Making a Wrist Watch

Hi-dee-hi! Firstly, thank you for the wonderful project and sorry if this is in the wrong sub-forum (I didn't think this belonged in Projects as it is questions about one rather than anything finished).

I was wondering if it would be possible to use the Espruino to create a small wrist-watch; with its battery capabilities, the screens available and the use of JavaScript, it seemed an ideal choice, but I am still a little unsure on how to approach it.

Where could I acquire a screen approximately the size of the Espruino and, furthermore, is the battery good enough to provide power to the Espruino and the screen for multiple hours at a time?

Thank you in advance for any input :)

EDIT: to expand, I am experienced with JavaScript but a novice with hardware. Additionally, the display needn't be anything fancy, just something like the display of an old Nokia would be fine if that's what would work best.

Update: I have found this article on a Nokia screen, which will likely prove very useful in the screen area, so I guess we can mark that as solved. :)

The only question that remains now is how long the Espruino's battery could power itself and the display for.

Update 2: Found an article on batteries. Hopefully I'm all setup to give it a go now :)

The espruino doesn't come with a battery, unless you bought the ultimate kit that came with one, though if you're driving a non-backlit LCD, the battery life will probably be decent. Espruino can run in very low power sleep mode if you tell it to.

I think the Espruino would make for a rather bulky wristwatch, especially if you wanted to add anything beyond a screen (and with just a screen, it doesn't seem very useful...)

Any recommendations on the best battery to use, bearing in mind my need for it to be very portable?

The smallest LiPo battery that will provide the required battery life ;-)

I'd get everything else set up on the bench and measure how much current it draws running off battery voltage, and let that be my guide. Picking the battery now is putting the cart before the horse. (Though I can't blame you for wanting to get orders placed now, speaking of carts and horses...)

Okey-doke, I'll see what I can get setup and see if it reaches a reasonable size. I suppose at least, if it is too big, I'll have a nice clock :P

You could use one of the old Nokia phone batteries... I've got a BL-5B here and it fits quite nicely within the footprint of the Espruino.

Also you're spot on with the Nokia display mentioned above - power draw from that is tiny. If you update it every second, Espruino will still end up drawing around 12mA (it's unlikely to be able to enter deep sleep at the moment because it can only wake itself up once a second from the RTC and it won't think it can enter sleep mode and wake back up in time).

So, updating every second with the Nokia battery you'd still comfortably be looking at over a day of usage.

If you had it updating every two seconds it should enter deep sleep and you'll be looking at well over a week I think.

Only other thing to say is that while Espruino uses its own RTC, it uses a built-in RC oscillator which isn't very accurate. There are some pads near the chip where you can solder on a wired 32kHz watch crystal though - and I've got some JavaScript code which will enable this crystal - but sadly I haven't managed to get it to switch the RTC over to use it yet! If someone could have a play and get that working it'd be awesome though :)