Ghost digits

[digibloke]

I've got a couple of old watches which have "ghost" times on the display when there isn't a battery in them. For example I have a epsa-optel segtronic which shows a faint 8:17 without juice. There's no leak to the lcd, it shows 8:17 even without the top polarizer present and with a battery it runs and displays great (the 8:17 is so faint that it hardly disrupts the display when the watch is running).

Does anyone know what causes this? (I very much doubt it can be cured -I'm just curious).

[rewolf]

Elelctrolytic decompostion by DC?
LCD are driven by AC. When DC voltage is applied to an LCD, the LC decomposes due to electrolysis. This is irrecoverable. I don't know however what the results of this process look like.
Maybe some day the oscillator stopped at 8:17

[digibloke]

I guess DC would do it but I can't think of any practical situation where this would lead to 8:17 being left burned into the display.

I'm wondering if it's perhaps been left in the frozen set mode at 8:17 (intended for time synchronization during setting) at some point and has stayed like that until the battery ran out. Perhaps if the same segments were on for a year with no change to the display then this would lead to the liquid crystal under those segments degrading?

Whenever I get my hands on a working epsa-optel display it all goes a bit "Sophies choice" as to which watch it goes onto...

[rewolf]

...but I can't think of any practical situation where this would lead to 8:17 being left burned into the display...

If the oscillator stops at 8:17 for some reason (e.g. weak battery, defective crystal), then you have this situation. The AC for the LCD is derived from the 32kHz oscillator. The whole circuit will simply stop in its current state. There is voltage applied to the active segments, but it doesn't alternate any more.

[digibloke]

Interesting. I've always been under the impression that LCD watches had pretty much instant shutdown once a battery stops supplying the required voltage (as opposed to LEDs which tend to get dimmer displays as the batteries run down). I guess it's feasible that a weak battery has caused my 8:17 after all (the oscilator circuit is fine so it wasn't a quartz failure).

I least I can be pretty sure as to what time it was when the display developed its problem.

[rewolf]

Interesting. I've always been under the impression that LCD watches had pretty much instant shutdown once a battery stops supplying the required voltage (as opposed to LEDs which tend to get dimmer displays as the batteries run down). I guess it's feasible that a weak battery has caused my 8:17 after all (the oscilator circuit is fine so it wasn't a quartz failure).

It depends on where the "weak point" of the circuit is. If it's the oscillator that fails first with decreasing voltage, things can happen as described.
It's the same thing that happens to LED watches when they "freeze" and show a single bright digit: no oscillation = no multiplexing.
However, it's just an idea - I haven't ever observed this behaviour on any LCD watch (but on other circuits with LCD) - but I haven't seen any "burnt-in" LCD segments either.

More thoughts on this:
Some LCD watches (at least older ones) use a charge pump circuit to generate a higher voltage (~5V) for the LCD, also driven by the oscillator. So if the oscillator stops, then the charge pump also stops and there is no LCD voltage, thus no harm can happen.
If the watch in question has one 1.5V battery, then it's likely to have such a booster circuit. A 3V-driven watch usually doesn't need one as 3V is enough for the LCD.
Just for "fun" you could try to slowly reduce the supply voltage until the watch fails and see whether the display freezes or goes blank.

I least I can be pretty sure as to what time it was when the display developed its problem.

Like in Hiroshima when all clocks stopped at the impact time because the bomb's EMP magnetized the movements.