Convert a Battery Powered LED Lamp to USB Power.

Greetings,

My desk at work has a hutch on it with a window opposite, and after a phone system change we lost the backlit LCD panels on our phones.  Since my phone is positioned under my hutch and I didn't care to rearrange my desk, I put a cheap battery LED lamp above the phone to illuminate it so I can see the LCD panel and buttons during the daytime.  Unfortunately I had a bad habit of forgetting to turn the 3 AAA battery powered LED lamp off when leaving, and it would be next to useless the next day.  Rather than continue wasting batteries or investing in a charger, I thought surely it wouldn't be difficult to convert it to USB power, there's only .5 volt difference and probably already a resistor in the lamp.  The lamp was about $5.00 for a pack of two and I had spare USB cables lying around.  The external monitor connected to my laptop had some ports that would be handy to use.  So here's how I went about the conversion... 

Step 1: Materials & Tools Needed

Ok so here are the tools and items I used to complete the conversion:
   A small phillips screwdriver (from a precision screwdriver set)
   A pair of small sidecutters
   A retractable blade box cutter knife (any sharp knife or other instrument to make a channel for the cable will do)
   A small cordless screwdriver/drill with a drill bit (You can get by without this though)
   A soldering iron and some 1mm 60/40 Rosin Core solder
   A 50mm fold back clip (used as a clamp for the wire)
   One small cable tie  (could probably be omitted)
   One 3xAAA battery operated 4 LED lamp
   One USB cable from a Samsung mobile phone (just about any USB cable will do)

You could probably benefit from using a pair of wire strippers - I didn't happen to have mine available at the time.

Step 2: Get the USB Wire Ready to Connect.

Ok now I use my side cutter to snip off the end of the USB cable that I won't be needing.
Then if you have wire strippers, now is a good time to use them to strip a small length off of the end of the USB cable, remove any shielding in place and separate the wires.  The red and black wires should have a small section stripped as well, and if necessary twist the exposed wires into a nice neat strand.  You could remove the Green and White ends now if you like or you can snip them off later.  They are not required.

Your USB cable should have a minimum of 4 wires in it, being:

Red  (+5V)
Black (GND)
Green (Data+)
White (Data-)

Some USB cables have braided metal shielding around a foil shield surrounding the wires.
This particular cable had only a foil shield, and 5 wires in it.  The 5th looked like a simple unshielded metal strand.  I don't know the exact purpose of it but I don't care either - only the Red and Black wires are important for this conversion.

Next I will get the LED Assembly ready and connect the wires.

Step 3: Remove the LED Assembly From the Lamp

In the lamp I have, there are three small screws holding the LED assembly in.  They go directly through the battery holder and into the plastic behind the lamp lens.  The battery holder wasn't actually secured to the LED circuit board in my lamp - if it is in yours then this might complicate things for you but shouldn't be insurmountable.  Depending on your specific lamp, it may be possible for you to complete the conversion without worrying about the battery holder.

Once you remove the screws, the LED assembly should come out without any trouble.  Set the lamp housing aside in a safe spot for later.

The battery back is connected to the circuit board with two wires, one for positive (red) and one for negative (blue).  Positive is marked on the circuit board as well.

Step 4: Desoldering, Then Soldering... Then Test.

NB: if you want to retain the ability to use a battery in your lamp, you'll need to omit the part about disconnecting the battery terminals.  For my purposes however, I didn't care about using the battery capability any further.

So using the soldering iron I desoldered the connections from the battery holder to the circuit board.

I then desoldered the connections from the battery terminals to the wires, though you could omit this step if it suits. 
Later on I ended up completely removing the battery terminals from the battery holder (they might come in handy for another project later), so I could have left the wires connected on those. 

After that I made sure the circuit board was clean enough for new connections from the USB cable.  Note in the picture you can see through the holes.
I then threaded the red wire from the USB cable through the positive side on the circuit board, and the black wire through the negative.  I bent them around to help hold them in place while soldering.  You might find a clamp to hold your wire and/or circuit board helpful for this.  I used an upturned 50mm fold back clip to help hold the cable in place.

Once the wires were connected I gave the solder a chance to cool down for just a short while, then plugged the USB cable into a port and pressed the switch on the circuit board. 

If your LEDs fail to light, I'd suggest checking the connections again, make sure you did a thorough job soldering the wires to the board, also check for breaks in the wires - particularly the strands near the soldered joints as this is the most fragile area.  If still no joy here it might be wise to try another USB cable.  Also don't forget that the device you are plugging into may need to be powered on!

Mine was all working alright so I can proceed to remount the LED assembly...

Step 5: Re-mount the LED Assembly

Now that I have a working USB LED, I can put it back into the housing.

The battery holder in my case was going to interfere with the USB cable, so to make that less of an issue I snipped sections of the holder off.  If you plan on keeping battery as an option you'll have to find another way.  I did end up putting a cable tie around the cable, securing it somewhat to the battery holder.  This could probably be skipped and in my case I'm less than confident in any safety this offers ;)  I also removed the battery terminals, figuring I might be able to make use of them on some other project perhaps.

As I was mounting my lamp to the underside of a hutch, I wanted the cable to come out from one of the sides.  So I used my cordless screwdriver with a drill bit and the box cutter to cut a little notch for the cable to go through.
Another option would be for the cable to come straight out the back where the mounting hole is, if that was an option you can use.

Originally this lamp was designed to be able to be turned 360 degrees and also has a tilt lens.  With a cable mounted like this right to the circuit board, the range of motion for rotating will be significantly reduced.  In my case this wasn't an issue.  The tilt feature still works just fine, and I can still rotate the lamp a bit.

And there you have it.  Good luck if you are doing something similar!