This is a fairly simple design, but would probably benefit from having a light sensor (something I didn't have easy access to.). I plugged mine into a light timer, which is now running my hydroponics setup. I was disappointed with how directional the light ended up being, go figure since I used parts from a flashlight, but it should still work well for seedlings.
Step 1: Gather Parts
LEDs salvaged from a Maglite (LED conversion kit)
DC Wall wart**
Small Dowel (From the craft section of the Dollar Tree, originally for another project)
Scrap metal (PCI slot cover from cheap computer case)
Straw (Optional, not shown)
Electrical Tape (Not shown)
Soldering Iron & Solder
Drill Bit (And drill of course)
Knife or scissors (to cut straw)
**NOTE: Match your wall-wart with the power needs for your LEDs. I grabbed the closest I had on hand, (5V DC) and the circuitry running the LEDs overheated fairly quickly. I will be looking for a 3V DC PS next time I hit RS or Goodwill.
Step 2: "Arm" Assembly
The idea was for the weight of the LED to make the metal catch on the wood so it would be adjustable in height, but still hold. Turned out the dowel was too small. A quick simple fix with a straw, and I had a perfect fit!
TIP: if you fold the end of the straw into itself and push it as far through the hole as you can, then you can simply push the dowel through the straw and it creates a tight fit. At least it worked for me :)
Step 3: Attach LEDs
A quick (and not so great looking) soldering job attached my wall-wart to the LEDs.
To hold it all in place and keep the wire out of the way I wrapped the arm in electrical tape. I probably should have covered the wire leads as well.
Step 4: Final Assembly
Since I don't have the proper power supply for my LEDs yet, I don't know exactly how good this really works.
As you can see in the last photo, I zip tied the wire to the dowel to help keep it corralled.
As for parts, since that will probably be where most of the comments will be aimed, I used what I already had laying around the house. I do believe you could build a better looking, if not better working, light with a little planning and shopping. If you have comments or improvements I would love to hear them. I would also like to hear about experiences from people who have tried LEDs for grow lights.