How much current does an LED take?

S

Sea Squid

Guest
I want to experiment the parallel port with eight LEDs tied to
a cut parallel port cable, then send instructions with Visual Basic
to create some patterns. Is there any danger to my laptop?

Thanks.
 
I found PP is unable to drive such LEDs, which needs 20mA, but what is the
converter chip I shall order?

Thanks





"Sea Squid" <Sea.Squid@hotmail.com> wrote in message
news:423928c3@news.starhub.net.sg...
I want to experiment the parallel port with eight LEDs tied to
a cut parallel port cable, then send instructions with Visual Basic
to create some patterns. Is there any danger to my laptop?

Thanks.
Click to expand...
 
Sea Squid wrote:
I found PP is unable to drive such LEDs, which needs 20mA, but what is the
converter chip I shall order?

Thanks
Click to expand...
Look here:

http://www.logix4u.net/parallelport1.htm

There is a schematic for doing exactly what you want to do.

(comp.arch.fpga?)

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.
 
On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid wrote:

I found PP is unable to drive such LEDs, which needs 20mA, but what is the
converter chip I shall order?

It is? I thought PP used TTL outs. You should be able to drive an
Click to expand...
LED through a current limiting resistor. Is it backwards? Does the
printer still work? Can you light the LED with a battery or whatever
and a resistor? Are you sure VB and your OS will let you write the
port? What is the Vf of your particular LED?

Also, such a massive x-post really sucks for this question and if
you don't see this in the group you want, you're in the wrong group.
That's one down side of Hamster nntp server. It won't send to
unsubscribed groups and I'm not going to pull a bunch of headers
from groups I don't read.
--
Best Regards,
Mike
 
In article <423928c3@news.starhub.net.sg>,
Sea Squid <Sea.Squid@hotmail.com> wrote:
I want to experiment the parallel port with eight LEDs tied to
a cut parallel port cable, then send instructions with Visual Basic
to create some patterns. Is there any danger to my laptop?
Click to expand...
Yes, parallel ports are relatively easy to damage by shorting them
out, etc. I've done this a few times. :-/ Serial ports are usually
more goof-resistant, but of course they have fewer pins...

Re your other post, the LED will still light if you feed it less than
20 mA; it'll just be dimmer. Even 1 mA should still produce an easily
visible glow. What you need to do is insert a resistor in series
with each LED to limit the current to the amount that the parallel
port can supply.

LEDs (and diodes in general) have an exponential current/voltage relationship.
To a first approximation, this means that above a certain voltage,
they'll pass all the current you can throw at them (possibly overheating
and burning up in the process); below that voltage, they'll pass very
little current. (Including for negative voltages.) Another way of looking
at this is that, if more than a little current is flowing, the voltage across
the diode will be almost constant for that diode. This is the diode's
"forward voltage drop", Vf.

So let's say you have an LED and a resistor connected to your parallel
port. You want to size the resistor so that (for example) 1 mA is flowing.
The parallel port is supplying 5 volts. The forward voltage drop of
the LED is in the neighborhood of 1.5-2v. That leaves 3-3.5 volts across
the resistor. You know the voltage across the resistor, and you know the
current you want; using Ohm's law you can divide in order to find what
the resistance must be (in this case, about 3000 to 3500 ohms).

--
Wim Lewis <wiml@hhhh.org>, Seattle, WA, USA. PGP keyID 27F772C1
 
Wim Lewis wrote:
In article <423928c3@news.starhub.net.sg>,
Sea Squid <Sea.Squid@hotmail.com> wrote:

I want to experiment the parallel port with eight LEDs tied to
a cut parallel port cable, then send instructions with Visual Basic
to create some patterns. Is there any danger to my laptop?


Yes, parallel ports are relatively easy to damage by shorting them
out, etc. I've done this a few times. :-/ Serial ports are usually
more goof-resistant, but of course they have fewer pins...

Re your other post, the LED will still light if you feed it less than
20 mA; it'll just be dimmer. Even 1 mA should still produce an easily
visible glow. What you need to do is insert a resistor in series
with each LED to limit the current to the amount that the parallel
port can supply.

LEDs (and diodes in general) have an exponential current/voltage relationship.
To a first approximation, this means that above a certain voltage,
they'll pass all the current you can throw at them (possibly overheating
and burning up in the process); below that voltage, they'll pass very
little current. (Including for negative voltages.) Another way of looking
at this is that, if more than a little current is flowing, the voltage across
the diode will be almost constant for that diode. This is the diode's
"forward voltage drop", Vf.

So let's say you have an LED and a resistor connected to your parallel
port. You want to size the resistor so that (for example) 1 mA is flowing.
The parallel port is supplying 5 volts. The forward voltage drop of
the LED is in the neighborhood of 1.5-2v. That leaves 3-3.5 volts across
the resistor. You know the voltage across the resistor, and you know the
current you want; using Ohm's law you can divide in order to find what
the resistance must be (in this case, about 3000 to 3500 ohms).
Click to expand...
depends how shitty the LED is. I've just had an unfortunate experience
with some 0603 orange LEDs, that at 20mA were extremely dim, and no
detectable light at 1mA. cf some of the high-efficiency LEDs I use that
are really bright (calibrated to a traceable standard eh wot) at 3mA.

Cheers
Terry
 
"Active8" <reply2group@ndbbm.net> wrote in message
news:95w78i1wjhys$.dlg@ID-222894.news.individual.net...
On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid wrote:

I found PP is unable to drive such LEDs, which needs 20mA, but what is
the
converter chip I shall order?

It is? I thought PP used TTL outs. You should be able to drive an
LED through a current limiting resistor. Is it backwards? Does the
printer still work? Can you light the LED with a battery or whatever
and a resistor? Are you sure VB and your OS will let you write the
port? What is the Vf of your particular LED?
Click to expand...
It is the convention to cascade an LED with a resister. PP can only provide
approximately
1mA current. This current is insufficient to drive an LED, however I can't
find any hardware
in my hand now. VB can write to the parallel port via some dynamically
linked libraries,
which interfaces directly with the hardware. I have no information on the
Vf, since I have no
choice but the LED that I cut out from a spoilt cellphone.
 
"Sea Squid" <Sea.Squid@hotmail.com> wrote in message
news:423962ee$1@news.starhub.net.sg...
"Active8" <reply2group@ndbbm.net> wrote in message
news:95w78i1wjhys$.dlg@ID-222894.news.individual.net...
On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid wrote:

I found PP is unable to drive such LEDs, which needs 20mA, but what is
the
converter chip I shall order?

It is? I thought PP used TTL outs. You should be able to drive an
LED through a current limiting resistor. Is it backwards? Does the
printer still work? Can you light the LED with a battery or whatever
and a resistor? Are you sure VB and your OS will let you write the
port? What is the Vf of your particular LED?


It is the convention to cascade an LED with a resister. PP can only
provide
approximately
1mA current. This current is insufficient to drive an LED, however I can't
find any hardware
in my hand now. VB can write to the parallel port via some dynamically
linked libraries,
which interfaces directly with the hardware. I have no information on the
Vf, since I have no
choice but the LED that I cut out from a spoilt cellphone.




If ya only have 1ma then a buffer chip or discrete components and a power
Click to expand...
source will be the answer, mebbe the USB port ??

--
Regards ..... Rheilly Phoull
 
On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid top-posted:

I found PP is unable to drive such LEDs, which needs 20mA, but what is the
converter chip I shall order?

Thanks

ULN2803 - Eight darlingtons in a DIP
Click to expand...
http://www.st.com/stonline/books/ascii/docs/1536.htm

Of, course, you'll need a separate supply - there is no reliable +5V. Vcc
at the LPT port.

Good Luck!
Rich
"Sea Squid" <Sea.Squid@hotmail.com> wrote in message
news:423928c3@news.starhub.net.sg...
I want to experiment the parallel port with eight LEDs tied to a cut
parallel port cable, then send instructions with Visual Basic to create
some patterns. Is there any danger to my laptop?

Thanks.
Click to expand...
 
On Thu, 17 Mar 2005 03:06:34 -0500, Active8 wrote:

On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid wrote:

I found PP is unable to drive such LEDs, which needs 20mA, but what is the
converter chip I shall order?

It is? I thought PP used TTL outs. You should be able to drive an
LED through a current limiting resistor. Is it backwards? Does the
printer still work? Can you light the LED with a battery or whatever
and a resistor? Are you sure VB and your OS will let you write the
port? What is the Vf of your particular LED?

Also, such a massive x-post really sucks for this question and if
you don't see this in the group you want, you're in the wrong group.
That's one down side of Hamster nntp server. It won't send to
unsubscribed groups and I'm not going to pull a bunch of headers
from groups I don't read.
Click to expand...
You're supposed to crosspost your answer - once - and set "followups-to"
to the group you are posting in. (from?) That way the OP will see the
answer in any group, but the rest of that particular thread will just go
to the FUT group(s).

Cheers!
Rich
 
Use a series resistor of at least 3.3K Ohm to keep the current under 1
milliAmp. Most LEDs will give out enough light at this current to be
visible.

Glenn.

Sea Squid wrote:
I want to experiment the parallel port with eight LEDs tied to
a cut parallel port cable, then send instructions with Visual Basic
to create some patterns. Is there any danger to my laptop?

Thanks.
Click to expand...
 
On Thu, 17 Mar 2005 19:05:36 +0800, Sea Squid wrote:

"Active8" <reply2group@ndbbm.net> wrote in message
news:95w78i1wjhys$.dlg@ID-222894.news.individual.net...
On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid wrote:

I found PP is unable to drive such LEDs, which needs 20mA, but what is
the
converter chip I shall order?

It is? I thought PP used TTL outs. You should be able to drive an
LED through a current limiting resistor. Is it backwards? Does the
printer still work? Can you light the LED with a battery or whatever
and a resistor? Are you sure VB and your OS will let you write the
port? What is the Vf of your particular LED?


It is the convention to cascade an LED with a resister. PP can only provide
approximately
1mA current.
Click to expand...
Who told you that? It's in disagreement with everything I've seen or
read.

--
Best Regards,
Mike
 
Robert Monsen wrote:

Sea Squid wrote:
I found PP is unable to drive such LEDs, which needs 20mA, but what is
the converter chip I shall order?

Thanks


Look here:

http://www.logix4u.net/parallelport1.htm

There is a schematic for doing exactly what you want to do.

(comp.arch.fpga?)
Click to expand...
Off the top of my head I would say a 500-1K Ohm resistor in series, and the
schematic here is using 1K.

If you try to connect directly without a resistor it will work but you could
harm your PC.

gtoomey
 
On Thu, 17 Mar 2005 14:57:23 +0800, "Sea Squid" <Sea.Squid@hotmail.com> wrote:

I want to experiment the parallel port with eight LEDs tied to
a cut parallel port cable, then send instructions with Visual Basic
to create some patterns. Is there any danger to my laptop?

Thanks.



Have a look at
Click to expand...
http://www.boondog.com

I also recommend Paul Bergsman's book
"Controlling Your World With Your PC"
ISBN 1-878707-15-9
 
On Thu, 17 Mar 2005 14:57:23 +0800, "Sea Squid" <Sea.Squid@hotmail.com> wrote:

I want to experiment the parallel port with eight LEDs tied to
a cut parallel port cable, then send instructions with Visual Basic
to create some patterns. Is there any danger to my laptop?

Thanks.
Click to expand...
I highly recommend a book by Paul Bergsman
"Controlling THe World With Your PC"
ISBN 1-878707-15-9

Also,have a look at
http://www.boondog.com
..
 
In article <423962ee$1@news.starhub.net.sg>, Sea Squid wrote:
"Active8" <reply2group@ndbbm.net> wrote in message
news:95w78i1wjhys$.dlg@ID-222894.news.individual.net...
On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid wrote:

I found PP is unable to drive such LEDs, which needs 20mA, but what is
the
converter chip I shall order?

It is? I thought PP used TTL outs. You should be able to drive an
LED through a current limiting resistor. Is it backwards? Does the
printer still work? Can you light the LED with a battery or whatever
and a resistor? Are you sure VB and your OS will let you write the
port? What is the Vf of your particular LED?


It is the convention to cascade an LED with a resister. PP can only provide
approximately
1mA current. This current is insufficient to drive an LED,
Click to expand...
I know LEDs that glow almost bright enough to put a spot in your eyes,
although these are high-brightness high-efficiency LEDs whose
efficiencies are favored by low currents.

I have in mind two green models by Nichia:

NSPG520S - a green one in a 5 mm package with a nominal beamwidth
("viewing angle") I believe 45 degrees - the widest really common viewing
angle in a 5 mm round-tip "bullet" non-diffused package. Should be almost
"nice-and-bright" at .125 mA, although I have experienced an
"unreliability" at currents this low that is mostly just a significant
variation in brightness from one unit or one lot to another when the
current is so low.

NSPGF50S - a small rectangular one with "viewing angle" around 110
degrees - very wide angle. Viewing angle is more like 140-150 degrees if
you allow brightness into directions so far off axis to be "valid" if
about 1/4 that of the on-axis brightness. I consider these "adequately
bright" at about .2-.25 mA.

In my experience, both of the above models as well as most other InGaN
"regular size" LEDs have brightness adequate to "downright bright" with
reasonable reliability at .5 mA. Even most "cheapest" InGaN LEDs at 1
(maybe as much as 2 in worse cases) mA are brighter than most LEDs of
vintage 1982 or older are at 20 mA.

Anyone wanna compare a 1976 vintage "prime grade" green LED at 20 mA to
a modern green one by Nichia (or a competitor such as ETG) with similar
viewing angle and being run at .5 mA?

As for small quantity purchasing: http://www.nichia.com, and look for
"order online"

- Don Klipstein (don@misty.com)
 
On Sun, 20 Mar 2005 07:29:19 +0000 (UTC), Don Klipstein wrote:

In article <423962ee$1@news.starhub.net.sg>, Sea Squid wrote:
snip
It is? I thought PP used TTL outs. You should be able to drive an
LED through a current limiting resistor. Is it backwards? Does the
printer still work? Can you light the LED with a battery or whatever
and a resistor? Are you sure VB and your OS will let you write the
port? What is the Vf of your particular LED?

snap - devil's scissors - Christopher Wagner, Faust

Anyone wanna compare a 1976 vintage "prime grade" green LED at 20 mA to
a modern green one by Nichia (or a competitor such as ETG) with similar
viewing angle and being run at .5 mA?
Click to expand...
No. I understand some of the high efficiency LEDs are great. I'm
still waiting to hear more about the OP's PP that can't handle more
than 1 ma. :) My troll meter's going haywire.
--
Best Regards,
Mike
 
NSPG520S - a green one in a 5 mm package with a nominal beamwidth
("viewing angle") I believe 45 degrees - the widest really common viewing
angle in a 5 mm round-tip "bullet" non-diffused package. Should be almost
"nice-and-bright" at .125 mA, although I have experienced an
"unreliability" at currents this low that is mostly just a significant
variation in brightness from one unit or one lot to another when the
current is so low.
Click to expand...

Thank you for the information, but graph titled "Forward current vs relative
luminisity"
showed the current scale is from 0 ~ 120mA, so maybe it still needs a 20mA
to get a resonable light.
Are you sure that 0.125mA can light it up?
 
On Thu, 17 Mar 2005 22:57:22 GMT, Rich Grise wrote:

On Thu, 17 Mar 2005 03:06:34 -0500, Active8 wrote:

On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid wrote:

I found PP is unable to drive such LEDs, which needs 20mA, but what is the
converter chip I shall order?

It is? I thought PP used TTL outs. You should be able to drive an
LED through a current limiting resistor. Is it backwards? Does the
printer still work? Can you light the LED with a battery or whatever
and a resistor? Are you sure VB and your OS will let you write the
port? What is the Vf of your particular LED?

Also, such a massive x-post really sucks for this question and if
you don't see this in the group you want, you're in the wrong group.
That's one down side of Hamster nntp server. It won't send to
unsubscribed groups and I'm not going to pull a bunch of headers
from groups I don't read.

You're supposed to crosspost your answer - once - and set "followups-to"
to the group you are posting in. (from?) That way the OP will see the
answer in any group, but the rest of that particular thread will just go
to the FUT group(s).
Click to expand...
Right. I tried that, but my Hamster isn't subscibed to those groups.
I found a fix after pouring over the docs again. You just select
those groups and disable news pulls - hopefuly before the next
scheduled pull.

I wonder how many readers check the followup header. Dialog gives me
a warning that a followup header has been set, but IIRC, I have to
remove the other groups. It also warns me about x-posting. How come
you don't set followup headers? I think I usually eliminate the
non-SED groups when I reply to you because I know you'll be around
to see the response.

I'd still like to bs with you sometime. I visited those 3 groups you
x-post to and the only one that wasn't full of crap was rec.puzzles.
I can imagine the responses we'd get. Is there a group for "I'm
bs'ing with Rich and the rest of you can f' off" ? :) Yeah, it's
called email :)

--
Best Regards,
Mike
 
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