Ed KF6DXX

Discussion in 'General Electronics Chat' started by kf6dxx, Aug 2, 2014.

  1. kf6dxx

    Thread Starter New Member

    Jul 23, 2008
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    Hello,
    I'm working on a visible light project using a small mirror (3x5-inches) to direct a beam of sunlight to a small telescope with a photo-diode mounted on the eyepiece. Current from the diode is then fed into a small amplified speaker.
    Currently I'm using a small DC fan at the mirror/transmitter end of a 650+ foot path to chop/modulate the mirror's light beam and generate an audible tone at roughly 600Hz.
    This produces nice loud pulses in the speaker but I would like to get rid of the fan as it blocks too much light. Without the fan I can use a larger mirror and get far greater range.
    I need suggestions on a better way (very simple, please) to modulate my photo-diode's ouput current of about 2-3volts. I've been working on this project for a long time and any help is greatly appreciated.
    Thanks, Ed KF6DXX!
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    Say more about your photo diode. Is it a solar cell or do you feed it voltage some how?
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Use a simple 555 timer circuit that can easily be set up to pulse an LED at 600 hz (or what ever frequency you want). Could you explain more about the project. It sounds pretty rudimentary right now, we might be able to help you fairly quickly with some circuit design ideas or off-the-shelf module ideas.

    Tell us what you are doing and why. Maybe a drawing or photo will help too
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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  5. vk6zgo

    Active Member

    Jul 21, 2012
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    Ed is using sunlight as the source he is modulating,so the easy forms of modulation are out.

    For simple Amplitude Modulation,this leaves a choice between a electromechanical shutter or mirror of some kind & some esoteric device which can change its light transmission capabilities with a modulating signal.

    An LCD could do this,but would it be transparent enough to let a reasonable amount of light through?

    Another possiblility is to change the light polarisation with modulation.
    Maybe a quartz crystal,or a mechanically variable polarisation filter?

    What happens if you drive a very thin mirror with the guts of a speaker?
     
  6. KJ6EAD

    Senior Member

    Apr 30, 2011
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  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    One option is to chop the signal going to or from the amp, perhaps by using a CD4066 or FET.
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    another way would be to vibrate the mirror. possibly glue a small mirror to a speaker cone and feed with 600 hz.route the reflection through a slot or hole to the telescope.
    also, did you know that led's detect light near their own color? actually output voltage like a photocell.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Have we lost the O.P.?

    I've been waiting to ask him why on earth he needs to modulate a beam of sunlight into a telescope into a photodiode. :eek:
     
  10. kf6dxx

    Thread Starter New Member

    Jul 23, 2008
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    THE_RB likes this.
  11. kf6dxx

    Thread Starter New Member

    Jul 23, 2008
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    Thanks for your response. Yes, I have put together quite a notebook on free space optical communications, which includes many of Clint's designs and experiments and I have contacted him a couple of times a few years ago.

    Clint KA7OEI, along with a few other guys, is in a class by himself for light comm's experimentation. Obviously a sharp guy working with a good crew!
    Ed KF6DXX
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Clint and I go back 30 years... I talked to him on 40m just last Sunday.
    wa7ark
     
  13. kf6dxx

    Thread Starter New Member

    Jul 23, 2008
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  14. kf6dxx

    Thread Starter New Member

    Jul 23, 2008
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    I've been looking at this site http://www.levitationfun.com/sunsound.html as a possible solution. But from everything I've read about transformers they will not work with DC current.
    This schematic shows what appears to be a transformer being fed from a solar cell or "battery". How can this circuit actually work?
    Thanks, Ed KF6DXX
     
  15. to3metalcan

    Member

    Jul 20, 2014
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    This circuit is an oscillator...The transistor is getting positive feedback through the transformer, causing it to switch on and off, creating an AC signal!
     
  16. kf6dxx

    Thread Starter New Member

    Jul 23, 2008
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    Hello, I think I've found a circuit that will work for my project. It's at this site http://www.technologystudent.com/elec1/opamp3.htm
    "Example Circuit-Light Activated Alerter"

    Can someone explain what pin#2 on the 741 op amp is connected to, is it connected to the 100K resistor R2? Or is pin #2 N/C (no connection)?

    I have draw literally hundreds of scematics but have never seen one drawn like this. Thanks for any help, Ed KF6DXX
     
  17. to3metalcan

    Member

    Jul 20, 2014
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    I think R2 is a 100k pot!

    Those are some "creative" resistor colors...
     
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