18 Jun
2004
18 Jun
'04
8:59 a.m.
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.
However, for bar (as opposed to dot) mode, I'd prefer one resistor per
LED (since the total current obviously increases as the number of LEDs
goes up).
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.
-tony