Silent Q

Posted By azog on September 8, 2009

Have you ever heard of, or seen, a til-311 display?

These are interesting “mid-retro” displays. They can display hex digits (0-F), have both a left- and right- decimal point, and blanking and strobe inputs. The logic driver is built-in, which is the tiny circuit you see near the notch. Driving these displays is very easy by simply feeding 4-bit binary. And they’re handsome.

I picked up a stack of 8 of these for what I considered a decent price. You can still buy them new, but they’re pretty expensive. So what else would I do but build a clock around them?

The first thing I did was breadboard a concept to see if it would work. And you’ll forgive the messy breadboarding technique. It’s not like I’m being graded on my work.

Once I had the basic idea down, I had BatchPCB fab up a board for me. My first one didn’t do very well, for reasons which I’ll explain later. So this is actually the second revision.

And a quick 30 minutes later…

Since I got 8 of these and BatchPCB gave me to boards, I have enough parts to build another, if I so desire.

Some issues:

The decimal points are not driven by the display logic, they’re directly connected to the power rails, so you absolutely need to use a resistor. My first board did not use a resistor, and guess what? I burned out the left decimal points on four of these displays.

I tried to use an on-board 7805 5VDC regulator, but I really don’t know how to properly utilize these in anything beyond a simple breadboard.

And for a real /facepalm mistake, I tied the AVR RESET to ground. AVRs uses an active-low RESET, so the device never actually started to run. That actually had me scratching my head for a significant period of time.

So “version 2″ dropped the 7805 and I opted for a 5v switched wallwart from SparkFun for $6. I remembered to pulled RESET to VCC through a 1k resistor. I also included two limiting resistors on the left decimal points. I physically removed the socket pins for the right decimal points. I also used a trick I saw elsewhere: two large through-holes provide strain-relief for the wall-wart cable.

The time-base is a DS32KHZ, which I’ve used before, and love. The time setting switches are on a basic RC debounce circuit. Other than that, there is really nothing else on the board.

The clock is a basic 12 hour clock. On the minutes display, the left decimal point provides a blinking second indicator. On the hours display, the left decimal point provides an AM/PM indicator. Both of which you can see activated on the image above. The left-most hours display is also blanked depending on the hours.

There are a few issues with the software, but those can be addressed without concern over the hardware.

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