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

Discussion in 'General Electronics' started by [email protected], Mar 25, 2007.

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  1. Guest

    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
    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
  2. Scott Dorsey

    Scott Dorsey Guest

    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.
  3. Sjouke Burry

    Sjouke Burry Guest

    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....
  4. John Fields

    John Fields Guest

    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.
  5. Dig through back copies of MAKE:. I think they had such a project.
  6. Graham

    Graham Guest

    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.
  7. Bob Myers

    Bob Myers Guest

    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

    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.
  8. Rich Grise

    Rich Grise Guest

    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.

  9. Bob Myers

    Bob Myers Guest

    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
    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.
  10. Rich Grise

    Rich Grise Guest

    OK, in all fairness, I hadn't been keeping up with the thread that
    closely - My Bad! ;-)

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