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Why is there a 8x8 matrix display at the top ?

A project log for Tower clock

Freeform Attiny 816 driven clock

Pierre-Loup M.Pierre-Loup M. 04/23/2022 at 08:370 Comments

You may notice that while I mention a seven segment display in the description, and in other logs, there is no such display on half of the pictures of the clock. There is a reason to that.

A few weeks back, there was a beautiful project shared on hackaday, about a big led matrix becoming a smaller one (and another form factor) thanks to fiber optics. I've really loved it, and as I was planning the first clock I thought I had to try to display time on a seven segment display, from an 8x8 led matrix, using fiber optics. Ordered some fibers, some small 2x2cm matrix display, and wait for them to arrive.

The clock has been assembled with that in mind : the led matrix on top of the clock, and one fiber optic going  from each dot of the matrix to one segment of a 7 segment display. Each column of the matrix would be a digit, the lines would be the segments from A to G plus the decimal point. Displaying hours, minutes and seconds would use six of the height columns. That's two columns not used, which could stay off, or could display random patterns to add blinkiness, or be routed to other places to display other informations (am/pm, alarm, etc.)

The seven segment display has been made with 2mm acrylic sheet, cut with CO2 laser. It's about the same shape as any commercially available display, it just shows 6 digits and two double-dot separation. It's composed of two layers : on the front layer are segments shapes, on the back one are 1mm holes at the center of each segments. Both layers have been glued together (soldered is the right words as it's assembled with chloroform), then a white diffusive PU resin has been poured into each segment. The idea was that the fiber should go through the hole, and diffuse into the resin.

Unfortunately, it doesn't work. At all. The resin is probably too much opaque, or too thick, to properly diffuse light. There might be too much loss into the fiber. Or at the ends of it. The leds are also probably not powerful enough. I believe it could work, but it would need more tries.

So I just add a "real" 7 segments display to it. I've kept the 8x8 matrix, because it looks cool. And as they are wired together, you can literally read each byte for each digit. Plus seconds. :)

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