6″ HCD Wall Clock Design

Katie got me some giant 6″ 7-segment displays from Sparkfun for my birthday a couple months ago, and I’ve been neglecting them. So I started designing it! Since she got me 3, the plan is to make a hexadecimal-coded decimal (HCD) display, where the hours are designated as 0 through C and the minutes run from 00 to 3B (we like to keep things complicated around here).

The plan is to lasercut a sheet of acrylic for the backplate, since acrylic is one of the stronger materials on the list of allowable materials for the laser cutters at the architecture media lab. I’m in the process of measuring the actual segments to cut out of the front plate, although it’s still a possibility that a more opaque plastic cover could be used (for that retro feel we all know and love). Each display comes with a dot (.), so two of those will be transformed into a colon.

A PIC24FJ32GA002 will be used as the controller, since I’d like to steer away from AVR controllers for now (or steer towards PICs, I suppose). I had to purchase a PICkit 3 for my microcontrollers class, and I enjoy using things I buy. This particular controller has just enough IO lines for its purpose: 9 for a binary representation of the time, 5 for the output control lines, and a few more for time setting buttons.  I’ve stumbled on several 32.768kHz clock sources, but that’s still on the table.

The displays are made of 7 separate segments, each containing 12 red LEDs (two parallel series of 6). This means the supply voltage needs to be around 12v, so I’ll need a way to interface the 12v segments with the 3.3v electronics. That way is the tpic6a259, an 8-bit addressable latch that uses open-drain DMOS transistor outputs that can handle up to 50v. Each digit gets a latch IC, and each one of those receives 3 address input lines along with a clear bit, a general control bit, and a data bit. The CLR and G bits can be tied together on all 3, while the addresses and D line are individualized.

So in the coming weeks, as free time comes (and goes), I’ll be drawing the mechanical structure of the clock and building it, laying out and creating the PCB, and (hopefully), an awesome new addition to a wall near me will be sitting on said wall!