Anyone tried to replace the CdS photosensor?

Chan Tran

Subscriber
Joined
May 10, 2006
Messages
6,821
Location
Sachse, TX
Format
35mm
Wikipedia said the is a ban in the EU. In the USA there are a lot of devices that use them as well as the cells for sale as components. The only thing is that nobody use the CdS cell to built a light meter any more.
 

Bill Burk

Subscriber
Joined
Feb 9, 2010
Messages
9,314
Format
4x5 Format
ic-racer said:
I have a few meter projects on-going. These involve using photodiodes and trying to drive galvanometers with Arduino MWM output and using tiny photovoltaics for driving galvanometers on their own.
Click to expand...

I’m still interested in an amplifying circuit for Weston’s. Can a buck converter raise the amperage to get a 2/3 stop higher reading from the needle? Even if it were a fixed circuit, resistor or mask could be used to bring down for calibration.
 

Bill Burk

Subscriber
Joined
Feb 9, 2010
Messages
9,314
Format
4x5 Format
koraks said:
A buck converter doesn't sound like an obvious candidate here. I'd suggest a simple opamp-based servo that's dimensioned to the desired range.
Click to expand...

I was hoping the galvanometer needle could be pushed higher than the current output of the weaker selenium cell normally pushes it to. I didn’t really want to use a battery inside the case.

But if it were a rechargeable battery on a circuit that charges anytime there’s enough light to peg the needle… that might be fun. Solar yard lights already have that kind of circuit.
 

qqphot

Subscriber
Joined
Jul 12, 2022
Messages
221
Location
San Francisco, CA, USA
Format
35mm RF
Yes, i was thinking a multi channel light sensor part so color response could be tuned, with a small stm32 or something with d2a output. Should be workable in terms of size for some handheld meters, I guess. I'd be tempted to do fancy stuff with user interface, which probably would be counterproductive. Opportunistic charging via an additional silicon solar cell behind the honeycomb lens is a cute idea too, but i'd worry about the long term health of the battery.
 

koraks

Moderator
Moderator
Joined
Nov 29, 2018
Messages
22,981
Location
Europe
Format
Multi Format
Bill Burk said:
I was hoping the galvanometer needle could be pushed higher than the current output of the weaker selenium cell normally pushes it to.
Click to expand...

Sure, that's what you'd use some kind of amplifier for. After all, the cell itself apparently doesn't push the needle far enough, so there has to be external power added to the system in order to make it go further.

Bill Burk said:
I didn’t really want to use a battery inside the case.
Click to expand...

A properly designed circuit would run for a looooong time on a single lithium coin cell. Why make things complicated? The alternative would be a fairly complex solution that would still come with the problem of rechargeable cell lifetime.

Bill Burk said:
Solar yard lights already have that kind of circuit.
Click to expand...

Yup. Those are really nice - the first year.
 

Bill Burk

Subscriber
Joined
Feb 9, 2010
Messages
9,314
Format
4x5 Format
I could go back to the idea of getting a fly-tying jig and wrapping additional windings around the coil.
 

qqphot

Subscriber
Joined
Jul 12, 2022
Messages
221
Location
San Francisco, CA, USA
Format
35mm RF
Alternately, check whether the meter was originally calibrated by masking off part of the cell, and remove the mask to increase output.

However - the idea of redrawing the scale has somewhat more merit, to a point, anyway, because the cell's degradation probably isn't linear. The few I've played with seem to plateau with increasing illumination so in full daylight they can't sense the difference between small changes at all.
 

Bill Burk

Subscriber
Joined
Feb 9, 2010
Messages
9,314
Format
4x5 Format
I’ve found one with a cat-eye mask and it’s about a third-stop. For that one it may work if I confirm it’s a third-stop low. But I am not sure now that I use the translucent screen touching the bubble glass of the cell. (Might be fully calibrated).

Redrawing the scale is an interesting idea! I have the graphic arts skills to be able to recalibrate the markings. And I can burn a silkscreen mask.

Don’t know if I would try to fix the figures, they would be hard to make as crisp. But I won’t be moving the marks very much. Maybe the numbers will still pretty much align. This definitely could work.
 

ic-racer

Member
Joined
Feb 25, 2007
Messages
16,548
Location
USA
Format
Multi Format
The battery issue I also thought could be solved by a permanent battery that can be recharged,but then thought just to use the solar charging cell to just drive the galvanometer.
 
You must log in or register to reply here.
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…