by Ring Engineering. It flashes on and off automatically. It draws
about 4 mA when on. However, during the 1 second that it is off,
it draws less than 1 mA, less than most detectors need to be acti-
vated. This can be overcome by adding a resistor wheel set to the
truck on the other end of the trailing car.
Now, how good is your planning? On our club layout, we set a
train limit of 15 cars plus two locos and a caboose. Why? Not
because we were being arbitrary, not at all. Our shortest pass-
ing siding was designed to accommodate a train that long.
Following in that thinking, if your shortest detection district is
longer than your longest train, you don’t need resistor wheel
sets on any other cars. The caboose will be in the detection dis-
trict before the loco leaves. If this fits your layout, you can jump
to “Types of detectors”, if not, continue to read this section.
If you want every car to be 100% detected, then the answer is
“Yes, one axle per car.”
However, there is a price to paid if you put one resistor wheel set
on every car on your pike: both the dollars spent and the power
drawn by all the resistor sets. Say you have 500 cars on your lay-
out. Even if you can use the highest value resistor (40 kohms),
you will draw almost ¼ amp just in the detection resistors.
Look for the shortest track length detection district on your
layout. Think about the locomotive having just left that dis-
trict. What’s the car that will have just come into the detection
5: An 8-car train with 3 cars having randomly assigned
resistor wheel sets.
district? How many cars are between there and the locomo-
tive? Say the eighth car back will have come into the district
(5). Okay, if you can assure that exactly every eighth car in your
consist has a resistor wheel set, you will know that you will
continue to detect the entire train. Do you want to be building
your consists by both destination and whether they have resis-
tor wheel sets? I don’t think so.
So, like so many things in model railroading, a compromise is
needed. Given a random mix of cars that have resistors and
those that don’t, I’d feel pretty confident that one with a resis-
tor wheel set is in the detection district, if we give a two or
three times fudge factor. That translates into every third or
fourth car for this example. Okay, this cuts the price down by a
factor of 3 or 4 from putting one on every car. Also, it cuts the
current draw down by the same amount.
Figure 5 shows an eight-car train with three randomly assigned
wheel set locations (red wheels). The red line is used to des-
ignate the detection district. Now that the loco has left the
district, there are three places that will keep the detector satis-
fied. By the time the last resistor wheel set is the only one in
the block, the caboose (not shown) will be in the district. The
entire train will be detected.
Types of detectors
There are two designs of detectors.
Digitrax implements the diode detector format. Their extensive
detection and signaling offerings are on their website
. The signal between the
input and the output is run through a set of diodes, dropping
about ¾ of a volt when the DCC signal current flows. The detec-
tor senses this voltage drop as an indication of occupancy. The
DCC Impulses - 5
MRH-Aug 2013
