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
