KFRT's Dot Clock
Wiring the display


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Building the display
Wiring, Pt 2

Here's a cool diagram of the type of LED I'm using (swiped from Wikipedia). It is important to study this a bit because of the polarity. When we install the LED's into the ABS sheet, we need to make sure we have the polarity right on every single LED. 144 LED's, no mistakes. Note that there is a flat spot on the mounting lip that corresponds to the negative lead. We want to mount all of the LED's so that the flat spot points toward the outside edges of the clock.

For this clock, I'm going to use blue on the outer (seconds) ring, white on the center (minutes) ring and alternate blue and white on the inside (hours) ring. You can really do this however you want, using whatever colors you want. You could, for instance, put a different color in all of the 5-minute marks on the minute ring.

The LED's I am using are:
   Kingbright Water Clear Blue, Mouser P/N 604-WP7113QBC/D
   Lite-On Water Clear White, Mouser P/N 859-LTW-2S3D8

You can do the glueing any way you wish. Just make sure you install from the SMOOTH side (see picture, below). I find that it works well to "install" a full ring of LED's and then inspect the whole ring for proper orientation before gluing. I set the ABS sheet on four of the spacers (we'll use later) to hold it above the table (because the LED's stick out a fraction of an inch). Once the LED's are in place and inspected for proper orientation, place a small drop of Super Glue on the back of each one (again, on the smooth side of the ABS). The Super Glue acts as a plastic weld and will actually melt the LED to ABS. This should be considered permanent! Go very easy on the glue -- you don't want any of it seeping through to the front side. One small tube of Super Glue should be enough for the whole project.

Here is the back and the front detail of the ABS sheet, once the LED's are glued in:

Another note about the SuperGlue... On this display, I used a tube that's been laying around for a couple years. It had partially gelled, and a few of the LED's popped out when I was bending the leads. It might be best to use a fresh tube.

Add Schematic here

If you thought anything up until this point was tedious, here's where the fun begins. The display is wired as a 12 x 12 grid (144 LED's total). We group the LED's in 12 groups of 12; five groups for the seconds (outer) hand, five groups for the minutes (center) hand, and two groups for the hours (inner) hand. The PIC driver will run one group at a time, scanning the display at 1KHz (1,000 tmes a second). This multiplexing is what allows us to only have 24 (12 + 12) wires for control. This simplifies the wiring and driver electronics, but complicates the firmware in the PIC (it has to scan the display).

Up until now, we haven't cared much about which edge is the top of the display. Wiring the 12 buses will define this. Here, we wire the outside leads of the LED's together, as in the following diagram. The yellow line indicates our "zero" position. This consists of the three LED's that are perfectly perpendicular with the edge of the ABS. Any of the four edges are okay, so if you have a "sense" of which edge should be at the top, now is the time to choose. If it helps, mark the "zero" set of LED's with a piece of tape or something.

The green lines (apologies for my ragged drawing) illustrate the 12 groups. The display must be wired this way in order to match how the firmware scans. Note that the groups start at the zero point and run counter-clockwise. This is because we're working from the back and the display is scanned clockwise.

Here is the same picture without the lines, and all 12 buses wired up. Once done, we have 144 leads sticking up, instead of 288.

Here is the "knit one, pearl two" of wiring the 12 buses. We start by bending an outer LED lead towards the outside edge of the clock. There should be about 1/8" above the LED, before the bend. I find it easier to do all the lead bending first and solder it all later.

Another bend is then made towards the next LED in the bus. Again, leave 1/8" or so between the bends. Don't waste time measuring this, just leave a little room.

A bend is then made in the next LED so that it overlays the lead we just bent. Then, the lead from the first LED can be bent over (and clipped, if it's too long).

Another set of shots, from above.

Once this is done for all 12 buses, you can solder it all. You'll probably find that the leads on the hour hand just barely reach. In this case, there isn't a real good mechanical connection, and the solder will have to suffice on it's own.


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Page last updated 1 August 2010