Last, not really related to this forum I suppose, but I was wondering about battery life. I mean, with charlieplexing only one LED at any given time is on, so I shouldn't have too much current draw - but I still have no clue what power source to use. I want to make this a wristwatch, so a coin cell (perhaps multiple) would be ideal, but I really have no idea. Any suggestions?
LAST: I want to use blue LED's, but if battery life makes that impractical, what kind would you suggest, what specs, etc. If blue LED's would work, once again, what kind/specs should I look for?
Firstly get ultra-bright LEDs and use a small current (1mA or less should be fine).
Secondly what voltage? Coin cells have a tiny tiny capacity, a small LiPo cell would be much better (and gives 3.6V or so in one package) - however I'd not be too happy with a LiPo cell strapped to my wrist given the fire risk.
Note blue green and white LEDs take a higher voltage than red/orange.
If you use a RTC (real time clock) chip or module then the display driving circuit can be powered off until its needed, RTC chips like the DS1307 (or DS1338-33 for 3.3V) will run for years off a Li coin cell because they take about 300nA. A separate battery for the MCU and display is desirable, adding complexity. If the display is only powered up when pressing a button switch a coin cell might be enough for that too.
Sparkfun do an RTC module incidentally.
For reference I've made a timepiece (little bit big to be a watch really) like this - it uses an RTC chip for keeping time and a ATmega168 to drive small 7-seg LED display. One AAA NiMH 1000mAh cell and a boost module to 3V3 powers the display for about a day only. If the 168/display is activated when needed it would last months (till the cell self-discharged). The RTC runs off a CR1225 coin cell and should last 10 years.
LAST: I want to use blue LED's, but if battery life makes that impractical, what kind would you suggest, what specs, etc. If blue LED's would work, once again, what kind/specs should I look for?
I don't know what you consider to be impractical. This past holiday I received the Binary Watch sold at ThinkGeeK as a gift. It has blue LEDs but I have no idea what the display method is. They are so bright I wouldn't be surprised if they are directly driven. (However because of how even the lighting is regardless of the time, I suspect there is some multiplexing.)
With two months of telling time + lots of extra demonstrations the coin cell is holding well. For me, the battery lasting one month was all I expected. Like I said, I'm on almost two already.
Well, are the LED's SMD in the watch you have? I think they use lower mAh. I'm thinking of using 1 AA and boosting it to 3.3v - a bit clunky for a wristwatch, but it'll do. As to using an RTC module, an interesting idea, but I want to keep this project more simple (I.E., clock off the ATTiny). Battery life from the AA would depend on the draw of the ATTiny - boosting a 2000mAh 1.5V AA up to 3.3v at roughly 90% efficiency will give me about 900mAh at 3.3v to work with - just driving the LED's, that's a little over a month straight. I found some Surface-Mount green LED's that use 2.1V @ 1mAh, and I might be able to solder to them... hmm...
I've just been working on the actual charlieplexing part in EAGLE, and the circuit board layout is a pain.
Alright, I think I've got both Charlieplexing and the other clock code/displaying in binary worked out. My final concern is battery life - boosting a single AA to 3.3V at 80% efficiency (about the best I can hope for, I think) I'm getting about 1Ah, total. The LED's will draw around 20mAH, I think, but how much does the ATTiny draw? That's the last thing I need to know.
Base on the datasheets if operate 1MHz 1.8V it draw 300 micro Amps. so if you operate at 3.3V map be about 1mA.
Please read the section 7 on power consumption issues. like disable ADC, all port pins was avoid unconnected, have a defined level. Connecting unused pins directly to VCC or GND is not recommended, since this may cause excessive currents if the pin is accidentally configured as an output.
Well, I'm using the 2 remaining pins for buttons to set the time, and then a button to display the time - to save battery, the LED's will be off unless I press I button that tells the arduino to display the time.