Anyone tried to replace the CdS photosensor?

[Chan Tran]

DigiKey lists about 70 CDS cells:

Yes the CdS cells are still very popular in many applications just not light meter. I didn't know there is a ban.

 

[Laurent]

Not sure if there is a ban, McHobby has some https://shop.mchobby.be/fr/electronique-divers/58-photo-resistance-3232100000582.html

I use this shop for other components, never tried this specific CdS cell

 

[Chan Tran]

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]

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.

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.

 

[koraks]

Can a buck converter raise the amperage to get a 2/3 stop higher reading from the needle?

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.

 

[Bill Burk]

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.

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]

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]

I was hoping the galvanometer needle could be pushed higher than the current output of the weaker selenium cell normally pushes it to.

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.

I didn’t really want to use a battery inside the case.

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.

Solar yard lights already have that kind of circuit.

Yup. Those are really nice - the first year.

 

[Bill Burk]

I could go back to the idea of getting a fly-tying jig and wrapping additional windings around the coil.

 

[Chan Tran]

I could go back to the idea of getting a fly-tying jig and wrapping additional windings around the coil.

Redraw the scale.

 

[qqphot]

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]

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]

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.