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Electronics Constant DC from varying DC (in a small package)

Discussion in 'Modding' started by Krikkit, 30 Dec 2008.

  1. Krikkit

    Krikkit All glory to the hypnotoad! Super Moderator

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    Hi guys, I've got a bit of a tricky problem here. (To my thin knowledge of electronics anyway!)

    I want to replace the bulb found in some scalextric cars with a single LED. (The bulb light goes through some perspex tubes to come out of the headlights etc)

    Can someone help me make something to provide a fairly constant DC voltage from the varying DC voltage that you get from the track?

    I measured the various outputs, and it varies from about 8V-16V fairly quickly, depending on the throttle position at the controller.

    Ideally I'd like something fairly simple that I can solder onto a single LED leg/small board to stick inside the car?

    There are some 3rd party circuits out there, but they're full-board type jobs which need fixing into the car, which I'd rather not do. Especially because they're ~£10 each.

    Upshot: I want a white LED to provide a near-constant brightness. :)

    Thanks in advance! :D
     
  2. capnPedro

    capnPedro Hacker. Maker. Engineer.

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  3. cpemma

    cpemma Ecky thump

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    That's the way to do it, not.

    What you want is a Constant Current Source, which is easy-peasy with a 317 and a resistor.

    For 20mA LED current the resistor value is 1.25/0.02 = 62 ohms

    Circuit is
    Code:
    8-16V ----------- 317 --RES----+--------+
                       |           |        |
                       |           |        V LED
                       +-----------+        -
                                            |
     0V ------------------------------------+
    Look out for the LM317LZ 100mA rating in a wee TO-92 package. Rapid have it, as do Maplin at only double the price.
     
    Last edited: 30 Dec 2008
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  4. capnPedro

    capnPedro Hacker. Maker. Engineer.

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    Damn you and your university degree!
     
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  5. cpemma

    cpemma Ecky thump

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    It's in Metallurgy... :lol:
     
  6. Krikkit

    Krikkit All glory to the hypnotoad! Super Moderator

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    Nice, thanks cpemma - I'm assuming, from the data sheet, that pin#1 on the LM317LZ isn't connected in that circuit. :)

    Should make for a tiny circuit, hurrah! :D
     
  7. mvagusta

    mvagusta Did a skid that went for two weeks.

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    I was thinking it could be done with a zener diode and a resistor? since it only has to drive one led and this would mean a very small size & weight.

    The to-92 package is very small and would have like a 50x better result, so definitely go with Cpemma's suggestion - as if that needed saying :D

    As for how to hook it up, check out the bottom of page 8 on the lm317 datasheet, where it shows how all three pins are used. You could use a smaller resistor like 39 ohm or 30 ohm to run the led at say 32 or 42ma, which can make some white leds really shine.. i dunno what type you have however? It wouldn't make a difference to how much power the motor receives btw.
     
  8. Krikkit

    Krikkit All glory to the hypnotoad! Super Moderator

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    Course it does! Duh!

    Well, that's certainly a great piece of work there guys, even at Maplin's extortionate prices it only comes to £2.31 per car with a fancy 3mm white LED.
     
    Last edited: 31 Dec 2008
  9. Alekoy

    Alekoy Ostekake!

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    maybe add a 100uF capacitor before the resistor for the leds? so that the lights stay on a little while without the car moving, and in turns (does anyone actually release the trottle in turns?)
     
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