Has anyone ever made their own light meter?

Discussion in 'Camera Building, Repairs & Modification' started by PhotoPete, Oct 17, 2005.

  1. PhotoPete

    PhotoPete Member

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    Thinking about this...looking for advice or plans...very low-light operation or accuracy down to the lumen is not necessary...just something that is more accurate than my eye for days when the sunny 16 rule is no help...
     
  2. Dave Parker

    Dave Parker Inactive

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    We did them back in highschool for our photography class, if I remember right we bought the parts from the local Radio Shack store, but now a days, I doubt they would even carry a light sensitive cell...they used to have the plans available in one of their little shop series books that they charged like a dollar for, you might give your local store a call.

    Dave
     
  3. jnanian

    jnanian Advertiser Advertiser

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    i used to be very good at casting a shadow from my right hand onto the back of my left hand and getting my exposure right on. once you do it a few times, and see the intensity of shadow, it is really very easy to figure out the exposure ... try it, its ez ...
     
  4. dphphoto

    dphphoto Member

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    Why not just go on eBay and get a cheap light meter? A Gossen Pilot would be a good choice. It would probably be more accurate than anything you could build yourself.
    Or...B&H Photo sells something called a Black Cat exposure guide. (I was thinking of getting one, then decided I didn't really need it.) It's only $20.
    Dean
     
  5. PhotoPete

    PhotoPete Member

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    Buy it? Where's the fun in that? :smile:
    I own a lovely Luna SBC, but I am looking for a little shoe-mounted number that I can slip onto the top of a Holga that I have modified to allow for different shutter speeds and aperatures. I could buy one, but I hate to buy a $100 meter for a $20 camera...

    I have the good fortune of living near You-Do-It Electronics...which is like what the Shack used to be...full of transistors, LEDs and CB radio parts. I'm sure they will have all the parts I need, if I can only determine which parts those are.
     
  6. Don Mills

    Don Mills Member

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    The Black Cat is a great device! Got a new one on Ebay for only $3.00.
     
  7. Calamity Jane

    Calamity Jane Member

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    The simplest form of light meter would be a photovoltaic cell and a small moving coil meter with a resistor in series. The problem is that the photgraphic scale is logrythmic and the meter is linear - i.e. it would only cover a narrow range. The other problem is that all photocells are "colour sensitive" and that would become an issue for other than B&W.

    Photoconductive cells are much less colour sensitive and much less linear. The problem is that you need a power source (battery) with a stable voltage OR you need to use a bridge circuit to remove the battery voltage variations from the measurement. All of the inexpensive light meters I have seen use a photoconductive cell in a bridge circuit. Of course the cost of a bottom-end light meter is less than that of a moving-coil meter! (The advantage of buying by the thousand!)
     
  8. BradS

    BradS Subscriber

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    I made one in Highschool. Bought the parts at Radio shack...must have cost all of $3.50 and an hour of my time to solder 'em up. Worked like a champ. I'll bet they still sell the CDS cells too.
     
  9. htmlguru4242

    htmlguru4242 Member

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    Good idea, I've been pondering this myelf recently, as well.

    By the way, what do you mean by saying that photovoltaics are "color sensitive"?
     
  10. dphphoto

    dphphoto Member

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  11. Calamity Jane

    Calamity Jane Member

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    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.

    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" :wink:
     
  12. Loose Gravel

    Loose Gravel Member

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    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.

    Good luck.
     
  13. Calamity Jane

    Calamity Jane Member

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    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.
     
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  15. Dan Fromm

    Dan Fromm Member

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    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.
     
  16. synthetase

    synthetase Member

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    hi photopete-
    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-
    lisa
     
  17. Curt

    Curt Subscriber

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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.
     
  18. sanderx1

    sanderx1 Member

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    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 :tongue:
     
  19. ras351

    ras351 Member

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    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.

    Roger.
     
  20. PhotoPete

    PhotoPete Member

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    I would love to see those plans. Thanks so much.
     
  21. PhotoPete

    PhotoPete Member

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    Thanks for the great link. I didn't even know that was out there.
     
  22. John Ossi

    John Ossi Member

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    Home-made light meter

    I would guess that it wouldn't be difficult to improvise an extinction light meter using neutral density filters.
    John Ossi
     
  23. ras351

    ras351 Member

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    I can't even recall myself how I came across them. :smile: They're not available retail in Australia but the distributor here was really helpful and sold me a few at wholesale prices for which I am extremely appreciative.

    They're a very easy chip to use - especially if you have a microcontroller but even then it's not necessary. Output is linearly proportional to light so, in the case of the TCS230, a decrease of half the output frequency correlates to a one stop increase in light. For a really simple light meter you could probably use one of the light-to-voltage chips and connect the output to either a small moving coil or lcd type panel meter. A few capacitors/resistors, a power supply and the calibration of a reference point and you're done. Not a lot different to the usual CdS type amplifier but it takes care of linearity issues.

    Roger.
     
  24. htmlguru4242

    htmlguru4242 Member

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    Those chips seem REALLY useful. Roger, what'd they charge you for those chips in small quantities?
     
  25. ras351

    ras351 Member

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    I paid AUD$5.33 (~US$4) per chip about 18 months ago and I bought five of them to distribute the small shipping cost (~AUD$3) although they would have sold me less. I was expecting them to be more expensive.
     
  26. Eric Mac

    Eric Mac Member

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    Why are screwing around with an external meter for your Holga. With a little modifications, you could put one inside your camera.

    http://www.textklick.demon.co.uk/leicattl.html

    Enjoy this site. This guy is my hero.

    Eric