/2".
Plain black goes to red; black with white stripe, goes to black. Yellow is ground and gets nothing, although you may wish to just put a bit of electrical tape over the end anyway. Twist the stripped wires together and cover with wire nuts or with electrical tape. Plug in and test. It should run smoothly and near-silently.
Step 4: Calibrate Your Thermometer/hygrometer
If the humidity is higher or lower, however, you will need to make a note of the % by which the hygrometer deviates, and always remember to mentally add or subtract that percentage during incubation to maintain the desired humidity:
For example, if your humidity reading is 80%, you must always subtract 5% from whatever you see on the hygrometer. If it says 40%, you will know the real humidity is 35%.
Step 5: Make Wire Floor for Incubator
Measure the dimensions of the bottom of your cooler. I say it this way because some coolers (like mine) are narrower at the bottom than the top, and if you only measure the top and make a wire floor from those measurements, it won't fit. It may also, if you are using styrofoam, simply tear up the inside of the cooler, trying to fit it in there.
Cut a piece of wire to size, plus 2" in each direction.
Cut a 2" square out of each corner.
Fold down the sides to a height of 2", so that the corners meet neatly. and the floor is as flat as you can make it. Place an empty, shallow water dish on the bottom of the cooler, then Install the wire floor over and above it, with the thermostat on top of the wire floor at the same level the eggs will lie. Make sure the water dish underneath is shallow enough that it doesn't interfere with the lay of the wire floor. DO NOT ADD WATER YET.
Step 6: Create a 'safe' Area for the Light Bulb, Fan, and Thermostat
Put a piece of aluminum foil between the light bulb and the wire, to deflect the direct heat away from the nearest eggs. You want a steady temperature on every egg.
Step 7: Cut Ventilation Holes
Eggs "breathe" and developing chicks need lots of oxygen. You need adequate air exchange inside the incubator for the chicks to live and grow.
You may have noticed in my intro, that I also put a ventilation hole in the lid, and covered it partially with duct tape. I find this lets me regulate humidity better without opening the lid (can uncover the top hole, as rising heat carries excess humidity away until the hygrometer reads the desired number, then cover partially and check frequently at first to make sure it holds the % humidity you want.)
Step 8: Add Humidity
If your humidity remains too high despite fiddling with the 'exhaust' ventilation hole in the lid, you may need to remove surface area (ie, take out the large wide dish, and put in one with a smaller bottom/less surface area for evaporation). NEVER LET THE BOWL GO DRY. It may help to leave a couple bits of styrofoam floating on the surface of the water. When you look in, make sure they're still floating - you'll be able to see the water level by where they are.
Water also functions as a thermal mass, helping to regulate the temperature inside the incubator.
For further information on the scientific technicalities of incubating eggs, I refer you to the "Now What?" page at www.PoultryHelp.com
Step 9: Stabilize Your Temps and Humidity
Run your new incubator for at least 2 days, empty except for water in the bowl, until you are satisfied that the temperature and humidity will remain stable (temperature 99-100F, humidity 50%) without you constantly fiddling with it or having to fuss over it. During this time, you can adjust the thermostat a tiny bit at a time, and adjust the number of open or partially-open ventilation holes, waiting an hour or two between adjustments to see how the change will affect things.
Be patient. I'm sure you want to put eggs in instantly, but the incubator needs to be set up as a stable environment first. You'll have better chances of a good hatch if you can take the time to make sure it's holding steady.
Step 10: Add Eggs...and Wait
In a commercial incubator, eggs are held upright, big end upward, and turned mechanically several times a day. If you want to stand them upright, you can put them in an egg carton (with the bottoms of the compartments cut off for better air circulation). Cut some 1.5" - 2" blocks of styrofoam from your left over piece, and use these to alternately prop up the long sides of your cartons. The intent is to put the eggs first at one 45 degree angle and then the other, every 12 hours at least.
Alternately, you can mark the eggs on opposite sides with a pencil, then lay them directly on the wire, turning them at least twice a day (gently flip or roll) so (supposedly) the chick doesn't stick to the inside of the shell.
Note: when you first put the room-temperature eggs in, the incubator will take several hours to come up to working temperature. This is okay, and normal. The incubator has to bring the eggs' own thermal masses up to 99-100F. The more eggs you put in, the longer this can take. This incubator will hold at least 2 dozen eggs in cartons. I think it will hold more if they're just lying on their sides.
On day 18, stop turning the eggs, or remove the tilt-blocks (if using the egg carton method), add more warm water to the bowl in the bottom, use your spray-bottle if necessary, bring the humidity up to 70%, and do not open the lid. If you want to observe, observe through the window. The chicks need the eggs to hold still, so they can orient themselves properly to peck themselves free of their shells.
Normal hatching time is 21 days. If your temperature is a little high, they may hatch in 19 or 20 (not desirable). If your temperature is a little low, they may hatch in 22 or even 23 days (again, not desirable). If your chicks don't hatch on day 21, yet you hear pecking or peeping, be patient and DO NOT HELP. If you feel a desperate need to help, check the forums at BackYardChickens.com for advice. Human help usually kills hatching chicks.
Do not open the lid until all chicks have hatched. If you MUST open the lid, use your sprayer to bring the relative humidity back up to 70% immediately - even a minute of dry air can 'shrink wrap' a chick inside the membrane inside its pipped shell.
Step 11: Candle for Viability After 4-5 Days
At the time of this writing, I have successfully hatched a single duck egg in this incubator (his name is Sherman and he has gone back to the farm to live). I now have 14 chicken eggs in it, with a hatching date of July 30.