Roll your own detection wheelsets

Block occupancy detectors designed to “sense” the presence of a locomotive or other load on the track do so because the load causes current to flow through the feeder wire the detector is connected to. A locomotive or lighted car such as a passenger car or caboose, is usually a large enough load to achieve this, however what about other rolling stock?

You can use cars with special wheelsets that have resistors installed on them. The basic idea is to place a 5-10K ohm resistor on the axle between the wheels with the end of the resistor electrically connected to the backside of the wheels. The resistor will conduct a small current between the rails and allow the occupancy detector to “see” the presence of the train. But why would you need these if you have a locomotive pulling the train drawing current? Well, what if the loco is across the gaps and in the next block ahead? Or what if the loco is working a local siding that is not included in the detection block? You need a car with detection wheelsets on the tracks connected to the detector so it can see if there is a train sitting on them.

Some folks install detection wheelsets on all their cars. Often this only requires replacing one set of wheels on each truck. However, you could also just do a random number of cars, say 10%, and assume that at least one will be in each train. Personally I plan to see if having just a loco and a lighted caboose will do the job and add more detection wheelsets if needed. Also, if you have lighted passenger cars they will conduct a current and be detected.

So where do you get these wheelsets? Several manufacturers offer them with various resistor ratings. If you are already planning to replace old plastic, sintered metal, or worn wheelsets then that would be a good time to add in some detection wheelsets. However, if you have more time than money, or pretty good wheelsets on your cars, you can make your own detection wheelsets, all it takes is resistors, glue, and conductive paint. Let me show you how it’s done.

First you’ll need some resistors. I use surface mount resistors since they are not easily seen once installed but 1/4 watt resistors are easier to see and handle so many folks use them. Also you’ll need a little CA, epoxy, or other glue, and some conductive paint or glue. Most electronic suppliers sell strips of surface mount resistors for next to nothing, and I got the conductive glue from All Electronics. Locate the wheel that is insulated from the axle, if the axle is plastic or other nonconductive material then this isn’t necessary. Sand or file a clean spot on the back side fo the wheel and the axle.

Place a little glue on the back side of the resistor and attach it to the axle/wheel surfaces with one end resting in each spot you just cleaned. Once the glue sets up, apply the conductive paint or glue to each end of the resistor and onto the clean spot on the axle or wheel. If the axle is plastic or nonconductive just draw a line along it from the resistor to the other wheel using the conductive paint or glue. Once the conductive paint dries, use your ohm meter to test the resistance from wheel to wheel to make sure you got good contact. The resistor provides the load for the detector to “see”. For those of you with a copy of my wiring book there is a drawing in page 91 showing a completed wheelset that makes this all very clear as well as the lead diagram here.

Depending on the characteristics of your detectors you may need one or two wheelsets per car and multiples in a train for detection to work. Most detector manufacturers offer tips on the number required and the resistor rating, along with ways to tune the detector to your layout wiring. For example the NCE BD20s have holes where desensitizing resistors can be mounted. Also, don’t buy a ton at first as you may find that for your particular layout and wiring, smaller resistors may be necessary. After some testing, I found that I will need a pair of 10K resistors on each car to get the LEDs in my signals to light up bright enough.

As I described in a previous post, I spent a couple days figuring out how to adjust the BD20 sensitivity for the circuit I am using. The BD20 assumes a 10K pullup resistor and small resistors will decrease the sensitivity of the unit. If the circuit is compatible then you can do things like adjust the number of times the track feeder passes through the current transformer. With a 5 amp booster you can have anywhere from 1 to 4 turns. If too much sensitivity is a problem there are ways to desensitize it using resistors or reducing the number of turns, but that is explained in the instructions. You may need to experiment a little with resistors to find your sweet spot so be patient.


  • What size surface mount resistors do you recommend?

  • Thanks Larry for the explanation on this. I have well over 200 cars and was wondering if I was going to have to do all of them which might warrant not installing a signal system at all. Now I see that I might be able to just rely on the locomotive and lighted caboose or a few axles in each train which makes more sense. Once again I appreciate your help and will refer to your book which I have in my collection.