How to add ~130 mV to output of solar cell in old camera exposure system

Thread Starter

patstar

Joined Jan 10, 2023
2
Hi internet,

First time posting here so apologies if there is somewhere that would be more suitable for this query.

In short, I have a solar cell connected to a moving coil (that moves a needle) and I want to add ~130 mV onto the whatever the solar cell is outputting. I.e. if the solar cell reads 200 mV, I want 330 mV across the moving coil. Adding a voltage source in series would achieve this right? I just don't quite know how I'd go about introducing this relatively low voltage.

This is part of an autoexposure system in an old camera that uses a solar cell to gauge the light level and set the aperture and shutter speed accordingly via moving needle. I want to change the range of film that I can use in this camera by tricking the camera into taking photos when it would normally be too dark. I have worked out that the solar cell voltage scales linearly with light "stops" and that a change in voltage of ~43 mV corresponds to a change of one stop. Overall I want to shift the exposure system by 3 stops, hence I want to add ~130 mV.

Ideally, I'd like to be able to hide any extra electronics away inside the camera - there is a small amount of space in the camera for something like a button cell battery or a DIP8 package/comapct circuit, etc.

Other misc. info.: the resistance of the moving coil is 7.2k ohms; the solar cell and the moving coil are the only two components in the circuit.

If anyone has any suggestions I would be very grateful! Let me know if there's any more info. I can add too.

Cheers :)
Pat
 

Ian0

Joined Aug 7, 2020
9,677
You might be able to do something as simple as this
A8D6A943-F69C-40BB-B980-4E781C78B39B.jpeg
using a 1.5V silver oxide cell.
As light levels increase, the amount added would decrease, but that would mean that the camera would work almost as normal at normal light levels.
The value of R would depend on the resistance of the meter.
 

Ya’akov

Joined Jan 27, 2019
9,078
Hi internet,

First time posting here so apologies if there is somewhere that would be more suitable for this query.

In short, I have a solar cell connected to a moving coil (that moves a needle) and I want to add ~130 mV onto the whatever the solar cell is outputting. I.e. if the solar cell reads 200 mV, I want 330 mV across the moving coil. Adding a voltage source in series would achieve this right? I just don't quite know how I'd go about introducing this relatively low voltage.

This is part of an autoexposure system in an old camera that uses a solar cell to gauge the light level and set the aperture and shutter speed accordingly via moving needle. I want to change the range of film that I can use in this camera by tricking the camera into taking photos when it would normally be too dark. I have worked out that the solar cell voltage scales linearly with light "stops" and that a change in voltage of ~43 mV corresponds to a change of one stop. Overall I want to shift the exposure system by 3 stops, hence I want to add ~130 mV.

Ideally, I'd like to be able to hide any extra electronics away inside the camera - there is a small amount of space in the camera for something like a button cell battery or a DIP8 package/comapct circuit, etc.

Other misc. info.: the resistance of the moving coil is 7.2k ohms; the solar cell and the moving coil are the only two components in the circuit.

If anyone has any suggestions I would be very grateful! Let me know if there's any more info. I can add too.

Cheers :)
Pat
Welcome to AAC.

Which make/model of camera is it?
 

sghioto

Joined Dec 31, 2017
5,381
Thank you so much! Out of curiosity, how do D1, R1, R2, and R3 set the 130 mV voltage? Looks like a voltage divider but I couldn’t make the maths work…
Correct, it's a voltage divider. D1 acts as a voltage reference of appx 550mv with R1 at 10K. R2 and R3 drop the voltage down to appx 130mv. These voltages were confirmed on my test setup.
However I like DickCappels suggestion of using a LM185 for D1. That will change the values for R1-R2 and use less current.

1673450892554.png
 
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