How much current does an LED take?

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.

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Best Regards,
Mike

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 , Seattle, WA, USA. PGP keyID 27F772C1

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

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

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

the

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.

provide

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

Who told you that? It's in disagreement with everything I've seen or read.

--
Best Regards,
Mike

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

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:

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

Have a look at

I also recommend Paul Bergsman's book "Controlling Your World With Your PC" ISBN 1-878707-15-9

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

Also,have a look at

.

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:

and look for "order online"

- Don Klipstein ( snipped-for-privacy@misty.com)

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?