Re: PC port question

Gareth wrote:

Randy Day wrote:
Before I plug this circuit into my PC, I'd like
to know if it's going to blow things up.

I'd like to use a 9v battery to power it,
rather than a 5v supply, since I'm using spare
parts for this. Can I use 9 volts and 1k
resistors, rather than the 5v and 330 ohms I've
seen in schematics? I know the current won't
blow the LEDs, but will the 9v cause problems
for the port circuitry? Are PC ports active
pull-up, or open-collector devices?

TIA

printer 1k LED
port ___
D0-|___|--|<--|
___ |
D1-|___|--|<--|
___ |
D2-|___|--|<--|-- +9v
___ |
D3-|___|--|<--|
___ |
D4-|___|--|<--|
___ |
etc.-|___|--|<--|

--- | |--
--- ---
- -
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Not sure if it will do any damage, but I'm sure it wont work (assuming
you want to switch the LEDs on and off?)

When the parallel port pin is low current will flow from the +9v
though
the LED and the LED will be on. That bit should be OK.

When the parallel port pin is high, it will be 3.3V (unless you have a
very old PC). You then have 9V - 3.3V across the LED and resistor, so
current will still flow and the LED will still be on, though not quite
as bright. You also have a problem that this current is going the
wrong way (into the parallel port when the pin is high). This may do
damage.

I know its academic, but where did you get 3.3 volts from ?

The parallel port uses TTL compatible signals < 0.7v - > 4.8v

PC parallel ports are NOT open collector.

Are you saying that you want to use 1k resistors just because you
don't
have 330 ohms? You could try the original schematic with 1k resistors
in series with the LED in place of 330 ohm. The LEDs wont be as
bright,
but probably still visible. Another thing you could try is
paralleling up the resistors - 3 X 1k in parallel would give 333 Ohms.

Gareth


--
Baron:
.

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