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Sci.Electronics.Basics -> Varying a pen laser voltage with audio signal how? tia sal2
There are 16 messages in this thread.
You are currently looking at messages 1 to 16.
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Author: temp@temp.comDate: 14:04 25-03-07
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Varying a pen laser voltage with audio signal how?
Greetings All
I'm trying to vary the voltage of a pen laser by using audio frequencies
(homemade analog switch /potentiometer I guess). I've created a laser
that will transmit audio
http://www.scitoys.com/scitoys/scitoys/light/light.html#laser_communicato
r
but I'm not sure how to fine tune it. I would like to have the audio
control how much power the laser gets.
Example: if the audio doesn't play the laser doesn't get any power. The
thing is I'm not trying to use the audio signale as a digital switch
on/off. I'm trying to use the audio signal to vary the power the laser
gets or at least use the audio signal like a potentiometer / analog
switch. Is this possible?
Tia SAL2
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Author: Michael BlackDate: 14:17 25-03-07
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I'm limiting this to the newsgroup where it belongs.
"temp@temp.com" (not@not.com) writes:
> Varying a pen laser voltage with audio signal how?
>
> Greetings All
>
>
> I'm trying to vary the voltage of a pen laser by using audio frequencies
> (homemade analog switch /potentiometer I guess). I've created a laser
> that will transmit audio
> http://www.scitoys.com/scitoys/scitoys/light/light.html#laser_communicato
> r
> but I'm not sure how to fine tune it. I would like to have the audio
> control how much power the laser gets.
>
> Example: if the audio doesn't play the laser doesn't get any power. The
> thing is I'm not trying to use the audio signale as a digital switch
> on/off. I'm trying to use the audio signal to vary the power the laser
> gets or at least use the audio signal like a potentiometer / analog
> switch. Is this possible?
>
> Tia SAL2
Why do you find the need to actually amplitude the laser? Is it because
you actually have a need for this, or because you think it will make
it simple, or because you don't know better?
There is good reason to not amplitude modulate it. At the very least,
when you amplitude modulate it the receiver will be open to extraneous
noise (ie varying amplitude light from the sun and artificial lights), and
may also suffer the further away the receiver is.
Frequency modulation ensures that the signal at the receiver will always
be as strong as it can be. And, because the amplitude of the laser is
constant, the receiver can put a limiter after the photodetector, so
that sunlight and artificial light can't cause problems.
Going to frequency modulation of some sort also means that the linearity
of the laser diode does not come into play. I don't have a clue, but it
may not work well at low voltage levels compared to the regular voltage.
That may get in the way of amplitude modulation.
IN the end, good design means knowing what you are trying to do. We
don't know that, so a solution is really impossible. (Trying to
amplitude modulate a laser is not what you are trying to do; you
are doing that for some other reason, but you've not given
us that reason.)
Michael
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Author: Scott DorseyDate: 14:29 25-03-07
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temp@temp.com <not@not.com> wrote:
>Varying a pen laser voltage with audio signal how?
You can't, easily. It's not very linear. Look into a PWM circuit instead.
Adjusting the duty cycle that it's operating at is easy, and you can then
integrate in the receiver.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Author: Sjouke BurryDate: 15:05 25-03-07
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temp@temp.com wrote:
> Varying a pen laser voltage with audio signal how?
>
> Greetings All
>
>
> I'm trying to vary the voltage of a pen laser by using audio frequencies
> (homemade analog switch /potentiometer I guess). I've created a laser
> that will transmit audio
> http://www.scitoys.com/scitoys/scitoys/light/light.html#laser_communicato
> r
> but I'm not sure how to fine tune it. I would like to have the audio
> control how much power the laser gets.
>
> Example: if the audio doesn't play the laser doesn't get any power. The
> thing is I'm not trying to use the audio signale as a digital switch
> on/off. I'm trying to use the audio signal to vary the power the laser
> gets or at least use the audio signal like a potentiometer / analog
> switch. Is this possible?
>
> Tia SAL2
I would connect the laser to a simple audio power stage,
just before the output electrolite between speaker and
power stage.(Replace the speaker with two Leds antiparallel
and about 1K in series, to have some readout of ac power).
You have a nice dc offset there, and can simply change
the modulation current trough the resistor/laser combo.
If the amplifier has a dual supply, connect between the +
supply and output.
The resistor for the laser should be calculated to limit
the mean laser current.
Oh, and to save on damaged lasers, use a colored Led to
test your rig. Much less expensive....
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Author: John FieldsDate: 15:25 25-03-07
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On 25 Mar 2007 18:17:08 GMT, et472@FreeNet.Carleton.CA (Michael
Black) wrote:
>I'm limiting this to the newsgroup where it belongs.
>
>"temp@temp.com" (not@not.com) writes:
>> Varying a pen laser voltage with audio signal how?
>>
>> Greetings All
>>
>>
>> I'm trying to vary the voltage of a pen laser by using audio frequencies
>> (homemade analog switch /potentiometer I guess). I've created a laser
>> that will transmit audio
>> http://www.scitoys.com/scitoys/scitoys/light/light.html#laser_communicato
>> r
>> but I'm not sure how to fine tune it. I would like to have the audio
>> control how much power the laser gets.
>>
>> Example: if the audio doesn't play the laser doesn't get any power. The
>> thing is I'm not trying to use the audio signale as a digital switch
>> on/off. I'm trying to use the audio signal to vary the power the laser
>> gets or at least use the audio signal like a potentiometer / analog
>> switch. Is this possible?
>>
>> Tia SAL2
>
>Why do you find the need to actually amplitude the laser? Is it because
>you actually have a need for this, or because you think it will make
>it simple, or because you don't know better?
>
>There is good reason to not amplitude modulate it. At the very least,
>when you amplitude modulate it the receiver will be open to extraneous
>noise (ie varying amplitude light from the sun and artificial lights), and
>may also suffer the further away the receiver is.
>
>Frequency modulation ensures that the signal at the receiver will always
>be as strong as it can be. And, because the amplitude of the laser is
>constant, the receiver can put a limiter after the photodetector, so
>that sunlight and artificial light can't cause problems.
>
>Going to frequency modulation of some sort also means that the linearity
>of the laser diode does not come into play. I don't have a clue, but it
>may not work well at low voltage levels compared to the regular voltage.
>That may get in the way of amplitude modulation.
>
>IN the end, good design means knowing what you are trying to do. We
>don't know that, so a solution is really impossible. (Trying to
>amplitude modulate a laser is not what you are trying to do; you
>are doing that for some other reason, but you've not given
>us that reason.)
---
Quite a nasty little post from someone who obviously doesn't know
what he's talking about.
Other than physically moving the laser back and forth (toward and
away from the receiver) at an audio rate, just how would you propose
going about frequency modulating the laser?
--
JF
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Author: John FieldsDate: 16:50 25-03-07
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On Sun, 25 Mar 2007 18:04:51 GMT, "temp@temp.com" <not@not.com>
wrote:
>Varying a pen laser voltage with audio signal how?
>
>Greetings All
>
>
>I'm trying to vary the voltage of a pen laser by using audio frequencies
>(homemade analog switch /potentiometer I guess). I've created a laser
>that will transmit audio
>http://www.scitoys.com/scitoys/scitoys/light/light.html#laser_communicato
>r
>but I'm not sure how to fine tune it. I would like to have the audio
>control how much power the laser gets.
>
>Example: if the audio doesn't play the laser doesn't get any power. The
>thing is I'm not trying to use the audio signale as a digital switch
>on/off. I'm trying to use the audio signal to vary the power the laser
>gets or at least use the audio signal like a potentiometer / analog
>switch. Is this possible?
---
Yes, and that's what the article shows you how to do, with a
technique called "Amplitude Modulation", or "AM". What happens is
that with the laser diode connected in series with a battery and
the 1000 ohm secondary winding of the transformer and no signal on
the primary, the diode will shine with a certain brightness. Then,
when an AC signal is placed on the primary, that signal will appear
on the secondary and cause the current into the diode to increase
when the signal on the primary goes positive and decrease when the
signal on the primary goes negative. That way, with a very faint
signal on the primary there will be a very small change in the
current into, and the quiescent brightness out of the diode, but
with a loud signal there will be a large change in brightness.
The scheme that you're proposing, that of the laser being off when
there is no audio and being brightest on the most positive peaks of
the audio signal is called "double sideband suppressed carrier
amplitude modulation" and has the problem, in this instance, that if
zero output from the laser corresponds to zero volts from the
source, then the negative-going portions of the input signal won't
be able to cause the brightness of the laser to change.
There are ways to get around that, but for your application straight
AM should be fine.
--
JF
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Author: William SommerwerckDate: 16:59 25-03-07
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Dig through back copies of MAKE:. I think they had such a project.
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Author: jasenDate: 06:19 26-03-07
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On 2007-03-25, John Fields <jfields@austininstruments.com> wrote:
> Other than physically moving the laser back and forth (toward and
> away from the receiver) at an audio rate, just how would you propose
> going about frequency modulating the laser?
Attach a mirror to a loudspeaker cone, it has less mass and you get twice as
much modulation, (but this is really phase modulation)
Hmmm... it's possible that a mirror attached to a piezo behind
a half silvered mirror could do a pretty good job of amplitude
modulating a laser.
Move the back mirror half a wavelength and the
interferance goes from constructive to destructive.
Bye.
Jasen
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Author: John FieldsDate: 11:51 26-03-07
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On 26 Mar 2007 10:19:36 GMT, jasen <jasen@free.net.nz> wrote:
>On 2007-03-25, John Fields <jfields@austininstruments.com> wrote:
>
>> Other than physically moving the laser back and forth (toward and
>> away from the receiver) at an audio rate, just how would you propose
>> going about frequency modulating the laser?
>
>Attach a mirror to a loudspeaker cone, it has less mass and you get twice as
>much modulation, (but this is really phase modulation)
---
No, it's FM. What you get reflected is light of a slightly
different wavelength because of Doppler shift, so it's a change in
frequency/color.
Also, if the rate of change and the amplitude of the mechanical
motion is the same in both systems the carrier deviation will be
identical.
--
JF
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Author: ehsjrDate: 14:21 26-03-07
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John Fields wrote:
> On 26 Mar 2007 10:19:36 GMT, jasen <jasen@free.net.nz> wrote:
>
>
>>On 2007-03-25, John Fields <jfields@austininstruments.com> wrote:
>>
>>
>>>Other than physically moving the laser back and forth (toward and
>>>away from the receiver) at an audio rate, just how would you propose
>>>going about frequency modulating the laser?
>>
>>Attach a mirror to a loudspeaker cone, it has less mass and you get twice as
>>much modulation, (but this is really phase modulation)
>
>
> ---
> No, it's FM. What you get reflected is light of a slightly
> different wavelength because of Doppler shift, so it's a change in
> frequency/color.
>
> Also, if the rate of change and the amplitude of the mechanical
> motion is the same in both systems the carrier deviation will be
> identical.
>
>
He needs to use the new mono==>multichromatic PLM (pulse
light modulator) chip. Rumor is that it will be announced
April 1 by Discoherent Industries Inc.
Ed
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Author: jasenDate: 06:59 27-03-07
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On 2007-03-26, John Fields <jfields@austininstruments.com> wrote:
> On 26 Mar 2007 10:19:36 GMT, jasen <jasen@free.net.nz> wrote:
>
>>On 2007-03-25, John Fields <jfields@austininstruments.com> wrote:
>>
>>> Other than physically moving the laser back and forth (toward and
>>> away from the receiver) at an audio rate, just how would you propose
>>> going about frequency modulating the laser?
>>
>>Attach a mirror to a loudspeaker cone, it has less mass and you get twice as
>>much modulation, (but this is really phase modulation)
>
> No, it's FM. What you get reflected is light of a slightly
> different wavelength because of Doppler shift, so it's a change in
> frequency/color.
but you can't maintain the changed frequency indefinately...
Hmmm, I think I see what you're saying the velocity of the cone is
reasonably proportional to the coil current, so in that way it's FM.
> Also, if the rate of change and the amplitude of the mechanical
> motion is the same in both systems the carrier deviation will be
> identical.
The moving mirror shortens the path by twice the distance it moves.
I see twice the effect.
Bye.
Jasen
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Author: GrahamDate: 16:44 06-04-07
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> Yes, and that's what the article shows you how to do, with a
> technique called "Amplitude Modulation", or "AM". What happens
is
> that with the laser diode connected in series with a battery and
> the 1000 ohm secondary winding of the transformer and no signal on
> the primary, the diode will shine with a certain brightness. Then,
> when an AC signal is placed on the primary, that signal will appear
> on the secondary and cause the current into the diode to increase
> when the signal on the primary goes positive and decrease when the
> signal on the primary goes negative. That way, with a very faint
> signal on the primary there will be a very small change in the
> current into, and the quiescent brightness out of the diode, but
> with a loud signal there will be a large change in brightness.
>
> The scheme that you're proposing, that of the laser being off when
> there is no audio and being brightest on the most positive peaks of
> the audio signal is called "double sideband suppressed carrier
> amplitude modulation" and has the problem, in this instance, that if
> zero output from the laser corresponds to zero volts from the
> source, then the negative-going portions of the input signal won't
> be able to cause the brightness of the laser to change.
I think you are taking the RF analogy too far.
But as light and RF are both part of the EM spectrum it begs
the question at what wavelength do modulation products
cease to be relevant.
--
Graham.
%Profound_observation%
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Author: Bob MyersDate: 17:24 06-04-07
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"Graham" <me@privacy.com> wrote in message
news:ev6bcp$3h9$1@news.datemas.de...
> I think you are taking the RF analogy too far.
> But as light and RF are both part of the EM spectrum it begs
> the question at what wavelength do modulation products
> cease to be relevant.
No, he's exactly right. You can't come up (easily) with
a scheme in which the laser is off during periods of no
audio, because that IS a suppressed-carrier form of AM,
and has precisely the problems that John mentioned.
There HAS to be some carrier (the laser beam) there at
all times, and so the most you could hope for is that the
laser JUST extinguishes at the most negative-going (or
positive-going, if you like - it makes do difference here)
part of the modulating waveform - which would be
plain old full-carrier AM at exactly 100% modulation.
Since you don't want to run the risk of clipping, you
would normally set things up with a little margin, and
so have somewhat less than 100%. But with no
modulating signal present, the laser will be at its
"quiescent" brightness (well, the current through the
diode will be, at least, and we are somewhat naively
assuming in this discussion that the brightness will track
the current linearly over the range of interest - another
reason NOT to try to get close to the "extinguish"
point at the one extreme). It will then vary around that
point as a modulating signal is applied, hopefully not so
much that our assumption of linearity is too badly
broken.
By the way - yes, this does mean that the frequency
of the modulated beam "spreads out" about the
nominal frequency (which, of course, is a wavelength
or "color" change as well). Good luck trying to
measure it, though...:-) (Note that a change of 1 nm
wavelength here would correspond to a modulating
signal with a bandwidth of around 500 GHz...)
Bob M.
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Author: Rich GriseDate: 18:00 06-04-07
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On Fri, 06 Apr 2007 21:24:35 +0000, Bob Myers wrote:
>
> "Graham" <me@privacy.com> wrote in message
> news:ev6bcp$3h9$1@news.datemas.de...
>> I think you are taking the RF analogy too far.
>> But as light and RF are both part of the EM spectrum it begs
>> the question at what wavelength do modulation products
>> cease to be relevant.
>
> No, he's exactly right. You can't come up (easily) with
> a scheme in which the laser is off during periods of no
> audio, because that IS a suppressed-carrier form of AM,
> and has precisely the problems that John mentioned.
> There HAS to be some carrier (the laser beam) there at
> all times, and so the most you could hope for is that the
> laser JUST extinguishes at the most negative-going (or
> positive-going, if you like - it makes do difference here)
> part of the modulating waveform - which would be
> plain old full-carrier AM at exactly 100% modulation.
> Since you don't want to run the risk of clipping, you
> would normally set things up with a little margin, and
> so have somewhat less than 100%. But with no
> modulating signal present, the laser will be at its
> "quiescent" brightness (well, the current through the
> diode will be, at least, and we are somewhat naively
> assuming in this discussion that the brightness will track
> the current linearly over the range of interest - another
> reason NOT to try to get close to the "extinguish"
> point at the one extreme). It will then vary around that
> point as a modulating signal is applied, hopefully not so
> much that our assumption of linearity is too badly
> broken.
Yeah - this is way too much like the RF analogy - you're
not detecting the laser by demodulating a signal at some
terahertz or whatever, you're looking at the brightness
with a photodetector of some kind. Take a look at a movie
sound track sometime - when there's no sound in the movie,
it's solid black, and a varying width strip appears (like
a scope display of an envelope) and lets the light through
to the photocell. Or, some sound tracks are varying intensity
all of the way across.
Cheers!
Rich
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Author: Bob MyersDate: 21:26 06-04-07
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"Rich Grise" <rich@example.net> wrote in message
news:pan.2007.04.06.23.01.17.804135@example.net...
> Yeah - this is way too much like the RF analogy - you're
> not detecting the laser by demodulating a signal at some
> terahertz or whatever, you're looking at the brightness
> with a photodetector of some kind.
Not really much difference, though. The photodetector,
of course, can't really respond to terahertz signals in the
first place, but even if it could, it would be the low-frequency
"envelope" of the resulting signal that would be of interest
- in a manner very analogous to what you do with the
output of a simple diode detector in the RF case.
Take a look at a movie
> sound track sometime - when there's no sound in the movie,
> it's solid black, and a varying width strip appears (like
> a scope display of an envelope) and lets the light through
> to the photocell. Or, some sound tracks are varying intensity
> all of the way across.
Sure - but the original point was what would be
coming out of a "laser modulator" such as the one
originally described. It turns out in that sort of design
to be very simple to do it via what DOES turn out to
be ordinary full-carrier AM.
Bob M.
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Author: Rich GriseDate: 21:50 06-04-07
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On Sat, 07 Apr 2007 01:26:16 +0000, Bob Myers wrote:
> Sure - but the original point was what would be
> coming out of a "laser modulator" such as the one
> originally described. It turns out in that sort of design
> to be very simple to do it via what DOES turn out to
> be ordinary full-carrier AM.
OK, in all fairness, I hadn't been keeping up with the thread that
closely - My Bad! ;-)
Thanks!
Rich
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