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

#### 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

#### sghioto

Joined Dec 31, 2017
3,940
This circuit will add the two voltages. Total current draw appx 1ma.

#### crutschow

Joined Mar 14, 2008
31,501
This circuit will add the two voltages.
Circuit looks good but I would use a voltage reference such as the 1.24V, TLV431 instead of the 1N4148 diode to minimize temperature variation of the added voltage.

#### sghioto

Joined Dec 31, 2017
3,940
Circuit looks good but I would use a voltage reference such as the 1.24V, TLV431
I figured someone would probably suggest that. I wasn't too concerned about the temp but more about current draw and component count.

#### patstar

Joined Jan 10, 2023
2
This circuit will add the two voltages. Total current draw appx 1ma.
View attachment 284952
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…

#### Ian0

Joined Aug 7, 2020
6,981
You might be able to do something as simple as this

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
7,017
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
3,940
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.

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