Simplified Block Detection without metal wheels : LUSENET : Wiring for DCC : One Thread

I have read many of the articles and postings on the subject of detecting whether an engine is present but not a car with insulated wheels. I would think a system would be available or have been described to detect the presence of a train and cars entering a block and similarly detect the absence of train/cars and provide a signal accordingly upon leaving the block. Current sense is nice for the engine and any cars providing an electrical "load". All cars will not have metal wheels or loads added which makes it difficult to detect the absence after the engine has left the block. Optical point sensing is good but at least two are needed, one at the entrance and one at the exit points. Does anyone have any thoughts on the practicality or availability of such a system for a HO system? Please let me know by return. Thanx in advance for your help.

-- Jim (, December 22, 2002


Yes such a detection system is practical. As you indicated two sensors are required at each entrance or exit. The sensors should be slightly staggered so that as a train passes, one detector is triggered before the other. One can use a relatively simple latching relay circuit to latch (occupied) or unlatch (unoccupied) as a train passes either end, in either direction. The sensors do not need to remain continuously active for the latching relay to hold the occupancy status of the block. It does not count cars so it won't tell you that the block is still occupied by a lost waycar!

See Look for file: LatchRelay BOD.doc

-- Don Vollrath (, December 23, 2002.

Thanks, Don. I waited for someone else to respond to see if they had any different wisdom. By all means, if anyone elese wants to jump in with their opinion and perspective on this one, please do.

Don's last comment is the reason I avoid the optical approach though initially this was the way I was going to go. That is, it won't tell if you a block is occupied by a car that came uncoupled. Optical is great for applications that need POINT detection - something passed the location of the sensor or is presently at the sensor. But after it passed that point, it's anybody's guess what is going on with the train.

This next statement may not be true any more. In the past, the sensors had to be above the layout, hidden in signal towers, buildings, bushes, the like. That was tough around turnouts where you had minimum clearance.

So I tried very hard to mount sensors under the track and look up between the ties. The light bounces off the bottoms of black cars better than you might think. Still, I couldn't get the light to bounce off all the different cars I had reliabily. So I abandoned this approach. Another friend of mine recently showed me a new optical sensor that he claims he has tested on all sorts of cars from under the track. It is reasonable that is now technically feasible. I knew why my attempts failed, but at the time, no one made the emitter or receiver with the right characteristics. If my friend is right, they are now made. You still have the problem of the POINT sensing mentioned aboved.

While optical emitters and receivers are cheap, the entire approach is not. If you use any receiver that points up, you may have to add circuitry that guards against being triggered by room light or someone bending over the sensor location. You need a set of optical devices at the entrance and exit area to be detected, plus you need some sort of latch, as Don indicated, to hold the state of the train that is between the sensors. In the end, you may find yourself saying, "Murphy did it to me again! This costs more!"

To be sure, putting metal wheels with resistors on every car is not cheap. Maybe just consider doing your cabooses or your cars with your FREDs on them. This will probably be your cheapest approach and if you train does come uncoupled, you will know it. If a train comes uncoupled while going over a turnout, you will not necessarily know it.

Like everything else, everything has it's price!

-- Allan Gartner (, December 23, 2002.

There are (or maybe used to be) optical sensors sold under the name IRDOT that used a synchronous pulse scheme to avoid problems with room light etc. They mounted under the track facing up between the ties. They are kind of pricey though.

You could consider having to install metal wheels as a benefit besides providing detection. Many modellers, myself included, find that metal wheels benefit operation in two ways: the trains run better with less derailments, etc. and your track stays cleaner.

Dale Gloer

-- Dale Gloer (, December 24, 2002.

Thanx for the responses. Your thoughts and experiences are exactly what I have been looking for, and will hopefully see some more. I'm an EE by academia, and will have fun experimenting and more importantly implementing what works. I'm of the opinion that I would prefer a system that would allow other visitors to bring their unmodified cars on my layout, and it would work. I run metal wheels and appreciate their benefits, but not everyone has metal underneath. keep your experiences and thoughts coming in, as I think there is a better solution out there, and I'm interested in making it work. Hope to hear from you soon. Thanx for your help.


-- Jim (, December 25, 2002.

I am in the process of converting from dc to DCC and have BD-1 block dectors ( optical ) not the Digtrax type. they incorprate a latch and relace type system this is more a question than an answer will they be able to be used with DCC?

-- James B. Hinkle (, July 23, 2003.

Let me start this off with a disclaimer. I currently do not have a train layout, or even a train for that matter, but I am looking to get into the hobby, and I am trying to learn as much as I can about all of it.

I said all that to say this. I have a couple of ideas that might be a situational solution. I am not sure how well it will work though, because I have not tried it, but is there a way that you could use a magnetic type of sensor, that could tell when there was something metal above it (it being mounted below the track). If your trucks have metal wheels this might be possible. My second idea is to but a time delay circuit in along with the sensor. So if you were using it to operate a signal then the signal could be turned on for maybe 5 seconds or what ever the time period that the circuit was designed for.

I am interested in this subject, because I can think of a couple of applications already to use it for.

I hope this helped, or maybe got you to think about a method that will help.


-- Gary Fuller (, February 07, 2004.

In factory automation, there are two types of sensors modelers don't know about. One is an inductive proximity sensor. It detects metal. I've used these, but it was to detect a lot more metal than a wheel. My gut feeling is that a wheel is not enough metal to be detected. Furthermore, if it had sensitivity high enough to detect a wheel, it would probably detect the track all the time. They have shield varieties of these, but in order for the shield to be effective, the sensor would have to be at least as high as the top of the rail. It may even have to be a little bit higher, which needless to say, renders it useless for model railroading.

The other type of sensor is a capacitive proximity sensor. This is like the circuit that is used in electronic stud finders. It can detect plastic. You have similar problems with this sensor as the inductive. It will detect your track if the sensitivity is high enough to detect the car. (Note, if it worked at all, it would detect the whole car. There definitely isn't enough plastic in a wheel to detect.

So even if you could afford either of these, I would say they probably won't work in model railroad applications.

-- Allan Gartner (, February 07, 2004.

There is yet another way. Use a track current detector for loco train detection in the middle of the isolated block, AND an optical detector at each end. OR couple the outputs so that that particular block is "occupied" if any of the 3 detectors sense a train. You only need one optical detector at the end of any given block. It can be diode OR coupled to the current detector for each adjacent block as a car standing at the gaps is in both blocks.

-- Don Vollrath (, February 09, 2004.

To detect the occupancy of a section of track, especially those that are hidden from view, try using an emmitter / receiver photo eye set- up like the one on the bottom of your automatically opened garage door. They will span a distance of 20' or more and will detect anything in their path. This is what I am using on my hidden storage yard. It won't bend around corners, but it will tell you if anything is on the track!!!

-- Phil Bode (, June 21, 2004.

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