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  1. #11

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    Chan,
    That is already done courtesy of the properties of the Cadmium Sulphide.
    The surface resistivity reduces in an exponential way with an increase in the photon density. The photons raise electrons to the conduction bands and increase conductivity, and resistivity is the inverse of conductivity.
    The conventional exposure steps of shutter and aperture are also exponential.
    So we don't have to worry about that in the circuit which is why they originally were able to just use a passive linear circuit to drive the meter which is also linear.
    There is a circular slide rule on the lens barrel that allows conversion of the meter reading to shutter and aperture setting

  2. #12
    Bill Burk's Avatar
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    Quote Originally Posted by wombat2go View Post
    Tom, I remember pulling those old carbon zinc batteries apart as a kid, not sure if exactly that part number though.
    They consisted of rectangular cells stacked in series to make the "pile" and clamped by a wrap.
    I had a flash that required a similar, but shorter, 15 volt cell. You guessed it. Inside are 10 thin button cells. When I couldn't find 15 volt, I would buy a 22.5 volt and take out 5 of the cells and fold the case back around it...

  3. #13

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    Quote Originally Posted by Bill Burk View Post
    I had a flash that required a similar, but shorter, 15 volt cell. You guessed it. Inside are 10 thin button cells. When I couldn't find 15 volt, I would buy a 22.5 volt and take out 5 of the cells and fold the case back around it...
    I have some early radios from before the days of plug-it-in-the-wall. For a "B" battery of 135v, I went to Shadio Rack and purchased on sale 90 CZ "C" cells, which I soldered in series and tapped at 22.5v and 90v, to provide for all the high voltage needs of the various radios.
    The look I got from the clerk was absolutely priceless.

  4. #14
    Roger Cole's Avatar
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    It's funny to me to read a 9v battery referred to as a "smoke alarm battery" as I knew them as 9v "transistor batteries" for the then common pocket "transistor radio" long before I'd even heard of a home smoke alarm.

  5. #15

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    Quote Originally Posted by Roger Cole View Post
    It's funny to me to read a 9v battery referred to as a "smoke alarm battery" as I knew them as 9v "transistor batteries" for the then common pocket "transistor radio" long before I'd even heard of a home smoke alarm.
    I remember them as transistor radio batteries from when smoke alarms simply did not exist. They were developed and made specifically for transistor radios.

  6. #16

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    I have to amend, a little, my reply #11 to Chan
    The electrical circuit and meter are basically linear however I see that the meter scale is somewhat non-linear, probably to correct the transfer function of the CdS LDR to match EV.
    The EV scale of this meter ranges from 2 (iso 100 f/5.6 and 8 second) to 18 (iso 100, f/5.6 and 1/8000)
    As to whether, back in the day, they designed the scaling by trial and error , and whether the LDR CdS cells were specially made for exposure meters, I don't know.

  7. #17

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    Quote Originally Posted by wombat2go View Post
    I have to amend, a little, my reply #11 to Chan
    The electrical circuit and meter are basically linear however I see that the meter scale is somewhat non-linear, probably to correct the transfer function of the CdS LDR to match EV.
    The EV scale of this meter ranges from 2 (iso 100 f/5.6 and 8 second) to 18 (iso 100, f/5.6 and 1/8000)
    As to whether, back in the day, they designed the scaling by trial and error , and whether the LDR CdS cells were specially made for exposure meters, I don't know.
    I think you did a bang-up job on the project. Restoring any kind of exposure meter and achieving anywhere near original linearity is enough to make you pull your hair out. I've pulled it off a few times, but not without spending days at it. On many jobs you just have to pick on end of the dial or the other, or somewhere in between where your linearity situation "bunches up" on you and you just have to live with it. But when you think about it, I suppose a lot of them came out of the factory not accurate from the depths of a coal mine to the surface of the sun.

  8. #18

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    Tom, This is first time I have laid hands on an exposure meter. I will be trying it on weekend on some 120 Fuji 160 and 400.
    Like a new toy!
    I will report back, hopefully with good results!

  9. #19

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    Thanks a million for the great article. I got hold of a meter in good shape except for a sick photocell. I substituted a cell from a dead Gossen meter and followed your diagrams to build a circuit board. My cell has a different response slope so I had to change two of the fixed resistor values to get the calibration into range. After an hour of back and forth tuning the high and then low ranges it turned out to be amazingly accurate. It's within 1/2 stop of the readings from my Lunapro SBC reference meter over the top 4/5 of the scale on both ranges. I'm waiting for the weather to warm up so I can go out and play with it.

    Charles

  10. #20

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    Good job, Stan,
    Let us know how the photos exposed with it turn out.

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