A friend of mine has an old 70-ies camera that uses mercury batteries to power the light metering. And as we all know, mercury batteries are bad for the enviroment, mmkay, and above all hard to find. So. What I'd like to do is to rebuild the camera to use standard lithium batteries, but I'd need some sort of voltage regulator to keep the light metering calibrated (the voltage level constant until the battery is drained). What I'm trying to get to with all this rambling is:
Do you have any suggestions where I could find such regulators?
I've gotten a suggestion to use a battery with more voltage than I need, and to use a resistor. Being an electronics newb, what would the main difference be? How would you guys tackle this?
First thing you need to do is sort out the camera. Traditional 'match needle' systems (put the needle in the center of the scale and shoot) don't depend on voltage value. A big change doesn't effect the exposure, only the number of f stops it takes to get from the top to the bottom of the meter scale.
Otherwise, I'd look to using the silver oxide or similar cell that fits the holder and fiddling the ASA number on the meter calibration dial. Putting a voltage regulator where you need to (and the technical constraints on it) are against you....
thanks for your reply. Yes, it's a match needle type meter. There's only one part that I don't understand. What do you mean, "fiddling the ASA number"? [img]/ubbthreads/images/graemlins/smile.gif[/img] Do you mean that I have to use a external light meter of some sort to calibrate the camera's own meter after changing to silver oxide cells?
Yup, that would be one way. You could also just fish it out:
Bracket the daylights out of a scene with good contrasts, say three or four f stops above and below 'box rating' with the new cell. Take them to the drug store..........
For match needle systems you're basically balancing a bridge. On one side is the resistor representing ASA (the setting on the side of the camera), on another one representing the shutter speed, on the third, f stop, and lastly the photo resistor itself. You shift shutter speed and f stop ratios to match the ASA and photo resistor. Voltage has nothing to do with it, as long as there's enough to cause enough current difference to move the meter (which is 'looking at the difference' between the two voltage dividers). This means you don't change anything except the cell (not a "battery" because there's only one *cell*, right?), to one you can fit. Polarity is important in that the + and - on the meter will be counter intuitive, otherwise not an issue with a match needle system.