Maker Pro
Maker Pro

Varying a pen laser voltage with audio signal how? tia sal2

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
 
S

Scott Dorsey

Jan 1, 1970
0
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
 
S

Sjouke Burry

Jan 1, 1970
0
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....
 
J

John Fields

Jan 1, 1970
0
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.
 
W

William Sommerwerck

Jan 1, 1970
0
Dig through back copies of MAKE:. I think they had such a project.
 
G

Graham

Jan 1, 1970
0
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.
 
B

Bob Myers

Jan 1, 1970
0
Graham said:
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.
 
R

Rich Grise

Jan 1, 1970
0
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
 
B

Bob Myers

Jan 1, 1970
0
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.
 
R

Rich Grise

Jan 1, 1970
0
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
 
Top