THINK FIRST



MOUNTING THE LEDs



MAKING IT (Hey thats a great name for a magazine :-)

DRILL HOLES



MOUNTING LEDs



SOLDERING IT TOGETHER

DO I *REALLY* USE 360R AND 36R RESISTORS?



ADD A RIBBON CABLE

Before you do anything, have a think about the size you would like the project to be. In this version, I used high intensity LEDs. they have a beamwidth of about 10 degrees, which means that I had to mount the stencil about 30mm from the LEDs to get an even illumination. If you purchase 'Flat Top' LEDs, then you can make the clock much thinner.I decided to make the display board using a piece of fiberglass material, with the LEDs mounted into a series of holes that I pre-drilled. This is a different mounting system than the original, but I feel that it is probably more accessible, as it doesn't rely on another expensive PCB. The LEDs are soldered on the back of the board to a piece of 26 way ribbon cable that is terminated with a 26 way IDC socket.You can use a piece of thin plywood, or acrylic if you like - Just use whatever material you can that will hold the LEDs stable.I started with a square of fibergless sheet, upon which I measured a 12cmx12cm square. I divided that into a grid that was 13 dots x 9 dots (which turned out to have a spacing of 1cm x 1.5cm), and drilled starter holes using a small drill.Then I measured the LEDs, and they were indeed 5mm diameter, so I drilled LOTS of 5mm holes.....Before I started drilling, I was aware that there was going to be a very big mess. There was board material everywhere. I did the drilling outside, and made sure I wore a dust mask.So many little holes....Once that was done, I slid each led into a hole, being careful to make sure that they were all oriented the same way. A drizzle of hot melt glue was useful in holding the leds in place. When assembling, I mounted every second row on the board, and gently folded the leads down as time came to mount the final sets of rows.Then I trimmed the leads, and soldered in the current limiting resistors, and soldered the LEDs together, following the layout diagram exactly. When you are following the diagram, remember that you are working on the BACK of the board, so you need to mentally reverse the layout. Also - don't forget to use 36R resitors (see below) when you are connected to a set of 3 LEDs and 360R (see below) resistors when you are connecting to a set of 2 - It alters the current flow to make the LEDs a uniform brightness.As an update, I have been using Blue Flat Top LEDs, with a forward voltage drop of about 3.3v @ 20mA. The supply that I was using was about 10.5VDC. With the original design, with 2 LEDs, the voltage drop across the LEDs was 6.6v, meaning that 3.9v was being dropped across the resistor (11-6.6). With a 360R resistor, this limited the current to 10.8mA, and with 3 LEDs, the voltage drop across the LEDs was 9.9v, meaning that 0.6v was being dropped across the resistor (11-6.6). With a 36R resistor, this limited the current to 16mA.What I have subsequently discovered is that with supply voltages of 12V, the current through the leds ends up being way too high (15 & 58mA), which is a problem.In my most recent versions, I have setled on 680R for the 2 Leds and 360R for the 3 Leds, providing 7.9 and 7.8 mA respectively. Much lower current, and still ample illumination.When everything is finished, prepare and solder on the ribbon cable with the 26 way connector.The connector plugs into the controller board. (But I guess you figured that out!)The photos below should show the process that was used.To be completely honest, I think next time I will simply use the display board from the first WordClock that I built. Yes, it would mean making another circuit board, but it was so much simpler to make, and the result looks much nicer.