If memory serves me well, photovoltaic cells produce more energy per Watt illumination with blue light than with red because blue photons are more energetic whereas photoconductive cells have a minimum energy threshold and will conduct current based on the Watts/sq. meter in any wavelength above the threshold.
Originally Posted by htmlguru4242
Been 20-odd years since I have designed anything around phtocells for "broadband light" - usually just delaing with a single wavelength.
By the way, I said coloUr sensitive, not "color"
Photoresistors are probably CdS,which has about the same sprectal sensitivity as the eye. This means it sees the green well and fades off in either direction towards red or blue. Si photo diodes see from the blue to the near IR. Si sensitivity peaks around 850 to 900 nm (where 400 nm is blue, 550nm is green,and deep red is 700 nm) and is all over at about 1050 nm. This high sensitivity to near IR is the reason for concern about light meters being sensitive to IR or even red. Most b/w films are very sensitive to blue, diminishing in the greens and gone by mid-red.
If you want to design a light meter, i.e. you have too much time on your hands or need more fun in your life, you could use either CdS or Si. If you use CdS, your sensitivity will better match film, but your meter will suffer from aging and temperature changes (if not careful) and slow response. If you use Si, your meter will be very linear and unaffected by temperature changes, but you will need to design a filter that enhances the blue response and cuts off the IR response.
In both cases, you will need to change the linear circuit response to logarithmic. This can be done with an electro-mechanical meter or a linear-to-log circuit. Choice here depends on whether you want an LED readout or just a needle on a scale. The latter is easier, but some prefer the LEDs for accuracy and durability. Durability is probably the bigger concern.
If you are building an analog meter, then calibration will be done with a series of screw adjustments to control gain and offset and perhaps flatness. These will tend to drift, especially if the meter is dropped.
Thanks for stepping in there Gravel to fill in where my memory wasn't too sharp.
Using a photoconductive cell (cadmium Sulfide) in a bridge opposite a logrythmic potentiometer and using the meter only to detect balance ("zero") should give a pretty good light meter. That's pretty much what the cheap meters do.
Um, Clamity, back when a US magazine, probably Popular Mechanix, published a design for a DIY meter. I know because one of my friends made one. I now have it, I'm not sure it works. If you want it and I can find it, its yours for postage. IIRC, there's a copy of the article with it. If I can find it. Send me a PM if you want it. And don't tell me you'd rather spend more than the cost of a new one to have one you made yourself.
if you still need it.
i have an obsession with collecting random information. i actually have three volumes of old radio shack transistor projects. Volum 2 from 1974 has a "high sensitivity solar cell light meter" if you like i can scan and post jpegs of it.
let me know-
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Yes, back in high school with parts from a local electronics supplier. I think it was a Cds or the like. It was in an electronics class. We made electrical meters also.
I took woodworking, metal working, leather working, small engine repair, electricity, plastic making, all at the Junior high I went to. Then in High School I had more shop classes.
Most of the shop classes are gone now because "Johnny" might have an accident on the shaper or band saw or burn his finger on a soldering iron.
Instead of making something for mom or dad they learn the theory of how a tree grows.
I know, when the local school districts dumped shop equipment I acquired some nice Delta and other old but beautiful equipment that a shipyard would have loved. I am teaching my son how to use the equipment.
Not wonder we have to buy all of the crap from China through Walmart.
We had one foundry in our city that produce grates and manhole covers. The foundry workers, highly skilled, lost their jobs because we couldn't have that dirt place around. There are people who loved to do the work and liked the pay check too. Now we get the crap from slave labor in China.
I am glad that someone would even want to build a light meter.
Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand
If one was building a light meter themselves it might then be worth seeing if they could get some pieces of narrowband filter glass (or just a piece of your favourite filter) and get multiple readings out of it at the same time. Or add a switch and some duplication and you can get whole scene, spot and center-weighted metering. That is, if its fun and you want more than just a light level reading or want something special.
or, of course you can make a matrix meter with cmos sensor colour metering... digikey carries all you might need - or possibly more than enough to damn your soul :-P
I missed this the first time around but for those interested in more advanced designs I built my colour darkroom analyser around a TAOS IC and a Microchip PIC. In particular I used the TCS230 which is an RGB light to frequency converter, however they have other alternatives depending on your usage. The only nuisance is that to be accurate it needs a filter but for what I used it for it wasn't needed.
I would love to see those plans. Thanks so much.
Originally Posted by synthetase
Thanks for the great link. I didn't even know that was out there.
Originally Posted by ras351