KFRT's Dot Clock
Building the display


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Wiring the display
Wiring the display

After a couple of years, a few LED's in the second clock have burned out and a few others are "weak." The second clock was built using blue and white LED's at a time when anything but the common red / green / yellow LED's were very expensive. I found the cheapest blue and white LED's I could find at the time, and that may have been a mistake. The odd colors still cost more than the common colors, but they've come down in price, and I was able to buy 100 of each from Mouser for about $47. I hope these are better quality.

On this page, I'll try to do a better job of documenting how the display is built.

The display is a "sandwich" of three 12x12x1/4" panels. One panel is clear acrylic (Plexiglass), another is smoked clear acrylic, and the center of the sandwich is ABS. The LED's are super-glued into the ABS panel and the smoked acrylic panel is a cover for this. The clear acrylic panel sits "behind" it all and protects the electronics and wiring and makes the whole thing thick enough that it will sit on a shelf. When complete, the clock is 12x12" and about 2" thick.

I buy the acrylic pre-cut from estreetplastics.com and you can get the ABS pieces from eBay. In this case, I had enough left over from before, so didn't have to buy any. They're never perfectly sized and on this display, I thought I'd try to fix that. Below is a picture of the "sandwich" being perfected on a Bridgeport mill. Credit goes to my friend Dale for this (thanks, Dale!); a little after-hours "G" job at work. The mill squares things up perfectly and leaves a nice finish on the edges. This step is optional, but if you really want it "done right"...

The blue sheet is actually the clear acrylic. Sheet acrylic comes with a protective plastic cover on both sides (to be removed later). For some reason, the plastic they use on the clear stuff is blue. This helps keep the acrylic from getting scratched and dinged while we manhandle it.

On the opening page, I mentioned marking the "sandwich." This is not really optional. It ensures that the three pieces are always stacked the same way throughout the process. Use a Sharpie, or some other indelible marker. Once marked, there is only one way that the three pieces can be assembled and have the marks line up. It's hard to see the marks on the dark plastic in this picture, but they're easy to see with the naked eye. There will be a small amount of "slop" when we drill the holes, and it's critical that the holes line up. Marking makes it a lot easier to make sure the panels are assembled exactly the way they were drilled.

The next step involves installing the drill template to the back of the ABS plastic. The drill template will show us where to drill the holes for the LED's. This was originally drawn on a sheet of construction paper with a ruler and compass. From the center, lines are drawn every 6 degrees to the edge. Then, three circles are drawn, wth radius's (radii?) of 3.5, 4.5 and 5.5 inches. These correspond to the hour, minute and second hands, respectively. The holes are drilled at the intersection of the radial lines and the circles.

Since we need a bit of precision here and we'll be drilling through the paper, the template is disposable. Because it takes a lot of time to draw the template, my advice is to draw it out and then make copies at your local copy center (Kinko's, whatever). Then, use a copy for the actual template.

The picture below is that of a Kinko's copy. Note that the inner circle has marks (small cricles) for the hour hand, and small "tick marks" between these. The small circles are for the 12 hours and the tick marks allow for 12 more for a 24-hour "military" clock. Being a "ham," the software I wrote supports both formats.

Also note the four crosses in the corners. This is where we'll drill holes for the bolts that hold the three panels together.

To align the template on the ABS plastic, we start by putting four pieces of masking tape at the center of the four sides of the sheet (left picture, below -- that picture didn't come out real well!). Then, the center of each edge of the plastic is marked on the masking tape (right picture, below). Measure each edge of the ABS with a ruler, divide by two and mark it. I should probably mention here that the ABS sheet is smooth on one side and textured on the other. The textured side is the "display side" and we will be drilling on the smooth side.

To glue the template to the ABS, I use Krylon 7020 spray adhesive (available at most home improvement stores). Cut the template so that it's slightly smaller than the ABS sheet and test the fit before applying the adhesive. Krylon 7020 will give you a minute or two where you can move the template around easily, but this is a good place to pay attention. The trick here is to align the masking tape marks with the noon, 3, 6 and 9 o'clock marks on the template. Once aligned both horizontally and vertically, the template is also centered.

Spray the Krylon on the smooth side of the ABS and position the template just like you did in your dry run. It should look a bit like the picture below, when finished. Note how the 6 o'clock mark lines up with the mark on the tape. All four tape marks should look like this (or very close). Go real easy on the Krylon -- you don't want too much! Once the template is positioned on the glue, smooth it out with your hand to remove any big bubbles. The real key here is the alignment.

Once the template is in place, the ABS sheet is taped to the smoked acrylic sheet using eight small pieces of duct tape (two on each edge -- better yet, use three!). Be sure to check your alignment marks. The clear sheet is set aside, for now. Note that we don't remove the protective plastic from the acrylic until later.

The LED's I'm using for this project are the "T-1 3/4" size, which are 5mm (0.197") in diameter and about 0.29" from the mounting lip to the tip of the bulb. A 13/64" bit works well for these. Because the panels are 1/4" thick, we'll drill completely through the ABS sheet and about half-way into the smoked acrylic sheet. In order to get any consistency, this has to be done using a drill press with a depth limit. Set the depth limit so that the tip of the bit is about half-way through the acrylic. This should put the drill bit tip about 1/8" above the table on the drill press.

Drill ONE hole and then inspect it. You should be able to hold the panel up to a light and see through it. Insert an LED and make sure it bottoms out, with the mounting lip of the LED resting on the paper. Inspect the acrylic side, too. You shouldn't be close to drilling THROUGH the acrylic.

Some drilling tips:
   1] With a little practice, you can "nick" the paper with the drill for better accuracy. Nick it first, reposition if you have to, then drill.
   2] Laser crosshairs help, but are not required.
   3] The panel "sandwich" will jump when the drill goes through the ABS and into the acrylic. This is normal.
   4] TAKE YOUR TIME. Inspect frequently. You've come too far to screw it up now!
   5] Keep the workspace clean to prevent drilling at an angle. You will likely have to stop once in awhile to clean the plastic out of the drill bit. This is a good time to wipe things down.
   6] Drill the center circle first. This will get you used to the "jump" as the bit makes the transition from the ABS to the acrylic.

Accuracy counts here. If you're off by a tiny amount, the clock will still look good. If you're off by .05", the clock will be ugly. Four more holes to go.

To connect the three sheets, I use 1/4" x 20 2-1/2" black socket head cap machine screws. Four 1/4" holes are drilled in the corner "targets" of the paper template. The easiest way to do this is to tape the clear acrylic sheet to the two (already taped) sheets. Align the marks again and just tape the clear sheet under the other two. Make sure you relese your drill depth limit and drill all the way through all three sheets using a 1/4" drill bit. The protective plastic is still intact.

Once this is done, it is safe to remove all the tape and separate the sheets. The paper template can be peeled off and discarded.

The last step in this phase is to inspect and clean the panels. Remove any burrs from the holes using an Xacto knife. Blow any debris out of the half-drilled holes in the smoked panel. Any gum remaining from the tape or the adhesive spray can be removed with a rag and acetone (this will also remove your alignment marks, so be careful!). Don't worry too much about getting all the glue off the smooth side of the ABS perfect -- it won't be seen once the clock is finished. The important thing is to remove any burrs that would prevent the LED's from seating properly and making sure the holes in the smoked acrylic are free of any "specks" of plastic that would be seen once the clock is finished.

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Page last updated 16 July 2010