18 Jun
2004
18 Jun
'04
1:28 p.m.
On Fri, Jun 18, 2004 at 09:59:16PM +0100, Tony Duell wrote:
That works fine for showing load on a single CPU. The circuit I saw that
emulated the BeBox double-bargraph is coming back to me now - *two* 4-bit
resistor-ladder D-As and a bargraph generator chip for each, tune to give
you some combination of 0-15 or 0-16 or 1-16 LEDs per channel (I haven't
seen a real BeBox up close, so I'm not sure how they look when they are
showing 'idle').
It's a harder circuit to make by hand than the simple 8 or 10-LED one
that's easy to find, but when you get it working, it _is_ more impressive.
Besides... for a simple load meter, there's no reason to be able to
individually address the dots - the LED bargraph chip is the right tool
for the job. What I can't remember is what the common part number is
(it's something like LM3xxx).
I don't
know the one he's talking about but the most popular one all over
the web (and the one that i've got lying around from my old mail server)
just has 10 LEDs driven directly from the port with a common resistor.
If that's how they did it, then it's a lot more reasonable. And I'd probably add a buffer chip (something like
an 'LS540 or 'LS541,
whichever allowed me use it as a current sinking stage (LED + resistor
returned to the +5V line). Most printer ports can supply the current
needed be LEDs, but I'd not want to risk it, if that port was part of a
difficult-to-find multi-IO chip on the motherboard. For PCs like mine,
where the printer ports are all built from TTL on add-on cards, and where
I have schematics anyway, I'd just connect the LEDs directly to the port.
I'd recommend a buffer chip, too - I tried adapting one of the bargraph
designs to run an analog panel meter, but only my desktop could source
enough current (TTL ISA parallel-port card)... my laptop couldn't budge
the needle.
I gotta say that an analog load meter is pretty cool.
-ethan
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Ethan Dicks, A-130-S Current South Pole Weather at 19-Jun-2004 01:20 Z
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