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30730Re: [midatlanticretro] Building a Better Tape Reader

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  • Dave McGuire
    May 15, 2013
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      On 05/15/2013 03:15 PM, Mike Loewen wrote:
      >>>> When I was playing with my Arduino micro controller I read articles
      >>>> about using led?s as light detectors.
      >>>> It got me thinking .....
      >>>> led bar graphs displays have .1 inch spacing to fit in DIP sockets .
      >>>> My idea is to use 2 bar graphs one as a light source and one one as the
      >>>> sensor.
      >>> Interesting idea.
      >>> I have seen leds used successfully on on solar trackers and having used
      >>> leds for sensing in component counters before you might find you need a
      >>> mask over the sensor array, with a line of holes at 0.1" spacing to stop
      >>> bleed (laterally) from one track to its neighbour. Also to stop bleed
      >>> from one row (longitudinally) to the next as individual segments in led
      >>> bar-graph displays are usually quite longer than they are wide.
      >>>> I did bread board one segment feeding an OP Amp with a 10K feedback
      >>>> resistor and got a
      >>>> 0 to 3.7 volt change with my LED flashlight through a sprocket hole
      >>>> sized hole.
      >>> The emitter array may need some tweaking on the current as well to get
      >>> usable signals.
      >>> Food for thought though, might just try it, I have some old PDP-7 tapes
      >>> I want to read.
      >> Forrest Mims popularized the use of LEDs in photodiode mode in the early
      >> 1970s, and has done quite a bit with them.
      >> But...with real photodiodes and phototransistors available for pennies, I'm
      >> note sure of what the point would be, other than for hack value.
      > How about using optical fiber to position the output from the emitters,
      > and on the other side to the phototransistors? You should be able able to
      > get the desired spacing that way.

      I think that's an excellent idea. One could mold them into a little
      "tub" of epoxy or similar, to make an optical read head.


      Dave McGuire, AK4HZ
      New Kensington, PA
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