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  1. #1
    BetterSense's Avatar
    Join Date
    Aug 2008
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    North Carolina
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    My auto-exposure pinhole camera

    I have an 8x10 pinhole camera that I made out of foamcore. It works very well and takes good pictures. The guess method of exposure can work pretty well, but I have an "If it ain't broke, fix it till it is" attitude sometimes.

    I rigged up an integrating light meter with some simple parts. It uses a CdS cell bought at radioshack in a box of "assorted cells". It's my understanding that CdS cells have a fairly peaked green-sensitivity. The CdS cell is hooked up through a pot into a capacitor, which it charges at a speed proportional to the light intensity. An analog comparator IC monitors the voltage across the cap and turns an LED on (soon to be a piezo buzzer) when it reaches the voltage set by a separate trim pot. At that point you have to manually close the shutter. I considered building an an automatic shutter, but I wanted the ability to override the meter.

    The integrating nature of the meter is what makes it so useful...if the sun goes behind clouds, or is in the process of setting, the meter will automatically take more exposure time to compensate. Some people might point out that charging a capacitor is not in the strictest sense a linear process, but if you keep the threshold voltage to below one time constant, it is linear for practical purposes. You can trip the meter in about 2 seconds by shining a flashlight on the sensor from the front. A bug is that the meter will eventually trip after like 18 hours in the complete dark because there is a finite dark current through the LDR. After setting the trim pot to give 20s exposure in bright sunlight, it has been pretty much spot-on in every situation since then, and I have learned to trust it entirely



    You can see, next to the proper pinhole, down in the corner is the separate pinhole with a shrink-tube sunshade to give a center-weighted reading. The CdS cell is glued internally.


    Inside view

  2. #2

    Join Date
    Sep 2007
    Location
    Texas
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    Med. Format Pan
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    356
    Can we see a schematic?
    * Just because your eyes are closed, doesn't mean the lights in the darkroom are off. *
    * When the film you put in the camera is worth more than the camera you put the film in... *
    * When I started using 8x10, it amazed me how many shots were close to the car. *

  3. #3

    Join Date
    Nov 2007
    Location
    Washington DC area
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    596
    very cool

    I once used a CdS cell to make a simple circuit to control an old shutter froma Konica SLR....the variable resistance from the CdS cell controlled how long a capacitor took to discharge....while the capacitor still had a charge it provided current to a transistor which controlled current to an electromagnet which triggered a curtain to close which ended the exposure

  4. #4

    Join Date
    Jan 2009
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    really cool. i would also like to see a schem if you don't mind.

  5. #5
    keithwms's Avatar
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    Oct 2006
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    Charlottesville, Virginia
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    Excellent!
    "Only dead fish follow the stream"

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