| Author |
Message |
Sea Squid
Guest
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 Posted:
Thu Mar 17, 2005 12:57 pm Post subject:
How much current does an LED take? |
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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.
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Wim Lewis
Guest
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| Posted:
Thu Mar 17, 2005 12:57 pm Post subject:
Re: How much current does an LED take? |
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In article <423928c3@news.starhub.net.sg>,
Sea Squid <Sea.Squid@hotmail.com> wrote:
| Quote: | 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).
--
Wim Lewis <wiml@hhhh.org>, Seattle, WA, USA. PGP keyID 27F772C1 |
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Sea Squid
Guest
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| Posted:
Thu Mar 17, 2005 1:00 pm Post subject:
Re: How much current does an LED take? |
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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...
| Quote: | 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.
|
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| Back to top |
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Robert Monsen
Guest
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| Posted:
Thu Mar 17, 2005 2:01 pm Post subject:
Re: How much current does an LED take? |
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Sea Squid wrote:
| Quote: | 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?)
--
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. |
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Active8
Guest
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| Posted:
Thu Mar 17, 2005 2:06 pm Post subject:
Re: How much current does an LED take? |
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On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid wrote:
| Quote: | 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.
--
Best Regards,
Mike |
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Terry Given
Guest
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| Posted:
Thu Mar 17, 2005 2:36 pm Post subject:
Re: How much current does an LED take? |
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Wim Lewis wrote:
| Quote: | 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).
|
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 |
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Sea Squid
Guest
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| Posted:
Thu Mar 17, 2005 5:05 pm Post subject:
Re: How much current does an LED take? |
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|
"Active8" <reply2group@ndbbm.net> wrote in message
news:95w78i1wjhys$.dlg@ID-222894.news.individual.net...
| Quote: | 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. |
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Rheilly Phoull
Guest
|
| Posted:
Thu Mar 17, 2005 6:30 pm Post subject:
Re: How much current does an LED take? |
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|
"Sea Squid" <Sea.Squid@hotmail.com> wrote in message
news:423962ee$1@news.starhub.net.sg...
| Quote: |
"Active8" <reply2group@ndbbm.net> wrote in message
news:95w78i1wjhys$.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 |
source will be the answer, mebbe the USB port ??
--
Regards ..... Rheilly Phoull |
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Glenn Baddeley
Guest
|
| Posted:
Fri Mar 18, 2005 1:54 am Post subject:
Re: How much current does an LED take? |
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|
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:
| Quote: | 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. |
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| Back to top |
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Rich Grise
Guest
|
| Posted:
Fri Mar 18, 2005 4:18 am Post subject:
Re: How much current does an LED take? |
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On Thu, 17 Mar 2005 15:00:59 +0800, Sea Squid top-posted:
| Quote: | 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 |
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
| Quote: |
"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.
|
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|
| Back to top |
|
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Rich Grise
Guest
|
| Posted:
Fri Mar 18, 2005 4:57 am Post subject:
Re: How much current does an LED take? |
|
|
On Thu, 17 Mar 2005 03:06:34 -0500, Active8 wrote:
| Quote: | 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).
Cheers!
Rich |
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Active8
Guest
|
| Posted:
Fri Mar 18, 2005 6:10 am Post subject:
Re: How much current does an LED take? |
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|
On Thu, 17 Mar 2005 22:57:22 GMT, Rich Grise wrote:
| Quote: | 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).
|
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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Active8
Guest
|
| Posted:
Fri Mar 18, 2005 6:10 am Post subject:
Re: How much current does an LED take? |
|
|
On Thu, 17 Mar 2005 19:05:36 +0800, Sea Squid wrote:
| Quote: | "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.
|
Who told you that? It's in disagreement with everything I've seen or
read.
--
Best Regards,
Mike |
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Gregory Toomey
Guest
|
| Posted:
Fri Mar 18, 2005 1:00 pm Post subject:
Re: How much current does an LED take? |
|
|
Robert Monsen wrote:
| Quote: | 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?)
|
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 |
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dmm
Guest
|
| Posted:
Fri Mar 18, 2005 2:12 pm Post subject:
Re: How much current does an LED take? |
|
|
On Thu, 17 Mar 2005 14:57:23 +0800, "Sea Squid" <Sea.Squid@hotmail.com> wrote:
| Quote: | 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 |
http://www.boondog.com
I also recommend Paul Bergsman's book
"Controlling Your World With Your PC"
ISBN 1-878707-15-9 |
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