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  1. #11
    Joe VanCleave's Avatar
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    Conventional resistive light dimmers, built for incandescent bulbs, don't work on LEDs because the LED has a minimal bias voltage it needs to maintain in order to work, since it's a semiconductor device. Flourescent lamps have a similar problem with not being able to be dimmed using a resistive-type dimmer switch, since they too require a minimum voltage in order to ionize the gas into plasma.

    A better idea worth trying is a DC motor control module, the type used to vary the speed of a motor by chopping the duty cycle. For use with an LED, the dimmer would merely pulse the LED off and on, the duty cycle of which will determine average exposure.

    ~Joe

  2. #12
    BetterSense's Avatar
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    What you describe is pulse-width modulation. It's the most power-efficient way to do it, but I accomplish my LED safelight (and LED flashlight) dimming with a common 7805 voltage regulator IC, available at radioshack for a couple dollars. It can easily be wired up into a linear current source, with a simple pot to adjust the brightness.
    f/22 and be there.

  3. #13

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    Quote Originally Posted by BetterSense View Post
    What you describe is pulse-width modulation. It's the most power-efficient way to do it, but I accomplish my LED safelight (and LED flashlight) dimming with a common 7805 voltage regulator IC, available at radioshack for a couple dollars. It can easily be wired up into a linear current source, with a simple pot to adjust the brightness.
    I hope you are dimming the current and not the voltage. Varying the voltage and letting the current float is usually a bad idea and can cause your LEDs to lose life.

  4. #14
    BetterSense's Avatar
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    No, you can easily wire up a 7805 as a current source. I don't think I've ever used one as a voltage regulator, actually.
    f/22 and be there.

  5. #15
    CBG
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    Quote Originally Posted by Struan Gray View Post
    There are LED light panels sold for - don't laugh - discos and DJs which would make a nice light source for contact printing. They dim, can have variable colour, and will even pulse on and off in time to your darkroom music. There are UV ones which are (just) deep enough to do some alt process work.

    http://prolight.co.uk/item/ledj63/
    http://prolight.co.uk/item/ledj66/

    You can of course make your own using simple electronics breadboarding kits and packs of LEDs and limit resistors from eBay. This is a nice project (hint: it doesn't have to be UV):

    http://www.instructables.com/id/UV-LED-Exposure-Box/
    I need to learn more about how one calculates the correct circuitry for LEDs. Unfortunately I am electronics illiterate; the explanations I have read on the web make no more sense than Sanskrit to me so far. I need to keep reading till I find a way into the subject. There is so much basic theory and terminology I am missing. I have no idea, for example, what a bias voltage is, or what chopping the duty cycle by using a DC motor control module implies.

  6. #16
    CBG
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    Quote Originally Posted by jgjbowen View Post
    I have found a vacuum easel much easier to work with than a printing frame.
    That's a great idea!

  7. #17
    BetterSense's Avatar
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    When I said 7805, I meant to say LM317.

    f/22 and be there.

  8. #18
    paul_c5x4's Avatar
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    Quote Originally Posted by BetterSense View Post
    When I said 7805, I meant to say LM317.
    Even although your sketch shows an LM117 :o
    That aside, the circuit will work for any 3pin fixed voltage regulator including the 78xx & 79xx series.

  9. #19
    Steve Smith's Avatar
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    Rather than repeating the information, have a look at my thread about using the LED boards from an LED stage light: http://www.apug.org/forums/forum43/6...ht-source.html

    Steve.

  10. #20
    Struan Gray's Avatar
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    Quote Originally Posted by CBG View Post
    I need to learn more about how one calculates the correct circuitry for LEDs. Unfortunately I am electronics illiterate; the explanations I have read on the web make no more sense than Sanskrit to me so far. I need to keep reading till I find a way into the subject. There is so much basic theory and terminology I am missing. I have no idea, for example, what a bias voltage is, or what chopping the duty cycle by using a DC motor control module implies.
    I think that is why it is good that ready-made panels are now available. The ones I linked to include dimming, colour adjustment and a nice stable case, all for not much more than the cost of parts (in small quantities).

    Making your own can be fun - and with a basic electronics education not terribly hard - but getting a large array of LEDs to illuminate evenly and reliably takes a little care.

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