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Thread: Making a clip-on CdS exposure meter

  1. #1

    Making a clip-on CdS exposure meter

    Hi all,
    I need some help regarding making a CdS exposure meter. It needs to be fairly simple. Just to know the exposure +/- 1EV. I intend to use it with negative film and my meterless cameras such as Zorki. I think a small exposure meter the size of Voigtlander VC II but for less money would improve the usability of these great cameras.

    I was thinking about a CdS cell with resistance ranging from very small to about 50 kohm. Then a variable resistor (logarithmic or exponential) in series, which would work as a dial. The basic idea is to keep the sum resistance constant. When the R of CdS drops down, I would increase the R of resistor. The question is what to use as the indicator? I could not find any suitable voltmeter or ampermeter, since it needs to be analog, with sensitivity about 0~3 V.

    Another idea was to use another resistor parallel to the variable resistor and CdS. Then I would add a LED in each of the parallel paths. The idea is that both LEDs will shine eqally when the resistance of the CdS plus the variable resistor is the same as the resistance of the parallel resistor. The user just has to set the variable resistor to make the LEDs shine equally.

    Also another problem is the power source. The power supply needs to be eqal all the time... Maybe a stabilizer (stabilisation diode) ?

    Do you think it is usable? Any other ideas about the circuit? Anything would help...

  2. #2

    Making a clip-on CdS exposure meter

    I have an old McGraw Hill schematic book with a section on photo stuff.

    Many vintage parts are unavailable but circuit concept is there.

    Nothing wrong with an innovative approach but your approach seems a bit complicated to me.

    Let me see what I find. Main problem is calibrating to some reference. Maybe a 2nd meter for the first run.

    I saw a meter patent once that used a couple leds in series for 3 purposes.

    The LED voltage drop is very stable & was used as a regulated voltage; the rest of the circuit was powered between the top of the pair of LED's and ground. The second function was a battery life indicator and the 3rd was scale illumination for the analog meter. It used an op amp too. The first three points were patent claims. Those seem innovative to me.


  3. #3

    Making a clip-on CdS exposure meter

  4. #4

    Making a clip-on CdS exposure meter


    Ken Heflinger , nov 25, 2002; 02:27 p.m.
    Ah .. try both a used meter and making your own for grins.
    Actually .. what you might want to do is experment with making an incident meter. Spot meters are much more difficult.

    For grins try picking up a silicon photo cell, use a resistor to load it (that will make it pretty much linear). Then use a inexpensive digital meter to read the voltage from it. Calibarate it using the sunny 16 rule (f/16 shutter = 1/ISO in sunlight). And go from there. You should be able to get about 0.25 v in sunlight (any more than that and you are probably off the linear area .. load it more), and since even an inexpensive digital meter can read down to 0.001 V you should be able to go down to about EV 8 at ISO 100. So you will not get a lot of sensitivity using this method.

    As long as you keep a high load on the cell it will behave pretty much linear over a 7 or 8 stop range .. and thus calibaration is easy.

    But .. by all means check for a used light meter .. it will end up being much easier to use.


  5. #5

    Making a clip-on CdS exposure meter

  6. #6

    Making a clip-on CdS exposure meter

    Thank you for your replies.

    To make it clearer: I want to make a small, portable exposure meter, that would be small enough to slide into a camera's hot-shoe and the photographer would barely notice it's there. There exist many such meters - most of them are old Selenium meters from 50 years ago. I know of only one such device produced nowadays - the Voigtlander VC II meter. But it would cost me about 200 and that's just too much, so I'm looking for a cheaper way. I already own a self-built digital incident light meter, but I need a reflected light meter.

    BTW: if you're interested in the digital incident light meter, maybe you could look here: (it's in Czech, but you might be intersted in the pictures and circuits)

  7. #7

    Making a clip-on CdS exposure meter

    Oh, just answered this on the other meter thread.

    Simple version: you want a bridge meter circuit. You'll need two fixed resistors, fairly well matched, one variable that covers approximately the range of 0-10k ohms, and a suitable CdS cell, plus a small galvanometer (aka milliammeter) movement and a battery holder for a button cell or AAA. A case to mount it in, some means of controlling the acceptance angle for light striking the CdS cell, and you're ready to start. Bring your own solder and calibration equipment...

  8. #8

    Making a clip-on CdS exposure meter

    Did I get it right? I think you meant this:

    Attached files

  9. #9

    Making a clip-on CdS exposure meter

    That looks perfect, Spyderman, though you do have to match the polarity of the CdS with that of the battery (I think). The CdS packaging should have information on how the cells like to be hooked up, if you buy the cell from Radio Shack; otherwise, there are almost certainly various texts and documents available that will confirm that.

    What this essentially does is use the meter movement to measure the difference between the CdS and potentiometer; since only the *difference* affects the meter, it's independent of battery voltage, but you will see less deflection as the battery voltage drops off with age. This circuit or a very similar one is used in some cameras with "center needle" meters (like my Spotmatic). Put an aperture and diffuser in front of the cell and you can measure at various f-stops; center the needle with the aperture and a simple table will read out the LV. Add one or more neutral density layers for multiple ranges, or you could use a graded neutral like Polaroid does in their lighten-darken controls instead of an aperture. Or, you could use a switched step resistor -- but the simplest method is to turn the variable until the needle matches a previously calibrated mark, and then read the LV off a scale indicated by a mark or pointer on the potentiometer shaft.

    Note you have to do it this way, rather than reading the scale from the meter movement needle, because the needle will deflect less from the calibration point as battery voltage drops. If you want to read the needle like a selenium meter, you're back to needing a very stable voltage source, because you have to have the same deflection for a given change in light at all times.

  10. #10

    Making a clip-on CdS exposure meter

    thanks a lot. that really helped me. now I only needto get to work

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