Where do I find a suitable LED display for a DIY LED watch?

[your_man_in_Hamburg]

Hi gang,

Does anyone here in the workbench forum know if, and where, one can buy suitable LED-displays for wristwatches? To be specific, I am looking for a 4 x 7-segment red LED display with a colon in the middle.

Since there are new retro-watches with this type of display on the market, I assume the displays themselves can be bought somewhere.

Yours

Johannes

[J Thomas]

[rewolf]

Since there are new retro-watches with this type of display on the market, I assume the displays themselves can be bought somewhere.

Rather not. I've been searching for years, but without success.
The chinese "modern" LED displays are usually made the way JThomas described. Besides that, these displays never look as nice and sharp as the original 70's ones.

If you don't insist on the colon, you can scrap an old LED calculator. Texas Instruments' are cheap, but it has to be an early one with single-digit display like SR 10, SR 11, SR-50 (SR-51 would also do, but considering that an SR-51 in mediocre condition sold for $227 last night this is no good idea). The later 'A'-types (SR-50A, SR-51A) are not suitable. But watch out - once you've started acquiring LED calculators you might get into collecting them

The lens and black frame in front of the LEDs are easily removable.

This is an SR-51, the SR-50 looks the same (sorry, the image is a bit blurred):

[bruce wegmann]

I have a local source for these individual LED digits. They're 70s-early 80s vintage, brand new [old stock], but not cheap [$6 each]. PM me if interested. Looked like only a dozen or so available, but they're there.

[rewolf]

I have a local source for these individual LED digits. They're 70s-early 80s vintage, brand new [old stock], but not cheap [$6 each]. PM me if interested. Looked like only a dozen or so available, but they're there.

You can get one of the mentioned calculaotrs for $10 shipped - and it has 10 or 14 digits.

[Old Tom]

Hi Johannes, I have some Hewlett Packard 4 & 5 digit calculator displays in 0.3" DIP packs (the reference for the 4 digit one is HP5082-7414). These are common cathode multiplexed displays with one DP per digit (right hand). Hopefully the photo explains more!



Displays with a colon in are very rare and I must admit the only ones I have seen were in 0.3" and 0.7", 4 digit clock displays, no small watch sizes. Most of the LED manufacturers used surface mounted LED dice and then (after wire bonding) covered them with clear resin or clip on magnifiers.

If you would like a couple of the HP displays to play with PM me your address and I'll post them to you (I'll try to find the bit of paper with the pinout on as well).

[your_man_in_Hamburg]

I must confess two things.

First of all, I am merely in the "dreaming about" phase of the project. I really should/must turn down all generous offers until my design sits bread-boarded on my desk. Only then can I start thinking about squeezing it into a watch case and start buying small components.

Secondly, I must say that I would have preferred new parts that can be replaced if grilled/crashed, or bought anew, if anyone would like my design.

Fortunately, I recently found an interesting alternative, filament displays: http://www.decadecounter.com/vta/tubepage.php?item=17&user=0
Roger Riehl once told me that the first (prototype?) Synchronars were fitted with such displays. So, this type of display would be just right for my project.

Yours

Johannes

[Old Tom]

Can't imagine anyone ever used a filament display in a wristwatch- they are just too large and, more importantly, draw far too much current. Even the best filament display needs 10mA per segment (most much more, after all they are just light bulbs) so displaying an 8 would require 70mA and evenj a four digit watch would be a real battery killer (12:00 would be 200mA, if my adding is OK at this time in the morning- no coffee yet!). However they do make nice desk clocks- you can filter the display to whatever colour you want to match your room.

[your_man_in_Hamburg]

Well, at least Roger Riehl claimed he had used such displays in some of the early Synchronar (prototypes?).

Yes, they are power hungry, so I would have to resort to extreme power-saving measures. Read-outs would just be flashed shortly for the user to see, a mere fraction of a second, regardless of how long he/she holds the button.

I recently checked with CAD how four of the smallest unit sold by Wamco, the KW 304S, would fit into a standard Synchronar shell (http://www.wamcoinc.com/displays/display_specs.htm). They fit, but just barely.

All in all, I think it is possible. A challenge, yes, but possible.

Yours

Johannes

[rewolf]

...Yes, they are power hungry, so I would have to resort to extreme power-saving measures. Read-outs would just be flashed shortly for the user to see, a mere fraction of a second...

I doubt that this is possible with a filament display. It has to heat up and this will take time. Futhermore: the lower the filament temperature, the lower the resistance, that is the higher the current flow (as with any filament lamp). No button cell battery can deliver 200mA. The short-circuit current of a brand new SR44 is 500mA at best - but with no voltage left. You'd need a supercap to buffer the amount of charge required to heat up the filaments. 200mA for 0.5s is 100mC charge, requiring a 200mF capacitor if you allow 0.5V voltage decay.

But it coud be worth a try anyway.
Do you have any thechnical data of the filament display in question?

[your_man_in_Hamburg]

Basic spec's can be found at the URL I provided in my previous post. Some additional info is provided by this page:http://www.wamcoinc.com/displays/display_general-information.htm

The KW-304S burns 12mA per filament (at 4V), which amounts to a wopping 264mA when you try to display "2358". Fortunately, this is not the case all the time, and only when the display has to be viewed in direct sunlight.

In addition to this power-management problem, I will also apply some (no doubt well known and existing) trick to "pump" the measly 0.4V delivered by the solar cell, to something that can be used for charging the batteries. That problem, however, is slightly off topic...

Yours

Johannes

[rewolf]

One of the "benefits" listed on the page you linked is "Low power"

Anyway, these filament displays are even more retro than 70's LED displays and for this alone they're worth a try .
However they ARE power-hungry, 3-4 times more than LEDs (twice the voltage, twice the current).

You hadn't mentioned the solar cell yet. Is only 1 single cell (0.4V) a must? That makes boosting the voltage a bit more difficult. 2 smaller cells in series would probably lead to better efficiency.
TI has a nice boost converter IC that starts at 0.5V and operates down to 0.3V: TPS61200.
AFAIK there is no other boost IC that can operate at this low voltage. Of course "bootstrapping" (supply the converter from its output) is also an option, but gets finally stuck when the secondary side discharges below startup threshold for some reason - that was the big flaw of the Bulova Thermatron.

BTW: The average number of active LED segments is 18.1 for a 24h watch that is read out evenly distributed over the whole day:
average 2.86 segments in the 10h digit, 4.83 for 1h, 4.5 for 10min, 4.9 for 1min, 1 for the colon. That would be 217mA for the filament display.

[your_man_in_Hamburg]

One of the "benefits" listed on the page you linked is "Low power"

Yes, I kinda reacted myself as I saw that, but this last resort replacement LED pulls even more Watts (unless I have misread the spec's): http://catalog.osram-os.com/catalogue/c ... owBookmark That is, the glow digits cannot be half bad, even if there are other LED:s that beat them

Anyway, these filament displays are even more retro than 70's LED displays and for this alone they're worth a try .

Definitely! I think they are worth some sacrifices.

You hadn't mentioned the solar cell yet. Is only 1 single cell (0.4V) a must?

Eh, no, I just haven't spent much time on that aspect of the design yet. My first (and only) effort to find a suitable solar cell ended when I found a 20 x 20 mm piece.

...was the big flaw of the Bulova Thermatron.

The Thermatron does not look all that good, but its working principle deserves respect.

BTW: The average number of active LED segments is 18.1 for a 24h watch ...

I must say I am slightly impressed that anyone came up with the idea of calculating these numbers.
Talking about maximum currents, it occurred to me that the about 0.2 A display current can be reduced considerably if I, for instance, light the digits (quickly) one after each other. The battery only have to deliver about 50mA then, but for a longer time span. I suppose the best way of finding out the limits of the design, is to go ahead and bread-board it quickly, and try things out.

Yours

Johannes

[your_man_in_Hamburg]

No, no, no! The LED does not burn more Watts than the incandecent display. In one of my older posts, my brain merely tried to save some time by comparing 1/4 A with 1 W. In reality, the two displays appear to about equally power hungry.

Sorry for the slip-up.

Johannes