Light meter battery modification

Phil_F_NM

Phil_F_NM

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Has anyone ever undertaken the task of adding a diode between the battery and the meter cell to reduce the voltage from silver oxide down to old mercury cell standards? I need to drop 4.5v down to 3.9v in this new 8mm camera of mine. I've thought about making a button cell cradle with the diode built in as a battery adapter but would rather take care of the whole problem from the start and just do the thing the right way.
I'm assuming (probably wrongfully so) that if I get the voltage to 3.9v I won't have to recalibrate the meter. If so, this project is going to take a lot longer than expected.

Yes, I know this is RFF but it is also the most friendly, active and most trusted (by me) forum I have found. There's a ton of knowledge on here.

Thanks all!
Phil Forrest
 
Yes, years ago I fitted a Schottky diode in the meter circuit of a Canonet. This used a single cell, 1.4v. As far as I remember, the meter worked fine afterwards - I can't remember if I recalibrated it.
 
Phil,

On my Leica SL2-MOT I had Sherry do the battery conversion. Originally the SL2-MOT used Mercury cells, but now I can use readily available alkalines.

Not sure if a diode was used to drop the voltage, could of been a resistor. In metering circuits it usually is a resistor bridge. One source of that bridge is the variable resistance provided via a a photoconductor which is the CdS cell part of the light meter.

The other part of the bridge uses the battery voltage as a reference with resistors on the "legs."

It is really a simple and basic circuit, but it is really about doing math. Generally there is a pot on one of the legs to trim adjustment. Sometimes if you are lucky there is enough adjustment to rebalance the bridge without having to change any resistor(s).

I'm not sure what was done, but a voltage drop could be performed via adding a resistor, or increasing the value.

Also inquire about what Quality Light Metric does in Holliwood Ca. They can convert my Gossen Luna Pro which uses 525 Mercury Cells to Alkylines. Pretty sure it involves dropping the voltage and rebalancing the bridge.

I believe the circuit you need to understand and study is called a Wein Bridge but this stuff I learned and forgot 40 years ago. I use to be great in math, but now I don't think I could factor an equasion, basically simple algebra. The circuit I'm talking about in a schematic is basiclly a square of 4 resistances with two pairs of connections.

Alternatively could be as simple as changing a resistor to create the same voltage as the old battery. The voltage drop could be figured out with Ohm's law if that is the case.

All the best. Hope I didn't give you a headache. LOL.

Cal
 
If your device has a bridge circuit you don't have to recalibrate anything.
The bridge circuit opreates the same no matter what the voltage. Believe it.
Look up Wheatstone bridge.
 
Spavinaw said:
If your device has a bridge circuit you don't have to recalibrate anything.
The bridge circuit opreates the same no matter what the voltage. Believe it.
Look up Wheatstone bridge.
Click to expand...

I don't know if it has a bridge circuit. I remember that the Pentax Spotmatic was designed with a bridge circuit so it had a meter that worked with batteries of all vintage.

The camera I'm getting rolling is a ~1968 Elmo C-300 Tri-Filmatic Super 8 motion picture camera. I've gotten this moving film bug and I want to start shooting both Super 8 and 16mm.

I've read that using the modern PX-25 4.5v battery gives a hotter reading in the Elmo than the proper voltage battery but I haven't tested this one yet.

Phil Forrest
 
I am using the MR-9 Battery adapter... I think it's still available... you can take a look at one and just copy the design... works perfectly in the Pentax and Gossen Luna Pro... traveling at the moment or I would attach a photo...
 
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