Followups set to sci.electronics.components.
In sci.electronics.components DaveC said:
Would like to build a clock with very large display. I'd like 7 to 10-inch
high digits. The largest I can find is 5-inch. I've considered doing a
single LED array, but cost becomes an issue with large digits.
Maybe this is what you mean by a "single LED array", but you can get "bar
graph" arrays of rectangular LEDs that might do what you want. They will
probably make it a lot easier to get straight lines. Mouser (USA) sells
some Kingbright arrays that may work - DD12HWB has 12 rectangular LEDs with
their short sides together and DC20/20EWA has 20 rectangular LEDs with
their long sides together. Digi-Key (USA) has some Lumex 12-LED parts that
should also work. Here, I'm going to talk about the Kingbright parts. The
illuminated length of the 12-LED one is about 1.8" and of the 20-LED one
about 2.0". They aren't very bright; the 12-LED one is rated at 900
_micro_candelas minimum. and the 20-LED one is rated at 2200 _micro_candelas
minimum. Compare to a high-brightness single red LED of 2800
_milli_candelas or more. There is a higher-cost 20-LED version rated at
9000 _micro_candelas minimum. All of the Kingbright ratings are at 10
milliamps forward current. There is lots of information about LED
efficiencies (and LEDs in general) at Don Klipstein's excellent site:
http://members.misty.com/don/ledx.html .
You'd have to stack these up to get the size you want. The 12-LED one
might have a little bit of trouble with this, as there is about 3.4 mm of
"dead space" between the edge of the LED on the end and the housing. You
will end up with about a 1.5 LED-width gap if you stack two of these
displays end-to-end; this may or may not be important depending on the
proposed viewing distance of the clock. Also, this will yield a "line
width" of 1.5 mm, which might be too narrow for easy long-distance viewing.
The sketch of the 20-LED one also shows an unspecified gap between the edge
of the last LED and the housing, although it appears much smaller than on
the 12-LED one. This one will have a "line width" of about 5 mm, which
may be better for distant viewing.
With the 12-LED displays, four of them would give you about a 7.2" high
display (14 required per digit, cost per digit US$33) and six would yield
a 10.8" high display (21 required per digit, cost per digit US$49). With
the 20-LED displays, four would give you about an 8.0" high display (14
required per digit, cost per digit US$82) and six would yield about a
12.0" high display (21 required per digit, cost per digit US$124).
Probably you want to run a relatively high voltage so you can run the
LEDs of each segment in series. It will make the wiring a lot easier
in any case. Two 12-LED displays in series at 20 mA will drop 60 V max,
and three will drop 90 V max. Two 20-LED displays in series at 20 mA
will drop 100 V max, and three will drop 150 V max. Worst case, for
a 24-hour clock, you'll need to provide around 150 V at 0.52 A (about
80 watts).
I hope this helps!
Matt Roberds