Have you ever asked yourself what the tiniest LED package is? I did. Its called 0402 with dimensions of 400 x 200 microns. They are not only barely visible but also hard to solder. After one trial and error and some video tutorials (1,2,3) I got some enameled wire attached to them. But what`s the purpose of those small devices?
As the sketch indicates I had the idea to build a custom ultra-flat 7-segment display with them. The plan was to solder seven 0402 LEDs, align them as in a conventional seven segment display and fix them by a thin glass-fiber composite.
Now soldering wasn`t any problem after some training. Having the LEDs ready I prepared 2 thin glass-fiber fabrics and an appropriate measure of laminating epoxy. I laminated the first sheet of glass fabric on a clean glass plate (not to forget the form release agent 😉 ) and tried to position the LEDs on it. There I went into trouble…At first I tried to place them on the wet glass fabric which didn`t work out as the LEDs would always try to reposition themselves because of the flexuous wires. Then I tried to place them on a piece of adhesive film in order to put them on the glass fabric as a single unit later…didn’t work out either…Last thing: I took the second, still try piece of glass fabric and sticked each of the wires (@ a point very close to the LED itself ) to it with adhesive tape. The solution is probably the worst, as adhesive tape will prevent curing/adhesion of the epoxy partially but anyhow I had to get those damned LEDs to the fabric ;-). So I connected both pieces of glass fabric with some pressure and waited for the epoxy to cure. The result is a thin, still a bit flexible plate with the LEDs well covered in plastic in the middle, see the picture. The overall thickness is roughly between 600 and 750 µm, which is kind of thin I guess. The only problem is that the LEDs got misaligned during the whole process. So they were not looking like a 7-segment thing anymore which could be problematic when trying to display something.
In order to test I hooked all the LEDs up to the digital pins of an arduino uno with a 100 Ω resistor as a current limiter for each of them. I wrote a short sketch where I defined the different numerals form 0 to 9 (essentially a definition of which LEDs has to be on for each numeral). Whereas in the loop, I made these numerals to be shown as a countdown with decreasing delay time in between each numeral. (download the code here).
As the picture below indicates at least number one is displayed correctly. As I mentioned early most of the LEDs where a little bit misplaced which leads to an obvious unreadability of most numerals.
So although I have a working prototype there is still a lot of issues for further improvements:
- test if LEDs can be grinded on top in order to make them even smaller
- better method of fixing and aligning the LEDs before the composite fabrication
- bring LEDs closer together (in order to make the numeral more visible
- try to solder two or three LEDs together as a strip to make a bigger numeral
- find a glass fabric which is more diffuse for another look
If anyone is interested in this idea please feel free to grab, test and make something of it. Why not try to make a clock?? Go for it!