# Driving LEDs with a parallel port

D

#### dnrg

Jan 1, 1970
0
Please excuse my ignorance here, but if I use all available pins to
drive 8 LEDs, would I still need resistors for protection? Adding an
optocoupler for protection, whatever that is, is probably beyond my
present abilities to put circuits together.

I've seen some circuits driving 8 LEDs that use no resistors, and
other circuits using seemingly arbitrary resistor values. I think the
idea is to cluster and use 8 LEDs together rather than, say, two
superbright LEDs due to the small amount of amperage available per
pin. Apparently, one pin will only supply enough current to light one
LED dimly.

Also looking for an easy to understand tutortial on the web for
controlling circuits from the parallel port. Can I buy a spare
PCI-card parallel port so I don't fry my internal one? How much are
these going for these days if they're even available?

Thanks.

- Dana

L

#### Lord Garth

Jan 1, 1970
0
dnrg said:
Please excuse my ignorance here, but if I use all available pins to
drive 8 LEDs, would I still need resistors for protection? Adding an
optocoupler for protection, whatever that is, is probably beyond my
present abilities to put circuits together.

I've seen some circuits driving 8 LEDs that use no resistors, and
other circuits using seemingly arbitrary resistor values. I think the
idea is to cluster and use 8 LEDs together rather than, say, two
superbright LEDs due to the small amount of amperage available per
pin. Apparently, one pin will only supply enough current to light one
LED dimly.

Also looking for an easy to understand tutortial on the web for
controlling circuits from the parallel port. Can I buy a spare
PCI-card parallel port so I don't fry my internal one? How much are
these going for these days if they're even available?

Thanks.

- Dana

It is always a good idea to use a buffer between the output and the LED.
The buffer output should limit the LED current unless you want to
damage the lamps.

J

#### John Jardine

Jan 1, 1970
0
dnrg said:
Please excuse my ignorance here, but if I use all available pins to
drive 8 LEDs, would I still need resistors for protection? Adding an
optocoupler for protection, whatever that is, is probably beyond my
present abilities to put circuits together.

I've seen some circuits driving 8 LEDs that use no resistors, and
other circuits using seemingly arbitrary resistor values. I think the
idea is to cluster and use 8 LEDs together rather than, say, two
superbright LEDs due to the small amount of amperage available per
pin. Apparently, one pin will only supply enough current to light one
LED dimly.

Also looking for an easy to understand tutortial on the web for
controlling circuits from the parallel port. Can I buy a spare
PCI-card parallel port so I don't fry my internal one? How much are
these going for these days if they're even available?

Thanks.

- Dana

You get about 4V for a logic high out of the port. Leds need about 3V so add
a resistor that will drop a volt while limiting the current out of the port
to say 5ma. (the port naturally can't supply more than a few ma's anyway).
As you mention, the actual value is pretty arbitrary and something like
220ohm would be OK. More for port protection than anything else.

After a number of years use I managed to total my LPT port last month but
fortunately was able to buy 6 ex equipment IDE PCB's for the princely equiv
of 2$. These are for the older ISA bus and my old workshop PC. The PCI PCB's will be dearer but a spare is well worth having if you are serious about experimenting. regards john S #### Si Ballenger Jan 1, 1970 0 Please excuse my ignorance here, but if I use all available pins to drive 8 LEDs, would I still need resistors for protection? Adding an optocoupler for protection, whatever that is, is probably beyond my present abilities to put circuits together. You want to protect the parallel port on the MB. Getting an add on parallel card would be good protection. Not sure if the MB port could with stand a dead short on the pins with out damage. I've seen some circuits driving 8 LEDs that use no resistors, and other circuits using seemingly arbitrary resistor values. I think the idea is to cluster and use 8 LEDs together rather than, say, two superbright LEDs due to the small amount of amperage available per pin. Apparently, one pin will only supply enough current to light one LED dimly. Have the parallel port pins operate transistors that switch on/off an external power supply for the LEDs. Also looking for an easy to understand tutortial on the web for controlling circuits from the parallel port. Can I buy a spare PCI-card parallel port so I don't fry my internal one? How much are these going for these days if they're even available? I've got a web page (below) that has some simple ways to control the parallel port pins just using notepad. Note that NT/2K/XP do not allow direct user level access to the parallel port hardware. A program called userport can fix this. A google search for words like parallel port and relay, LED and such will provided many sites with info. The bottom link has a decent sounding parallel port card for$8.50. SIIG makes a nice dual parallel port card
for ~\$50.

http://www.geocities.com/zoomkat/ppswitcher.htm
http://www.compgeeks.com/details.asp?invtid=PCI2S1PIO-WB

R

#### Rich Grise

Jan 1, 1970
0
Please excuse my ignorance here, but if I use all available pins to
drive 8 LEDs, would I still need resistors for protection? Adding an
optocoupler for protection, whatever that is, is probably beyond my
present abilities to put circuits together.

I've seen some circuits driving 8 LEDs that use no resistors, and
other circuits using seemingly arbitrary resistor values. I think the
idea is to cluster and use 8 LEDs together rather than, say, two
superbright LEDs due to the small amount of amperage available per
pin. Apparently, one pin will only supply enough current to light one
LED dimly.

Also looking for an easy to understand tutortial on the web for
controlling circuits from the parallel port. Can I buy a spare
PCI-card parallel port so I don't fry my internal one? How much are
these going for these days if they're even available?

Thanks.

- Dana

I'd say it's a crapshoot if you want to hang an LED right on a
printer port pin - the schem. of the original shows an LS374
http://www.epanorama.net/documents/pc/parallelport_schematic.html
which can source 2.8 mA, and sink 24 or something. So you could
use them to drive the low side, but there's no 5V vcc coming
out the port. So the temptation would be to high-side drive them
and let the current fall where it may, but I wouldn't be
comfortable depending on a limitation of the totem-pole output
as my LED current limiter. What if somebody slapped in an HCT374?
And how long can an LS374 source this 2.8 mA at +2.4v? Per buffer.

If you can snag the PC's +5V somewhere, you could low-side drive
the LEDs, otherwise you'll need external power, in which case you
might as well build a buffer.

Good Luck!
Rich

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