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HO scale train LED question.

Discussion in 'Electronic Basics' started by Patrick O Shane, Oct 26, 2005.

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  1. Hello all,

    I recently discovered this newsgroup and I think it's awesome. Thanks for
    all the information posted.

    I have a question regarding LED lighting on an HO scale model train.

    I want to replace the headlight on the train with an LED. I was wondering if
    there is a simple circuit someone could recommend to keep the LED from
    flickering as the train moves on the tracks. The trains motor and lights get
    their voltage from the wheels making contact with the track, but it's not
    always a constant contact and sometimes you can see the light flicker. I was
    thinking maybe some kind of capacitor/resistor setup. My thinking is, if I
    connect a capacitor it will discharge slowly enough to where you won't see
    the flicker from the train failing to make contact at certain points on the
    track. I'm not that well versed on electronics so I could be way off here.

    Any thoughts?

  2. How 'bout a big capacitor charged through a diode (to keep the engine
    from sucking power out of the capacitor when the wheels pass over a
    dirty spot on the track) and the LED powered from the capacitor
    through a current regulator, to keep the brightness constant till the
    capacitor runs down too low.
  3. ehsjr

    ehsjr Guest

    Good idea. Here's one possibility:

    -----+ +---[100R]---+---[470R]---+
    | | | |
    _~_ | | |
    | |+ ---+ |+ |
    |BR1| [C1] [LED]
    |_ _|- ---+ | |
    ~ | | |
    | | | |
    -----+ +------------+------------+

    BR1 is a bridge rectifier so that it doesn't matter if the
    train is going forward or backward. C1 is 470 uF at 25
    volts (assuming the HO train runs at 16 v or less). If the
    train runs at more than 16 volts, the 470 ohm resistor
    needs to be increased, and the cap needs to be rated at
    ~10 volts over the maximum train voltage.

    If you want the headlamp (LED) to glow at the same brightness
    regardless of speed (but it has to be above some minimum
    speed) use the circuit below:

    -----+ +---[100R]---+---|LM317|---+
    | | | ----- |
    _~_ | | Adj [51R]
    | |+ ---+ |+ | |
    |BR1| [C1] +------+
    |_ _|- ---+ | |
    ~ | | [LED]
    | | | |
    -----+ +------------+-------------+

    The circuits above assume that the train gets no
    more than 16 volts, and that the (white) LED is
    rated for ~30 mA max current. The LED current is
    limited to less than 30 mA.

  4. Thanks for the tips guys. I'll try it out and let you know.

    Shane :)
  5. This may sound like a silly question but to be honest I just can't figure
    out in my head exactly how or what capacitors do. My wife is an engineer and
    I read some of her text books and now I'm more confused. My question is, I
    have some 35V @ 100 uF caps. What would be the difference between the 25v @
    400uF and the 35v @ 100uF? Please don't laugh I know it's probably a pretty
    basic question but I just can't grasp caps theory.
  6. JazzMan

    JazzMan Guest

    Capacitors are like a balloon that stores electrons. Imagine
    air flowing through a tube, a tube that has a balloon stuck
    in the middle of the run. If a suden spike of air pressure
    comes down the tube the balloon swells up and absorbs some
    or all of the spike. Air pressure is somewhat analogous to
    volts, which is essentially electron "pressure". That's a
    really coarse way of looking at it, but it does seem to help
    me visualize the concept.

    The uF rating refers to the ability to store electrons, and
    the voltage rating is the maximum the capacitor could be
    exposed to before being damaged.

    Please reply to jsavage"at"
    Curse those darned bulk e-mailers!
    "Rats and roaches live by competition under the laws of
    supply and demand. It is the privilege of human beings to
    live under the laws of justice and mercy." - Wendell Berry
  7. You can think of a capacitor as a tank that has a rubber diaphragm
    stretched across its middle, Charge (incompressible fluid) can pass
    through it, but only by stretching the partition, and only with
    increasing pressure drop across the two pipes on each side of the

    The general relation between voltage across a capacitor and the
    current through it is I=C*(dv/dt) or current (in amperes) equals
    capacitance (in farads) times the rate of change of voltage (in volts
    per second). So any time you pass current through a capacitor, it
    requires that the voltage across it change. The voltage rating of a
    capacitor is a maximum it can safely tolerate. and many capacitors
    have a dielectric layer between the ends that is created by
    electrolytic action that oxidizes a metal surface, so you can apply
    voltage only in one direction, or the electrolytic action reverses and
    destroys the oxide. These will have either a + or - sign on one lead
    to show the required polarity.

    For more information than you ever thought you wanted, see:
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