Drive LEDs with a soundcard?

[dnrg]

Any simple circuits out there for driving two LEDs from pulses of
sound (separate left and right channels) coming out of a PC soundcard?
I'd like to do this using the parallel port but am afraid I'll fry the
port.


Thanks.

- Dana

 

[Lord Garth]

"dnrg" <dananrg@yahoo.com> wrote in message
news:c1888d06.0308031111.33ebbc62@posting.google.com...

Any simple circuits out there for driving two LEDs from pulses of
sound (separate left and right channels) coming out of a PC soundcard?
I'd like to do this using the parallel port but am afraid I'll fry the
port.


Thanks.

- Dana

Is there a particular voltage or frequency that activates the LED?

 

[Robert Monsen]

Are you talking about driving them using the power from the speakers? I am
not sure how an LED in series with the speakers will affect sound quality...


If you just want them to light up, there are lots of circuit schematics for
sound detectors using electret microphones. There has to be one for lighting
up an LED based on sound volume. For these, however, you'll need to use
external batteries or a power supply of some kind.

"dnrg" <dananrg@yahoo.com> wrote in message
news:c1888d06.0308031111.33ebbc62@posting.google.com...

Any simple circuits out there for driving two LEDs from pulses of
sound (separate left and right channels) coming out of a PC soundcard?
I'd like to do this using the parallel port but am afraid I'll fry the
port.


Thanks.

- Dana

 

[cpemma]

dnrg wrote:

"Robert Monsen" <postmaster@BulkingPro.com> wrote in message
news:<TYlXa.43736$It4.26253@rwcrnsc51.ops.asp.att.net>...
Are you talking about driving them using the power from the
speakers? I am not sure how an LED in series with the speakers will
affect sound quality...

Nope, just power from the 1/8" speaker jack. I don't have a multimeter
so I don't know what the voltage would be coming out, but it ain't
enough to light an LED. Apparently I'd need a transformer to step up
the voltage.

Anyone else have a clue precisely how to light LEDs with output from
my soundcard?

There's only about a volt max on the line-out, a led needs over 2v and over

5mA, so you need some modest amplification. The transformer route will put
too much load on the output.

 

[dnrg]

dananrg@yahoo.com (dnrg) wrote in message news:<c1888d06.0308031111.33ebbc62@posting.google.com>...

Any simple circuits out there for driving two LEDs from pulses of
sound (separate left and right channels) coming out of a PC soundcard?
I'd like to do this using the parallel port but am afraid I'll fry the
port.

Using a radio shack audio transformer works; I attached the 8 ohm side
to the output from a walkman (for a test) and the 1000 ohm side to an
LED. Problem is that the LED doesn't light brightly enough. It's
definitely responding to what it's being fed (at full volume), but how
do I get the LED to light brighter? Presumably there's not enough
amperage. I actually put up to 4 LEDs in series and they all lit at
the same brightness; I'm a bit confused here. Help!

Thanks.

- Dana

 

[Robert Monsen]

Both LEDs and speakers are driven by current; however, LEDs only work with
current going one way, and further have a minimum voltage requirement of
around 1.5V.

Running your sound card at full volume will cause it to put out more power,
which will increase the LED brightness. However, transformers don't increase
power, they just change voltage and current. Thus, since power is equal
(ignoring losses in the transformer) you have I(in) X V(in) = I(out) X
V(out). Therefore, increasing V with the transformer decreases current in
proportion.

If you are driving 4 LEDs with this, The voltage must be greater than 4 LED
drops, which is about 6V. Since the voltage gain of your transformer is
about 11, and you couldn't see the LEDs light before using it, I'm guessing
that your sound card signal is between .6V and 1.5V peak to peak. Normal
LEDs light up with about 1mA of current, so your 11 gain means your original
speaker current might be around 11mA. Thus, if you got a transformer with a
smaller turns ratio, (ie, 250 ohm to 8 ohm, for example,) the current would
be twice as big, but your voltage would be 1/2. This would mean you could
drive fewer LEDs, but you would drive them more brightly.

Note that the current through the LEDs will be the same if they are in
series, which is why they all light with the same intensity.

You could also try a rectifier diode ($1 at RS) to put more of the voltage
on the positive side... that will ensure that the current is travelling in
the right direction for more of the cycle, thus causing the diodes to light
up for twice as long (they are flashing now with the frequency of the sound,
and only lighting when the voltage is positive). However, there is a voltage
drop across these rectifier diodes of around 1.2V that you'll need to take
into account. Put the AC leads (the ones with a tilde marking) across the
speaker wire outputs, and connect the long lead of the LED to the + end of
the rectifier, and the short lead to the - side.

Disclaimer: I'm a hobbyist, not an EE. You are on your own if you follow
this or any other advice I give you...

Regards,
Bob Monsen

"dnrg" <dananrg@yahoo.com> wrote in message
news:c1888d06.0308181636.21841322@posting.google.com...

dananrg@yahoo.com (dnrg) wrote in message
news:<c1888d06.0308031111.33ebbc62@posting.google.com>...
Any simple circuits out there for driving two LEDs from pulses of
sound (separate left and right channels) coming out of a PC soundcard?
I'd like to do this using the parallel port but am afraid I'll fry the
port.

Using a radio shack audio transformer works; I attached the 8 ohm side
to the output from a walkman (for a test) and the 1000 ohm side to an
LED. Problem is that the LED doesn't light brightly enough. It's
definitely responding to what it's being fed (at full volume), but how
do I get the LED to light brighter? Presumably there's not enough
amperage. I actually put up to 4 LEDs in series and they all lit at
the same brightness; I'm a bit confused here. Help!

Thanks.

- Dana

 

[Robert Monsen]

If you use a bridge, you can control the output using the volume control. it
just allows you to get more of the current from the card through your LEDs.
Once the voltage drops below 1.5V, you won't have any output, so the volume
control can turn off the LEDs even with this case.

As to where to buy the transformer, try digikey or mouser. You could also
try the surplus shops like all electronics or goldmine. Take a look at

http://www.ee.nmt.edu/~wedeward/EE382/SP02/ee382_suppliers.html

for some ideas on where to buy parts.

Regards

"dnrg" <dananrg@yahoo.com> wrote in message
news:c1888d06.0308190945.512bd8ec@posting.google.com...

"Robert Monsen" <postmaster@BulkingPro.com> wrote in message
news:<MOj0b.150398
If you are driving 4 LEDs with this, The voltage must be greater than 4
LED
drops, which is about 6V. Since the voltage gain of your transformer is
about 11, and you couldn't see the LEDs light before using it, I'm
guessing
that your sound card signal is between .6V and 1.5V peak to peak. Normal
LEDs light up with about 1mA of current, so your 11 gain means your
original
speaker current might be around 11mA. Thus, if you got a transformer
with a
smaller turns ratio, (ie, 250 ohm to 8 ohm, for example,) the current
would
be twice as big, but your voltage would be 1/2. This would mean you
could
drive fewer LEDs, but you would drive them more brightly.

Thanks Bob.

It sounds like the transformer you suggest is the way to go. I
actually only need to light two LEDs (preferably a pair of those
"super bright" ones used in LED flashlights); I used 4 as a test to
guestimate how much voltage was coming from the transformer I have.
Can you recommend a good online / mail order source for 250 to 8 ohm
transformers? And would you suggest some sort of a pot for additional
control of brightness beyond the volume dial?

You could also try a rectifier diode ($1 at RS) to put more of the
voltage
on the positive side... that will ensure that the current is travelling
in
the right direction for more of the cycle, thus causing the diodes to
light
up for twice as long (they are flashing now with the frequency of the
sound,

That won't work for me because I need to be able to directly control
the flash duration with the audio output.

- Dana

 

[Dave Walker-Sharpe]

I would suggest using a powered circuit, maybe taking 5V from the
parallel port (if you're feeling brave!) and using a pair of
transistor drivers to light up your LEDs. This would be the best way
of doing it without overloading your sound card's output.

Dave

"Robert Monsen" <postmaster@BulkingPro.com> wrote in message news:<tkw0b.54259$2x.20652@rwcrnsc52.ops.asp.att.net>...

If you use a bridge, you can control the output using the volume control. it
just allows you to get more of the current from the card through your LEDs.
Once the voltage drops below 1.5V, you won't have any output, so the volume
control can turn off the LEDs even with this case.

As to where to buy the transformer, try digikey or mouser. You could also
try the surplus shops like all electronics or goldmine. Take a look at

http://www.ee.nmt.edu/~wedeward/EE382/SP02/ee382_suppliers.html

for some ideas on where to buy parts.

Regards

"dnrg" <dananrg@yahoo.com> wrote in message
news:c1888d06.0308190945.512bd8ec@posting.google.com...
"Robert Monsen" <postmaster@BulkingPro.com> wrote in message
news:<MOj0b.150398
If you are driving 4 LEDs with this, The voltage must be greater than 4
LED
drops, which is about 6V. Since the voltage gain of your transformer is
about 11, and you couldn't see the LEDs light before using it, I'm
guessing
that your sound card signal is between .6V and 1.5V peak to peak. Normal
LEDs light up with about 1mA of current, so your 11 gain means your
original
speaker current might be around 11mA. Thus, if you got a transformer
with a
smaller turns ratio, (ie, 250 ohm to 8 ohm, for example,) the current
would
be twice as big, but your voltage would be 1/2. This would mean you
could
drive fewer LEDs, but you would drive them more brightly.

Thanks Bob.

It sounds like the transformer you suggest is the way to go. I
actually only need to light two LEDs (preferably a pair of those
"super bright" ones used in LED flashlights); I used 4 as a test to
guestimate how much voltage was coming from the transformer I have.
Can you recommend a good online / mail order source for 250 to 8 ohm
transformers? And would you suggest some sort of a pot for additional
control of brightness beyond the volume dial?

You could also try a rectifier diode ($1 at RS) to put more of the
voltage
on the positive side... that will ensure that the current is travelling
in
the right direction for more of the cycle, thus causing the diodes to
light
up for twice as long (they are flashing now with the frequency of the
sound,

That won't work for me because I need to be able to directly control
the flash duration with the audio output.

- Dana