Step 1: Parts & Tools
The pieces needed for a LED bar are reasonably cheap. Don't be alarmed at the amount of parts needed - nothing is expensive, all of them are pretty common and easy to use. You probably already have 3/4ths of this stuff right now.
Most of these parts were purchased at The Home Depot and Lowes.
Metal Wire Cover (Light Bar) $5.00 for (1) 5' bar. Used to keep people from tripping over wires in the home or office, I modified it to become my light bar. You could also use wood, PVC pipe, or another material. (Though I suggest something compact and tidy).
Rubber Insulated Clamps (3) $1.25 for 2 - These are used to mount the light bar to a surface.
Electrical Tape (1) $4.00 for 66' - Used to hold pieces together and insulate wires from bar.
Wet Rag - Any cloth or sponge will do, just soak it in water, its used to clean leftover solder off your soldering iron.
LEDs (18 for one light bar) $10 for 100- Pick whatever colors you want. I also suggest fading LEDS. You can use any voltage you want, though most colors fall in two categories, 1.9-2.1v(red, orange,yellow), and 3.0-3.4v (green,blue,white). Brightness is up to you, 10000mcd-18000mcd (Millicandelas) are plenty for night lighting, something like 25,000mcd may be too bright for night time, but good for accent lighting (glowing under furnitur, though 35,000mcd or higher can even be daytime lighting. Real life stores are far too expensive, so on EBay you can get them from Hong Kong for 1/20th the price. I suggest the sellers HKJE LED or LED-HK
Hot Glue Gun $5 - Get a lot of glue sticks, as they will hold things in place and insulate.
Power Supply (1) $1- Any source of power will do, though LEDs run on DC. Your voltage can be whatever you want, but you must choose your own resistors. (Supply Voltage should be higher than the LEDs Forward Voltage, around 300mA for one light bar (Milliamps are the max amount of LEDs you can have). I got three supplies for $3 at my local GoodWill charity.
Resistors (At Least 10) $3 for 100 on EBay, I suggest ResistorsPlus- These keep the LED from taking in too much electricity. It can change a 9 volt or 12 volt power supply into a 3.3 volt for an LED. For my 9 volt supply, I needed 150 Ohm resistors (9 Volts for 2 LEDs in Series). Calculate yours @ ledcalc.com A common rating is wattage, this simply means heat dissipation, you can always have the W number higher than recommended, but never lower. A higher wattage rating costs a tiny bit more, and is larger, for the most part 1/2 watt is fine, unless you begin using ultra-high power LEDs (like Luxeon Stars which can need 3-10W resistors).
20 Gauge Speaker Wire (Around 8-10 feet) - Used to connect the LEDs to the power supply.
Soldering Iron $10 (1) - Cheap, everyone should have one around. A 15 Watt iron from Radioshack works fine.
Solder (1) $3 at Radioshack- Solder with flux. I recommend silver solder at 0.022" thickness and a rosin core, it's easier to flow and more durable. Used to connect LEDs to the Speaker Wire.
Needle-nose Pliers - Used to bend LED legs.
Insulated Quick Disconnects (Optional) $2 for 12- This is used to easily plug the power supply into the light bar. You could just solder the power supply wires straight to the speaker wire, but then you always have the cord attached. (***Update, I now recommend using 2.5mm DC barrel plug connectors, they are much more durable, easier to plug in, and make a stronger connection. Buying them online is semi-random, try eBay as always**
Power Drill (1) - If you don't have one, ask a friend.
13/64" Drill Bit (2) - $1.50 for one. Used to drill the holes in the light bar. 13/64th" is the perfect size for a 5mm LED, it keeps them from going through the hole and holds them in place.
Wire Clippers - Used to cut the legs of LEDs. You can use some small scissors as well.
Awl - Something sharp with a fine point. I'm sure you can find something.
Scissors - Used to cut speaker wire and electrical tape.
Wire Stripper or Knife - Used to strip plastic insulation from the speaker wire.
If you are new to LEDs or soldering, I suggest viewing this guide @ llamma.com
Most of these parts were purchased at The Home Depot and Lowes.
Metal Wire Cover (Light Bar) $5.00 for (1) 5' bar. Used to keep people from tripping over wires in the home or office, I modified it to become my light bar. You could also use wood, PVC pipe, or another material. (Though I suggest something compact and tidy).
Rubber Insulated Clamps (3) $1.25 for 2 - These are used to mount the light bar to a surface.
Electrical Tape (1) $4.00 for 66' - Used to hold pieces together and insulate wires from bar.
Wet Rag - Any cloth or sponge will do, just soak it in water, its used to clean leftover solder off your soldering iron.
LEDs (18 for one light bar) $10 for 100- Pick whatever colors you want. I also suggest fading LEDS. You can use any voltage you want, though most colors fall in two categories, 1.9-2.1v(red, orange,yellow), and 3.0-3.4v (green,blue,white). Brightness is up to you, 10000mcd-18000mcd (Millicandelas) are plenty for night lighting, something like 25,000mcd may be too bright for night time, but good for accent lighting (glowing under furnitur, though 35,000mcd or higher can even be daytime lighting. Real life stores are far too expensive, so on EBay you can get them from Hong Kong for 1/20th the price. I suggest the sellers HKJE LED or LED-HK
Hot Glue Gun $5 - Get a lot of glue sticks, as they will hold things in place and insulate.
Power Supply (1) $1- Any source of power will do, though LEDs run on DC. Your voltage can be whatever you want, but you must choose your own resistors. (Supply Voltage should be higher than the LEDs Forward Voltage, around 300mA for one light bar (Milliamps are the max amount of LEDs you can have). I got three supplies for $3 at my local GoodWill charity.
Resistors (At Least 10) $3 for 100 on EBay, I suggest ResistorsPlus- These keep the LED from taking in too much electricity. It can change a 9 volt or 12 volt power supply into a 3.3 volt for an LED. For my 9 volt supply, I needed 150 Ohm resistors (9 Volts for 2 LEDs in Series). Calculate yours @ ledcalc.com A common rating is wattage, this simply means heat dissipation, you can always have the W number higher than recommended, but never lower. A higher wattage rating costs a tiny bit more, and is larger, for the most part 1/2 watt is fine, unless you begin using ultra-high power LEDs (like Luxeon Stars which can need 3-10W resistors).
20 Gauge Speaker Wire (Around 8-10 feet) - Used to connect the LEDs to the power supply.
Soldering Iron $10 (1) - Cheap, everyone should have one around. A 15 Watt iron from Radioshack works fine.
Solder (1) $3 at Radioshack- Solder with flux. I recommend silver solder at 0.022" thickness and a rosin core, it's easier to flow and more durable. Used to connect LEDs to the Speaker Wire.
Needle-nose Pliers - Used to bend LED legs.
Insulated Quick Disconnects (Optional) $2 for 12- This is used to easily plug the power supply into the light bar. You could just solder the power supply wires straight to the speaker wire, but then you always have the cord attached. (***Update, I now recommend using 2.5mm DC barrel plug connectors, they are much more durable, easier to plug in, and make a stronger connection. Buying them online is semi-random, try eBay as always**
Power Drill (1) - If you don't have one, ask a friend.
13/64" Drill Bit (2) - $1.50 for one. Used to drill the holes in the light bar. 13/64th" is the perfect size for a 5mm LED, it keeps them from going through the hole and holds them in place.
Wire Clippers - Used to cut the legs of LEDs. You can use some small scissors as well.
Awl - Something sharp with a fine point. I'm sure you can find something.
Scissors - Used to cut speaker wire and electrical tape.
Wire Stripper or Knife - Used to strip plastic insulation from the speaker wire.
If you are new to LEDs or soldering, I suggest viewing this guide @ llamma.com
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Aa far as calculating how many LEDs each can use, the 5-7v adapter can power one light bar, and the 12v adapter can power two, maybe three.
As far as calculating resistors, I punched the numbers into LEDcalc.com, and got these results:
For the 12v adapter, you would need 330 Ohm 1/8 W (Minimum) resistors per two LEDs.
For the 7v adapter, you would need 33 Ohm 1/8 W (minimum) resistors. Note that whatever resistor you get doesn't have to be perfectly that number, 330s are very common, if you can't find a pack of 33 Ohm resistors, 30, 35, 40, will all work; You'll find something. Have fun!
A resistor that is rated for 1/4W means it can give off .25W of heat into the air, a 1/2 .5W and so on. It is better to use a higher wattage rating if possible, the only cost will be the physical size of the resistor. You could even use a 3W 180 ohm resistor if you wanted, but it would be bigger than a couple LEDs together. A 3 W resistor can very easily dissipate .25W of heat, however a 1/4W resistor trying to dissipate 3 watts of heat will explode.
To sum it up, a bigger wattage rating is better, not worse.
Have you tried finding the metal wire cover on the home depot website? I couldn't, possibly cause HD doesn't put such random crap on their site? I found a plastic one at ACE http://www.acehardwareoutlet.com/(i53rvh55z4b5twbuutsmzw45)/ProductDetails.aspx?SKU=3139789 How well do you think this plastic will hold up?