Download - O scale trains

More on Realistic Passenger
Car Lighting
Ted Byrne
In Issue 31 of OST, I discussed simple electronic units to
control light emitting diodes (LEDs) in passenger cars (Photo
1). I would like to wrap up that topic here, so that I can move
on next time to cover the inexpensive sound systems that are
appearing for the smaller scales. Since sound does not have a
scale, they are also applicable in O Scale.
1
2
LEDs, and integrated circuits to control them in model railroads,
are not that new. The new factor is the decreasing cost
that makes their use attractive. The main advantage of LEDs is
their low power needs. It is quite possible to have a long passenger
train with internal car lighting wherever appropriate.
However, as with any car illumination technique, the light flickers
as the car wheels move over the track. If that bothers you, it
can also be fixed.
AC
IN
AC
IN
RECT.
RECT.
Figure 1
CAP.
.1
CAP.
.1
LM
317Z
RES.
68
LM
317Z
RES.
68
LEDs
LEDs
Figure 1 and Photo 2 are a circuit diagram and photograph
of my universal controller. I’ll show how it can be used to
eliminate flicker, but first let me walk you through the components.
As you can see by comparing it with the circuit in the
last installment, the controller consists of two rectifier/currentlimiter
circuits. There are two slight differences, though. I used a
single bridge-rectifier instead of four diodes (for simplicity), and
I put a capacitor between the rectifier and the current-limiter
to smooth out the DC voltage. As you recall from last time, the
317 regulator keeps the voltage across its resistor from exceeding
1.2 volts, so the current can be controlled by the value
of this resistance. This dual unit can be used to provide 0.02
amperes to each of two circuits of lights with 68 ohm resistors.
The plastic board can even be cut in half horizontally and the
two sub-units can be put in two different cars.
AC
IN
AC
IN
RECT.
RECT.
Figure 2
CAP.
.1
CAP.
.1
LM
317Z
RES.
68
LM
317Z
RES.
68
LEDs
LEDs
Figures 2, 3 and 4 show how this unit can be modified to
do different jobs by a few wire changes. In figure 2 one rectifier
is omitted and both current-limiters are connected to the other
rectifier. This can be used several ways. If you use DC cabcontrol,
thus variable track voltage, sometimes there might not
be enough voltage for all the lights that you want (Recall that
the rectifier and limiter take up about three volts.) We’ll put the
lights in two parallel strings with this circuit. Perhaps you might
want two illumination levels at the same time; an example
would be bright interior lights and dim marker lights. This can
be set by the value of the two resistors in the unit. You might
48 • O Scale Trains - July/August ’07