June 2017 63
a schematic or suggestion for a DIY
train transformer they can share?
Boy, are you lucky!!! I started
collecting Marklin HO Scale back in
1962, so a 1959 vintage should be
I believe what you are looking for
is the 16-20 VAC transformer Marklin
used to require. The basic power pack
was a variable voltage — 0-16 VAC
approximately — transformer. It had
a lever to rotate which increased/
decreased the voltage output.
To reverse the engine, the handle
was pushed down momentarily which
would send 20 VAC to the engine,
which toggled a latching relay to
reverse the ‘brushed AC motor.’
So, there are a number of ways
to control the locomotive. The easiest
would be to use a transformer output
through a potentiometer, driving a 30
or 40 watt amplifier (direct coupled).
This output would directly couple to
the track. The center spikes are the
Hot, with the rails being the Neutral.
If I were (and I might in the
future) to design a solid-state
replacement, I would select a PIC24F
or PIC33F series processor and
have it generate a PWM sign wave.
There are app notes at the Microchip
website specifically regarding the
digital generation of sine waves. Then,
you can vary the frequency for speed
control — rather than the voltage —
which would give much better low
speed response for the engine.
Grand Rapids, MI
[#1173 - January 2017]
Turn Signal Signal
I have a newly restored 1971
Honda CB350 motorcycle. One
problem is forgetting to turn off the
turn indicators. I've found a kit that
“beeps” every time the indicator
lights up, but it’s annoying as I sit at a
light. I'd like a circuit to alert me if the
indicator stays on for more than two
Sounds like a job for an Arduino
Pro Mini and a few discrete parts.
First, you need to convert the
on-off 12V turn signal levels from
the lamps or LEDs to 5V logic levels.
You can find level-shift circuits via a
Google search. Then, you’ll have two
sets of pulses: one from the left signal
and one from the right signal. Run the
5V logic signals to two inputs on the
Arduino Pro Mini.
Second, you need a control
program that determines what to do
with these pulses and when to turn
on an indicator (visual or audible).
When the software detects a pulse on
either input, it starts two timers: Timer
1 for two minutes; and another (Timer
2) for about 1.5 times the length of
a turn signal on-off period (1.5 times
the flash duty cycle). Each incoming
pulse restarts Timer 2. So, as long as
the Arduino Pro Mini receives turn
signal pulses, Timer 2 continues to
run. If at the end of the two minute
period Timer 2 is still running, the
indicator turns on. The indicator turns
off as soon as Timer 2 stops running.
That indicates no more pulses from
the turn signal.
As an alternate, an Arduino Pro
Mini could simply count the number
of pulses that occur for your vehicle
in a two minute period and when the
count equals that number, it turns
the indicator on. You might include a
“kill” switch for the indicator in case
you need to keep the emergency
flasher on for more than two
minutes. For more information about
the Arduino Pro Mini, visit: www.
[#1174 - January 2017]
How Much Tolerance Is Enough?
When designing circuits, is there
a rule of thumb for picking voltages
and tolerances of components? For
example, if my power source is 12
volts, is an electrolytic capacitor with
a 24V rating “better” than one with a
16V rating? What do good designers
use as a margin?
I used to design circuits for
military applications and in that
environment, a voltage rating
double the expected maximum
was considered adequate. In an
automotive application, the spikes
from the starter can exceed 60 volts,
so you need to keep that in mind and
Ripple current is another
capacitor parameter that needs to
be addressed. It turns out that the
ripple current rating increases with
voltage rating, so you might use an
electrolytic cap with 10 times the
needed voltage rating just to get the
ripple current rating.
In general, the MTBF (mean time
between failures) is calculated based
on the stress on the component.
A component (resistor, transistor,
transformer, etc.) that is rated 70°C
but is running at 150°C will have a
short life, but if it’s running at 30°C
the life will be normally long.
[#2172 - February 2017]
I have been using a Sony ICF-9740
AM/FM radio on my nightstand since
1974. Recently, there is increasing
interference on the AM band. It’s not
AC “buzz” or “hum” that one would
expect from old power supply filter
caps; it’s more of a high frequency
whine and it’s consistent even when
the volume is turned all the way
down. I could replace this unit but I
would really rather fix it. Any pointers
on finding the source of the noise or
theories on what might be causing it?
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