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I'd like to make my own binary watch, and something pretty small. Using surface mount LEDs would work but they use a lot of energy (~30mA a piece for something I'd consider reasonably bright) so there'd have to be a mechanism to "show the time".

Instead of this I'd like to only update my display once per time unit and leave it in a stable state in between - exactly like an e-ink display, thus massively reducing the power requirement.

I only have 6 binary display parts in my design so I really need individual e-ink cells, but I've no idea if this kind of thing even exists!

Does such a component exist? Is this the 'right' line of thinking? Thanks!

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    \$\begingroup\$ Did you try to contact eInk sales rep, to see if you could source eInk displays s.a. the ones used by Sieko watches. Here's the data-sheet. \$\endgroup\$
    – bdutta74
    Commented Sep 4, 2012 at 14:59
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    \$\begingroup\$ Not sure if "bunary" is a typo, but I think eInk's segmented EPD's like this one (5 for $50) may serve you purpose well, and they seem to be not very expensive. However, given that these are developer-samples, availability in larger numbers may be something to check. \$\endgroup\$
    – bdutta74
    Commented Sep 4, 2012 at 15:36
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    \$\begingroup\$ @icarus74 - That looks interesting enough to be an answer. I'll upvote if you post it. \$\endgroup\$
    – stevenvh
    Commented Sep 5, 2012 at 14:51

2 Answers 2

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Sadly, eInk is pretty difficult to make yourself. I know of nobody who has managed to make their own. LCD is also verging on impossible to home brew. Your only options are to look for an existing LCD that you can buy and use, or use LEDs.

You don't need 30mA for an LED. We have surface mount LEDs here that are annoyingly bright, even at 1mA! You could go down to 0.25mA and still have a perfectly visible display. These LEDs are HSMW-CL25 from Avago Technologies.

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    \$\begingroup\$ This has to be the best bet - the link quotes 5mA (but I'm assuming that's maximum current) but a 0.25mA LED would allow constant use of about 5 days on a 180mAh button battery, so with a micro tilt switch to detect when the watch is in "read" orientation would make this last for ages. Now to sort out the oscillator... \$\endgroup\$
    – JP.
    Commented Sep 4, 2012 at 21:39
  • \$\begingroup\$ Yeah, at 5mA those things are painfully bright. \$\endgroup\$
    – Rocketmagnet
    Commented Sep 4, 2012 at 22:10
  • \$\begingroup\$ How many LEDs would you be using @jphastings? \$\endgroup\$
    – boardbite
    Commented Sep 6, 2012 at 10:59
  • \$\begingroup\$ Six (4 hours, 2 hours, 1 hours, 1/2 hours, 1/4 hours, 1/8 hours - so that "6:30am" would be "HHL HLL", and 15:08 would be "HHH LLH") \$\endgroup\$
    – JP.
    Commented Sep 6, 2012 at 14:06
  • \$\begingroup\$ So worst case LED current consumption would be 6x0.25mA = 1.5mA. \$\endgroup\$
    – Rocketmagnet
    Commented Sep 6, 2012 at 14:08
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You could try eInk's segmented EPD range called SURF. Those are apparently used in Sieko watches, apart from bunch of other devices (such as smart batteries, to show charge levels). Development samples seem to be available for $50 (for a pack of 5), with three options 7-segment, 14-segment and a simple progress-bar.

Since these are developer-samples, availability in larger numbers may be something to check with the eInk sales rep. Especially so because some people have faced difficulties working with eInk, but situation might have changed now - YMMV.

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