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Thread: Old Selenium meter resurrection

  1. #1

    Old Selenium meter resurrection

    I just found an old Selenium meter that used to belong to my granfather. It was kind of semi-functional (the needle would move) but it gave wrong readings. So I decided to repair it and use it with my meterless cameras: like my pinhole, Flexaret, Zorki, Smena, etc.

    As I openned the meter there were only 3 important parts: Selenium plate, some resistor and a galvanometer. I removed everything but the galvanometer. In my electronics-junk-box I found a Silicone photo cell from a disassembled solar-powered calculator. I put the photo cell in place of the Selenium plate and added a 500 kohm variable resistor (for later calibration). I soldered it all together in series. The galvanometer needle was quite sensitive, so I fine-tuned it with the variable resistor. I set the resistance just high enough so the needle would reach its maximum at EV 15 (enough range for me).

    After I put the box back together I placed a sticker over the scale and at various EV in range about 5 to 15 marked the needle position. I also wrote the EV numbers to those marks and so the meter is usable again. No need for batteries, no Selenium getting old... But also poor low light sensitivity (starting at about EV 5), Anyway, a good way to turn non-working collector's item into a working exposure meter.

    (hopefully I'll post also some pictures)

  2. #2
    Administrator Tom Persinger's Avatar
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    Old Selenium meter resurrection

    very cool! thanks for posting. i hope you have some pics of the process, would like to see it.

    welcome to f295.
    tom

  3. #3

    Old Selenium meter resurrection

    The original selenium probably wasn't worth much in low light, either -- I've got a Gossen Sixtomat (selenium meter) that was probably made in the early 1960s, and it's only good down to about that level (though correctly, what you measure there is LV; EV is calculated from a combination of aperture and shutter speed, with the two having the same values at ISO 100).

    If you want to meter in low light, you need either a silicon photorestistor or a CdS cell, with suitable circuitry to convert resistance to a needle reading (and a very stable voltage source, which is why mercury cells were used before they were banned for environmental reasons).

  4. #4

    Old Selenium meter resurrection

    Thanks. I was also wondering if I could make some real exposure meter, sensitive also in low light. It need not be very acurate, say +/- 1EV would be fine. I need something small (the size of Voigtlander VC II) but for less money.

    I already have some ideas, but if anyone here has better ideas about such circuits it would be a big help.

    I posted a new thread with this question here: http://www.f295.org/DIYforum/cgi-bin.../m-1143527112/

  5. #5

    Old Selenium meter resurrection

    The simplest method is to use a CdS cell, galvanometer movement, and any old 1.5 V battery, plus a calibrating resistor, in a "bridge circuit" -- this circuit essentially just compares the resistance between the (fixed in use) calibrating resistor and that CdS cell, the latter of which is proportional to the light falling on the cell (more light, less resistance). The biggest advantage of a bridge circuit for a meter is that it's insensitive to supply voltage -- ordinary alkaline cells, which change voltage quite a bit from fresh to almost dead, aren't usable in many older meters, but they work fine in a bridge type meter; there's also no calibration change from mercury or zinc-air cells, to alkaline, to silver oxide, even though they vary by 30% in voltage.

    Your circuit would run from battery, split to the CdS and calibrating resistance (which is usually a fixed and variable resistor in series), through a pair of additional resistances (one on each leg), and then recombine to return to the battery; the meter movement is wired across the two legs between the first and second resistance on each. The diagram for this looks like two triangles base to base, the meter movement forming the shared bases, the CdS cell and adjusting resistance forming the other legs of one triangle, and two fixed resistances forming the remaining legs of the second triangle -- and then both apices connected to the battery.

    Of course, suitable component values are left as an exercise...

  6. #6

    Old Selenium meter resurrection

    Thanks.

    Though I know what a bridge circuit is I was not able to find a suitable analog V-meter. I think I'm gonna do some shopping for sensitive V-meters

  7. #7

    Old Selenium meter resurrection

    Pictures to the original thread post: Old Selenium meter resurrection

    Attached files

  8. #8

    Old Selenium meter resurrection

    This is what it looks like inside:
    • the red and yellow wire are from + of the silicone photocell to the galvanometer
    • the plastic rectangular thing is a multi-turn variable resistor
    • the black wire goea to a variable resristor
    • the green wire goes from variable resistor to the galvanometer
    • the orange and blue were meant to be a bypass of the variable resistor (there is a switch at the end) to increase sensitivity in lowlight, but it doesn't work at all.
    Attached files

  9. #9

    Old Selenium meter resurrection

    another view...

    Attached files

  10. #10

    Old Selenium meter resurrection

    ant this is from the front: you can see the calculator solar cell behind the honeycomb...

    Attached files

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