project idea: portable password safe

Felix sucotronic at
Mon Sep 9 09:29:39 EDT 2013

> > AAA
> > batteries are cheap, environment friendly (more than button ones) and
> have
> > more than enough energy for months (or years).
> What I really hate about them is their acid. The sooner or later,
> it'll come out and foul up your circuit.

Well, I don't have so much experience in that field. But I have some
duracell cells in a bike light, that sometimes gets all the day outside,
and they haven't break for months :P

> > Screen: 10x2 screen with backligth would be more than enough (rotating
> long
> > text of course), so the device size would be kept small.
> Or, better: an OLED. They're dirt cheap these days. And there's even
> a fab in China that has figured out how to please us lazy Western
> developers, with data sheets, and even offering matching connectors:

Wow, they're sooooo cheap! I said the LCD one because it has built-in
character display, so it's easy (at least for me) to interact with it.

> Meanwhile, I did some thinking in the direction of a device without
> keyboard, without USB host, and without dongle bay, but still with
> the other features.

Also can be developed in the future a usb powered mcu with usb-host (again
for keyboad plug-in) and radio that will communicate with the device :P

> The voltage of the boost converter for the LCD depends on what the
> panel wants. E.g., I have my eyes set on the ER-OLED013-1 (1.3",
> 128x64), which has a built-in DC-DC converter, but apparently they
> found it too weak, so the external supply would have to be 12 V.
> If the internal converter is sufficient, 3.3 V would do, which could
> then also be used for the memory card. Not sure if the memory card
> would be happy with just the battery voltage.

According to this[1] it would work until 2.7V and if you plan to use 2032
it corresponds with 75%[2] of battery capacity consumed (being very
optimistic, the measures were made with low current discharge), what is not
so bad.

> The idea of using a rotary encoder for input kinda grew on me. I
> first thought of the one that's coplanar with PCB (EVQWKA), but
> the iPad-style encoder may actually be more convenient. It should
> also be more comfortable to "dial" an item than to hammer a "next"
> botton a dozen times.

Agree with that. I remember to see a curious design in TI documentation[3]
using tactile sensors.

> And this is what the critter could look like:
> It looks great!

[3]: page 14

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