When I did the LED Light box project I bought a couple of sets of LED's. This set has the remote and is RGB not white which means they can change colours.
Cut out your image you wish to have back lit from the LED's. I used a scroll saw but depending on your image you choose you could use a jigsaw or copping saw etc. Take your time and get it as good as you can.
Then make a frame that is deep enough to hold the LED's and power pack, I also had to drill a small hole in the corner to poke the remote receiver through. And I cut and chisel small dado for the power chord at the bottom.
I then painted it mat black ... I mean Batman... :-) and the contrast of the Rimu wood frame makes it look nice even when the LED's are not on.
Then I put in some small cleats inside to hold the actual picture into the frame. Putting in the LED's around the edge of the frame using the double sided tape was easy and simply plug in.
the only thing to consider when making something like this is where to hide the power pack and remote receiver if you are doing a full on more detailed image I would try leave room at the bottom of the image for this.
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