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"noisy" flashlamp

Discussion in 'Electronic Design' started by George B, Nov 15, 2003.

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  1. George B

    George B Guest

    I recently attached a phototransistor to an audio amplifier (simple 4
    stage transistor) after reading about observing ("listening") to
    fluctuations in the light levels in the sky with such a device. If the light
    is collected through a telescope one can "listen" to the light reflected off
    clouds and the horizon. My amplifier, although it didn't have a great enough
    gain to hear anything in the dark, made what sounded like white noise when a
    torch light was directed onto the phototransistor.
    Can anyone explain this considering that the filament is fed directly from a
    battery(supposedly purely DC)? Could the noise be arising from radio
    interference or could it be due to the nature of the electrochemical action
    within the battery?
    Before anyone suggests that something may have been amiss with my amplifier
    I can assure you it was correctly biased and was able to amplify the 50Hz
    mains hum from the lighting within the house quite nicely (the noise
    disappeared if the photodiode was covered or taken outside in the dark).
    Incandescent lights produce a soft hum, whilst fluorescent lights produce a
    very loud and harsh 50Hz beat.
    If I have time I might observe the waveform produced from the flashlight
    using an oscilloscope.
    I plan on getting either some higher gain transistors or building an IC
    based amp to get the required gain in order to listen to fluctuations in the
    light levels in the night sky.

  2. I know nothing about that torch, but how does the sky sound like?
    Do you have a website with some wav-files?
  3. I read in that George B <>
    No, you are effectively hearing the thermal noise of the hot filament.
  4. Fred Bloggs

    Fred Bloggs Guest

    The "torch" biased the photo-transistor on to some collector current Ic
    and there is shot noise directly proportional to sqrt(Ic) associated
    with that conduction. Your amplifier was amplifying that shot noise- it
    is a white broadband noise. With Ic=0, at no light, there is no shot noise.
  5. Bill Sloman

    Bill Sloman Guest

    How much torch-light was actually hitting the photo-transistor?

    In other words, what was the level of the photon-generated base
    current in the transistor?

    You may well have been hearing the shot noise on this current.

    The Art of Electronics gives an equation for shot noise in section
    7.11 (page 432 in my copy).

    I(noise)rms = (2.q.Idc.B)^0.5

    where q is the the charge on the electron 1.6E-19 coulomb
    Idc is the (base) current in Amps
    and B is the bandwidth in Hertz.

    Hope this helps.
  6. Tim Auton

    Tim Auton Guest

    If you really want to listen to light have a look here:

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