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  1. #1
    olleorama's Avatar
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    Source for good quality photodiods?

    Hi, I'm planning to build a shutterspeed tester to test some of my older cameras. But to save me the trip to the electronic parts store I thought I could salvage some photodiods or phototransistors. Anybody know any electronic stuff that contain hi-quality photodiods? I read about cordless computer mice, but that those weren't so good. Anybody got more suggestions? ANy household appliances? I've got a full container full of electronic junk (computers, household stuff, some tools) in my apartment complex so I think there could very well be some lying around there.

  2. #2

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    Probably more effective & less stressful to just buy from Digikey or +. Certainly take a lot less time than searching through stuff.
    Heavily sedated for your protection.

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    John makes an excellent point.
    But there are bulb socket adaptors and similar gadgets with light sensors to turn on outdoor lights, and some of the little solar powered LED garden lights have photo cells to turn them off and on. Many of these are 10 $'s or so at home centers.
    However a photo cell from Digikey is probably less than the shipping cost with no experimentation needed to re-purpose it.

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    I've noticed that Frys has discrete components like that in one aisle. Have seen such since Radio Shack cut back on thier stock.
    Gary Beasley

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    olleorama's Avatar
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    Yeah, I know that it would be much cheaper. But still, I have some kind of idea not to buy new stuff all the time. I will probably do this time anyway, since I want it to be a high quality item.

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    Because I was planning to offer shutter tester DIY kits, I built a few prototypes myself, so I can speak from experience. One, you should use an infrared sensor on one side and an infrared diode on the other. Using random light sources for the light to illuminate the sensor yields inconsistent results. The good news is that many devices (like scanners, copiers) employ optical moment limit sensors, which are basically a photo diode and a LED, in most cases, infrared. Many of them can be separated in two by just cutting it, and many have mounting holes which then can be used to easily mount them on a stand. The more difficult part is what you use to measure the time, you really should be using a Schmitt trigger circuit, otherwise the measurement will be imprecise, especially for the high speeds.

    The other part to consider is that for lens shutters, the real exposure time is not easy to measure, as during the time the shutter takes to open and then to close the amount of light passing through varies and it adds to the light passing through when the shutter is open fully. The mechanical shutters were adjusted to take that into account, so it may seem like then run slightly slow if you just measure the time the center of the shutter stays open.
    Sorin

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    Quote Originally Posted by sv@diycamerakit.com View Post

    The other part to consider is that for lens shutters, the real exposure time is not easy to measure, as during the time the shutter takes to open and then to close the amount of light passing through varies and it adds to the light passing through when the shutter is open fully. The mechanical shutters were adjusted to take that into account, so it may seem like then run slightly slow if you just measure the time the center of the shutter stays open.
    If you use a computer with software that captures and displays the change in voltage against a time scale (audio software for instance), it's a relatively easy matter to determine the area under the curve (integrate, if you remember Calc 101) and compare it to the expected result. Although doing this reasonably quickly will yield a somewhat imprecise result, it'll certainly be within a 1/2 stop or less without too much work.

    Dan

  8. #8
    BetterSense's Avatar
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    But there are bulb socket adaptors and similar gadgets with light sensors to turn on outdoor lights, and some of the little solar powered LED garden lights have photo cells to turn them off and on. Many of these are 10 $'s or so at home centers.
    Those aren't the same thing, and they won't work. Most daylight-detecting circuits use CdS LDRs, which are far to slow.

    You can buy phototransistors at Frys and Radioshack. They will work almost anywhere a photodiode will.

    I have several hundred photodiodes in a big bag in my mad scientists' lab. If you PM me I'll send you some through the mail.

    I have 2 homemade shutter testers, an analog one that uses a phototransistor and a digital one that uses a photodiode and microcontroller. I use external sources of light for both. Any concerns about the unevenness of the shutter tripping can be taken care of by putting the measuring photodiode at approximately the film plane, which you should be doing anyway. Since the photodiode is approximately a point source, any concerns about the shape of the shadow cast by the shutter are taken care of. You are welcome to worry, I suppose, about the light pulse being not-exactly-digital due to lens effects from leaf shutters, but I'm sure not.

  9. #9
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    Quote Originally Posted by BetterSense View Post
    I have 2 homemade shutter testers, an analog one that uses a phototransistor and a digital one that uses a photodiode and microcontroller. I use external sources of light for both. Any concerns about the unevenness of the shutter tripping can be taken care of by putting the measuring photodiode at approximately the film plane, which you should be doing anyway. Since the photodiode is approximately a point source, any concerns about the shape of the shadow cast by the shutter are taken care of. You are welcome to worry, I suppose, about the light pulse being not-exactly-digital due to lens effects from leaf shutters, but I'm sure not.
    I did the same with mine. I have an IR photodiode mounted in a plastic plate and I place the camera with the back open onto this which places the diode at the film plane (or close enough). I use it on my kitchen worktop directly under a light fitting and I have found it works just as well with or without a lens fitted (35mm SLRs). It works fine with in shutter lenses too.

    Mine has a 7 digit (7 segment LED) counter and a 100kHz oscilator which is gated by the photodiode. It gives the shutter time in milliseconds to the nearest one hundreth of a millisecond.



    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  10. #10
    olleorama's Avatar
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    Quote Originally Posted by Steve Smith View Post
    I did the same with mine. I have an IR photodiode mounted in a plastic plate and I place the camera with the back open onto this which places the diode at the film plane (or close enough). I use it on my kitchen worktop directly under a light fitting and I have found it works just as well with or without a lens fitted (35mm SLRs). It works fine with in shutter lenses too.

    Mine has a 7 digit (7 segment LED) counter and a 100kHz oscilator which is gated by the photodiode. It gives the shutter time in milliseconds to the nearest one hundreth of a millisecond.



    Steve.
    Okay, can you explain the last part? Or do you have a wiring diagram? I'm not a electronic engineer, I'm more into phusics, so I thought I'd use a simple circuit which I found on the net and just plug it in to the soundboard and use MATLAB to analyze (or audicity, but I feel matlab could integrate better..), but your versions sound much more accurate. I have had some doubt which gives the better readings, photodiods or phototransistors. But now it seems as IR is the way to go? Why would that be more accurate than for instance an ordinary laser pointer?

    How would a circuit using a schmitt trigger look like? And how should I ground it? My original design was just a few diods and resistors in a battery box... As I said, I have only studied electronics on a high school level.

    And btw, since I live in europe I don't have a radio shack close to me, the nearest electronic parts shop is an hour away. But that wasn't the issue anyway, it just would seem like a good idea to recycle, even if it's just a tiny tiny component, I feel bad everytime I pass the electronic junk pallet, there is so much stuff that could be used, instead it's gonna sit on some deposit, poisoning the earth. Well, when the metal prices are high enough, those circuit boards are gonna be hard currency, at least that's a comfort. Maybe I should start saving old 486 motherboards in my shed?

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