# HO scale train LED question.

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

1. ### Patrick O ShaneGuest

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?

Shane

2. ### John PopelishGuest

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. ### ehsjrGuest

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|---+
| | | ----- |
| |+ ---+ |+ | |
|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.

Ed

4. ### Patrick O ShaneGuest

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

Shane

5. ### Patrick O ShaneGuest

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. ### JazzManGuest

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.

JazzMan
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7. ### John PopelishGuest

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
partition.

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.