Mechanical indicator

I want to make a Timestrip. What I mean by that is a very low power circuit that runs off a CR2032 or smaller that counts up to a certain time in hours or days. When the time comes, it turns on some kind of indicator and then waits for reset from a button or similar.

I set out to make this but don't know what to use for the indicator. I can't use an LED because LEDs are not low-power. What should I use instead? And is this circuit a good use for the 32.768KHz crystal driving mode?

orangeLearner: I can't use an LED because LEDs are not low-power.

It's possible to get very small LEDs that don't use much power. How much energy they use would depend how long you left them on after your time had elapsed.

I suppose that an LCD would use less power. Watches routinely run them for years on a small battery. I've no idea how to drive an LCD display but I imagine you could find out.

You're right - I didn't think of an LCD but I have seen them in Atmel product videos. I suppose I would use the Atmel Butterfly or an LED with a 1/1000 duty cycle, like Nick Gammon does in his low power circuits.

What you want is some kind of bi-stable indicator flag. So there is no power consumption except when you make the flag change state. Such a thing must exist I am sure.

I've seen something call (I think) a 'flip dot' or similar - basically a disc that could be flipped via a solenoid so that either face was up. I gather they're intended to be used to create very large dot matrix displays, but I'd have thought that one on its own would be good enough to show whether the timer had expired. No idea how current they take, though.

If can replace the "flip dot" with an electromagnet that opens a gate and makes something (e.g. ball) fall through. The electromagnet may take 10mA or so. A simpler solution is to involve a battery powered quartz clock. You power the clock (so the seconds hand moves) for as little as possible at the end of the time-cycle, but as much as to make a visual difference. Interesting challenge, I would like to see it working in practice.

You could use an e-ink display. These displays have little balls inside that are black on one side and white on the other. Flipping the dot takes power, but it is stable in either state. Most displays wold be way too big for your application, but they do make small ones. I have a flash drive that uses one as a capacity gauge. A Google search for "e-ink Arduino" came up with plenty of hits, so it has been done.

I don't see where super accurate timing is involved since you are talking hours and days. Like watching grass grow to a certain height. Blinking a low power led would seem a way to draw attention. Does not have to happen continuous just a few times for a few seconds then wait maybe 30 seconds or so and do again. Put the sucker to sleep between.