Original Cin

Found while I was searching for something completely different: Disk Clock – A new conception of time for the OS X Dashboard. Be sure to look at the online demo in Firefox; the clocks are pretty, if confusing, and you don’t need a Mac.

I just happened to find Ponoko, a site that will laser-cut things based on EPS files. Oh, and they have Inkscape templates available.

One thing I’ve been assuming for the clock is that I’d run it off of batteries. This may not be realistic.

I used to sell batteries, so I know a bit about them, albeit not nearly as much as I’d like. The capacity of a rechargeable battery is measured in amp-hours, or milliamps-hours as appropriate. According to the LED wizard I found online, a mere 12 of the LEDs I’ve bought for the model I’m building would make a circuit that “draws current of 960 mA”.

A rechargable NiMH cell I just happened to have around has a capacity of 2500mAh.

2500mAh/960mA = 2.6 hours

2.6 hours of operating time is not acceptable for a clock. Not even close. Having to “wind up” the clock by popping in new rechargeable cells every 3 or 4 days might be acceptably retro, but this is not.

Also, as a battery is used, its voltage decreases; the exact discharge curve depends on the battery, particularly the battery’s chemistry. Most batteries, IIRC, have a real voltage which is higher than their nominal voltage initially, but can fall dramatically at time goes on (the internet has plenty of charts of real batteries’ discharge curves).

Batteries will do fine for a model to show what the clock is supposed to look like, but probably not for the real deal.

Since I’m an impatient person, I’ve put a little PHP based version of the clock right in this blog’s sidebar. I’ve also changed the way the colors switch after realizing that Green changing into Red looked way too Christmaslike.

Last time, I wrote that it’d be very difficult to work through the clock’s logic if I used only 5 output pins. Fortunately, there’s no requirement that says I need to do so — leaving two pins available for input should be sufficient. I can use 1 pin to denote AM/PM, and that leaves 10 pins available.
In general, the fewer pins make up a combination for a particular hour, the easier the logic will be to work through, so I’m starting by assigning every available combination that uses only 1 or 2 pins first (for AM and PM, respectively) then I move on to 2 and 3 pin combinations:
0000000001 1AM/PM
0000000010 2AM/PM
0000000100 3AM/PM
0000001000 4AM/PM
0000010000 5AM/PM
0000100000 6AM/PM
0001000000 7AM/PM
0010000000 8AM/PM
0100000000 9AM/PM
1000000000 10AM/PM
1000000001 11AM/PM
1000000010 12AM/PM

The clock, as I’ve previously described it, can be in one of 24 possible states, because there are 24 hours in a day. Each hour “segment” of the clock can be in one of three possible states — red, yellow, or green, but red and yellow are really the same state based on whether the time is AM or PM.

I’m thinking of making an Arduino the heart of the clock. It’s a bit of overkill, but I guess I can have a fancy light display for the change of the hour or something like that.

The Arduino has thirteen digital outputs (or inputs, as the case may be). One way to hook things up would be to have each output control a single section of the clock, and use the final one to determine AM/PM. Unfortunately, this doesn’t leave any way to change the time.

There are 24 possible states for the clock face. The minimum number of outputs needed to represent this is 5, since 2⁵=32. However a lot of fairly complex logic would be required to get the clock working in this case: 12PM would be “01100″ and 4PM would be “10000″ — here’s a table of each possible output when using five pins this way, the corresponding time, the color that the “9′o’clock” segment should be.

So, the rule just to determine if the yellow LEDs for the 9 segment should be on would be something along the lines of “Light up if ((Pin 2 AND Pin 4) AND NOT (Pin 3)) OR ((Pin 2 AND Pin 3) AND NOT (Pin 4 OR Pin 5))”

Yikes. Complex! Poking around Wikipedia gives some hints on how to make this slightly better, but nothing based on an output of 5 pins is going to be simple. Fortunately, there are some things that I can do to make things easier for myself…

As part of my 2008 campaign to “Create It,” I’ve decided to try to build a clock of my own design. This is, quite frankly, crazy. As a matter of fact, I have little experience with anything that I might use to create this project other than some halfway-decent programming sense.

Anyway, on to what I have in mind: A ring, on a square board roughly two feet wide. The ring acts a sort of pie chart of time, and updates in one hour increments (no minutes in my clock, I don’t believe in sweating the small stuff!). At 12:00 midnight, the clock is completely green. The clock then begins turning red, one hour block at a time. At noon, the ring turns yellow, and the process repeats until it cycles back to completely green again at midnight. The drawing above shows 4:00 PM under this system.

I’m thinking of using LEDs to light up the clock face, and I plan on picking some up to play with the next chance I get.