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block detection

Block Detection

INFORMATION #10-1: A Very Brief Introduction to Block Detection.

Before I get into block detection, I should warn you that it is neither cheap nor easy. While there are reasonably priced solutions on the market and relative easy ways of implementation, the shear fact that you will probably need a lot of detectors and will likely need to program a computer, keeps block detection from being in the cheap and easy category. You will have a lot to think through and organize. If programming a computer isn’t your thing, block detection may be beyond your reach.

Block detectors are offered in two basic ways.  One is a complete unit that detects a train and reports back to your control system.  The other way is block detector modules that detect the train but do not report back to your control system.  These are about $10.  They require additional circuitry to send their output to your signaling system or whatever else you want to do with their output.  This extra circuitry is included with some rail switch control modules.

Both have their advantages and I use both.  The multi-detector module I use in my hidden yards where I have lots of tracks to detect.  The individual detectors are good for where you have a long stretch of track where you only want one or a few detectors.  They can be used in conjunction with a rail switch controller for a nearby passing track.  Or you may use them with such a controller when you need a few more detectors than a multi-detector might have to avoid buying another whole multi detector.

You should buy these now and install them now as you build your railroad.  I will discuss how to wire them to your current wiring in the track wiring section of this web page.  They are really simple.  For the Wangrow unit, just run the feeder to your track through the hole in the middle of the detection device!  For the Digitrax detector, connect one end to the track and the other end to your bus or sub-bus.

It may be possible to mix the single detector modules from several manufacturers.  I will be looking into this “one of these days.”  However, the circuitry that reports the occupied block back to your signal control system is manufacturer specific.

Digitrax information on wiring block occupancy.

SUGGESTION #10-2:  Detecting Trains and Rolling Stock.  Where Do I Have to Put a Resistor? 

Block detection usually requires some sort of load to be placed across the tracks.  This can be a locomotive, a lighted caboose or passenger car, or a car with a resistor installed between the wheels.


The back of this train, nearly the entire observation car, located in Block #1 would not be detected.  Could this cause a problem on your layout?

Many locomotives, cabooses, and passenger cars have an insulated wheel opposite a wheel that is picking up power.  In the case of a locomotive, power pick up may be the right front and the left rear.  This means the ENTIRE locomotive must be in the desired block before it is detected!  This is extremely important to understand.  To put it another way, when the front of the locomotive enters the block, it will not be detected.  Or, if a lighted caboose is overhanging the last block, it will not be detected.  It could be sitting on and fouling a rail switch!  If you have a simple need for block detection, you may be able to live with this situation.  The important thing is that you understand the situation and decide what is right for you.

Most of you will need to detect when the back end of a train is clear of a block or the front end has entered a block.  Those modeling the modern diesel era will need something on the last car to be detected since you don’t have a caboose.  You will need to detect an axle that is near the front of the train and the back.  In the case of a steam locomotive, it may not be essential that the very first axle of the pilot truck be detected.  The first driven axle may be adequate.  You need to decide this and plan where you place your breaks in the track accordingly.  Since I’m a steam person you are no doubt wondering, what do I do?  I’m going with the first driven axle.

What if your train is a single steam locomotive and it’s backing up?  Right, you may need a resistor across the back truck if you need to detect this situation.  You will need to examine your situation and make this call.  Some obvious scenarios:

Within a yard.  In my case, I will not be placing block detection in my yards.

Manual or automatic operation.  If you will not be using a computer to run your trains, you may be able to get by without worrying about it. You aren’t going to leave a train fouling a rail switch or turntable bridge.  On the mainline, the signal may not change exactly at the right moment.  In many cases this won’t be a big deal.  Will you have any that will make this a concern?  If you are using a computer to control train movements, you may or may not need this.  Think through all the train movements you intend.

In general, you will be better off  — meaning you will have less surprises — if detection occurs as near to the front and back of each car and locomotive as reasonably possible.  In the case of a car, that means the front axle on the front truck and the rear axle on the rear truck.

INFORMATION #10-3: Various Ways to Put a Resistor between Wheels. 

There are a number of ways to accomplish this.  Unfortunately, each method usually has some drawback associated with it.  This section will lightly go over some of them.  For my recommended approach, see the section below using conductive paint and resistors glued to the axle.

Use lighted caboose or tender wheels:  This is easiest to do.  Minimal soldering capability is required.  The drawback here is the entire car must be in the block to be detected.  Okay, well maybe not the entire car, but darn near!  You get the idea.

Use a lighted caboose or tender truck and an extra set of wipers:  Your local hobby shop probably carries wipers that can be added to trucks that have metal wheels that are insulated from the rest of the truck.  This isn’t a terribly bad way to add a detection resistor.  But don’t fool yourself.  This method isn’t necessarily as easy as it seems.  You may find that you need good soldering skills to make this neat and compact — especially if it needs to go on a flat car or hopper car where there aren’t many places to hide the wiring and resistors.

Use resistive paint:  This would seem to be the easiest way to get your resistor — just paint the insulated axle!  Besides the obvious problem of getting the right resistor value, I’m told this stuff easily cracks.  Bummer.

Resistor rubbing wheels or axles:  Basically, I’m referring to the multitude ways one can devise to get the resistor lead to wipe on a wheel or axle.  Some of these are effortless to implement.  I decided this method would ultimately suffer from bad contact.  Also, if the resistor is touching the wheel too tightly, it will affect its ease of rotation.  I’m almost certain this will be tricky.

INFORMATION:  Conductive Paint Instructions in English.

Conductive paint by Busch.  Silber-Leitlack 5900.  Walther’s part number 189-5900.

If you have never used conductive paint before, you will be in for several unpleasant surprises. First, is the cost. About $12 for a jar. Second, is how tiny the jar (vial) is.   I have Z scale tank cars that are bigger!  You begin to believe this stuff really has silver in it. The surprises aren’t over.  The instructions are in German! Okay, I see there are a few people in our international community here that are smiling.

The rest of us are having a heart attack. As expensive as this stuff is, we don’t want to waste a drop. We are further perplexed as to why there are two jars. Is this stuff like epoxy and be mixed in two parts? It is clear while there is some English on the package, there is far more in German. Something is being left out.

Fortunately, I discovered that the Alta Vista web search engine has a translate feature. Check it out:

Shrubs silver conductive paint make all materials (plastic, wood, glass of etc..) electrically conductive.  To the improvement of track impact contacts or to the repair of printed circuit boards, heatable autoback windows etc..  Unsichtbarnachen ” can become over old cothe conductive paint with almost all handusual synthetic resin colours (spray overs).

Verabeitungshinweise:  Strong-vibrate before use. Vibrate every 30 minutes again when long operating.  Silver conductive paint with fine brush, geeigneterr india ink feather/spring or by syringes or Stempelm lay on.  Consider:  In damp status no conductivity results!  Drying time:  approx. 2 Studen.  For diluting only use the enclosed solvent or xylene.  Brush cleaning and mark distance with solvent.   Xylene, Kunstharzoder cellulose solvent.  Surface resistance (depending upon job strength):  approx. 0.5 to 0.7 ohms/cm2 temperature-steadily to approx. 200 c.

Safety note:  Only for young people over 14 years and adults suitably.   Inflammatory.  In the case of processing of air, because injurious to health during the inhalation, swallowing and contact with the skin.  The skin can provoke.   From ignition sources keep away – do not smoke.  Contact with the eyes avoid.

Okay, so it isn’t perfect.  I was still impressed.

I believe Xylene is a carcinogen.  Use outside if you must use it!  I believe “temperature steadily” means it might burn if it reaches 392 degrees Fahrenheit so you should heed the advice of keeping it away from fire and heat sources that might ignite it.

I like “adults suitably.”  Presumably that means responsible adults.   I’m not sure what that says about those of us who play with trains! 

SUGGESTION #10-7:  Buy the Wheels With the Resistors Already Installed.

Wheel sets with 5.1k ohm resistors, suitable for use with DCC, are available from The Signaling Solution.

SUGGESTION #10-6:  What Value Resistor Should I Use?

See your manufacturer’s manual for their block detectors.  It most likely will say the resistor should be a maximum of some particular value.  Use that value on each axle that you place a resistor.  In my case, it was 10k ohms.  At 10,000 ohms and 12 volts, that resistor will draw 0.0012 A or 1.2mA.  That amounts to considerably less power than 1/8th of a watt, so go ahead and use a 1/8th watt resistor — typically the smallest resistor you can buy.  I got mine from Digikey.  See the Getting Electronic Parts section.

Instead of manufacturer’s manual of saying that the resistor should be maximum value, it instead might say it needs at least some minimal current. For example. It might say, minimum sensitivity is 3 mA or .003 amps. For HO, you would divide 14V by .003 A for a maximum resistor value of 4667 ohms. You can probably use the nearest available value of 4.7 k-ohms. Notice that I have used the term maximum resistor value and minimum current. Have no fear, you are reading everything right. It’s just a little thing called ohms law. You don’t need to know ohms law, just note my terms of maximum and minimum.

Some might debate whether you should use a higher value.  After all, a car would have 2 of these resistors; 1 at each end of the car.  So why not make the value twice as high?  Two reasons.  1)  If just one end of a car is in a block, it will be reliably detected.  2)  Because even using the value the manufacturer suggests for reliable detection would take over 1400 cars, yes cars, to use up the 3.5A of a booster.  Anyone have that many cars sitting on the track of one booster???  If that same booster was running two locomotives at an amp each, you will still be able to have over 600 cars on that one booster.  In short, even this recommended value draws so little current, it probably won’t be much of an issue for most users.

Note:  This section is a suggestion!  So if your particular situation warrants something different, by all means, do it.  It’s your railroad!  The only caution I add is that if you pick too high a value and find that maybe you should have used a lower value, you are faced with the formidable task of changing all your resistors.  Perhaps you could put another in parallel with it to lower its value — if the underside of your car has adequate clearance.  If you use the manufacturer’s value and have a lot of locomotives, you may find you need to add a booster.  More expensive than changing the resistors, but adding a booster is definitely far, far easier! 

SUGGESTION #10-9:  Glue the Resistors To the Wheels and Use Conductive Paint or Conductive Pens.

This suggestion comes from John Plocher.

“Instead of soldering etc, I use ACC to carefully cement a 10K surface mount resistor to the axle, and use silver conductive paint to connect it to the metal wheel sets.”  John also uses circuit board repair pens instead of conductive paint. See the section on Getting Electronic Parts for information on purchasing these.

Conductive paint 
1/16 watt, 10k ohm resistors, surface mount 
1/10 watt, 10k ohm resistors, surface mount 

Prices are in US dollars the last time I bought them.  They give discounts for quantity buys.  In most cases you can buy 1 of something.  However, the unit price will be higher than what I paid for the quantity I bought.  USE MY PRICE INFORMATION ONLY AS A GUIDE.  I DON’T INTEND TO CHECK FOR PRICE INCREASES.

Note: I have deleted the section on how to solder resistors to wheels. My preferred wheels are no longer available. Further, soldering leaded resistors to wheels sometimes hits the underside of the car. We are now using the conductive paint and the SMT resistors. Currently, we are trying the size called 1206. We are using Walther’s Goo to attach the resistors. We need to get these on more cars before we can offer any more advice.

Note: 1206 means the resistor is 0.12″ x 0.06″ – small! As far as wattage is concerned, you can use smaller. However, adding the conductive paint was a problem. We weren’t that good with a paint brush! Our goal is to find the smallest resistor that we can work with, but not hit the bottom of the car. If you are working in a scale larger than HO, especially O and larger, you can probably go to 2512 and not have any problem hitting. You can guess what 2512 means!

INFORMATION #10-8:  Where to Put the Block Detectors.

In case you have not discovered this yet, implementing block detection possibly means having nearly as much wire under the layout as you did for conventional DC wiring.  My hidden yard with its block detection definitely has a lot of wire.  Once you get over this realization, you wonder, where do I put the block detectors?

Locate the block detector out at the block to be detected and mounted on a terminal strip. Then I use thermostat wire to run the block detector back to the device that will send the detected block signal to your system. This avoids the miles of heavy wire and you could run several block signals through one thermostat cable. More importantly, now that modelers have discovered that there bus wires should not exceed 30′ (10m) in length, it is better to locate your block detector close to the actual block being detected. See the section on track wiring to learn more about keeping your bus short.

If you are using something like the BDL-162 (16 block detectors on one board), you may find that your bus lengths will be over 30′. If this is the case, use Digitrax’s remote sensing diodes, RD2, to avoid the long bus length.

Compensating Twisted Pair Buses for Reliable Block Detection

The following was contributed by John M. Smith,

Track wiring capacitance can cause a NCE BD-20 to indicate that the track is occupied, when it is not. A capacitor, connected as shown here, can compensate for the capacitance of the track wiring, and allow the BD-20 to operate normally.

Capacitor Compensation for Block Detection

The capacitance connections are critical. For discussion, in this figure the bus wire that passes through the BD-20 core is colored red, and the other is colored black. Make a connection from the bus red wire to one terminal of the capacitor. This connection must be on the same side of the core as the booster, as shown. Current through this connection to the capacitor must not have gone through the core first. Connect a wire to the other terminal of the capacitor, pass it through the core in the direction opposite that of the red bus wire, and connect it to the black wire (anywhere nearby).

The capacitor value is not critical. For a 21-foot track bus of twisted 16 AWG wires, compensating capacitors ranging in value from 62 pF to 1000 pF were effective. A 390 pF was chosen. Use a non-polarized capacitor!

INFORMATION #10-10:  Cheap and Dirty, Crude, Cheapskate Block Detector.

First, let me say that I am not proud of this detector.  It violates at least one design rule.  Yet it works.  A year after its creation, none of them have burned themselves up for violating that design rule.

What you have is probably the cheapest block detector you will find.  You can probably build it for $0.50.  This was designed for a friend who wanted something simple to indicate trains on his hidden storage tracks.  He didn’t want it to feedback to a DCC system.  He just wanted a cheap circuit that would light an LED.

It is simple and it is crude.  All it does is light one or two LED’s on a control panel.  Heck, if you wanted to, I suppose you could light three or four.

It is not sensitive.  Normal resistors on wheels, in the 5k ohm or higher range won’t work.  If you want to detect cars, you will need resistors less than 1k ohms.  Less than 100 ohms will be better.  Otherwise, you won’t see the light on the control panel very brightly.  The resistors will need to be 1/2 watt.

Obviously, all these low value resistors could add up to be a significant load on a booster.  Therefore, other than detecting your locomotive, I would only put resistors on the wheels of your cabooses, or better yet, light the caboose.

This detector, being crude, will cause a 2.8V drop on your track.  A train going from a block using this detector to one that does not, will noticeably speed up.  A train will noticeably slow down when leaving an undetected block to a detected block.  As cheap as this detector is, you could block detect all blocks.  The change in speed is noticeable, but you may decide it is tolerable and not worth worrying about.  Build a few of these detectors and try it.

If you don’t need an indicator for every block, but don’t want the speed change, you can leave off the LED’s.  In this case, the circuit just acts as a voltage equalizer.

For a super cheap circuit, my friend uses a 1A diode, like the 1N4001.  This will work for HO, N, and Z.  If HO, do not run multiple units.  You may blow the diodes.  For multiple lash-ups or larger scales, use a 3A diode.  However eight 3A diodes can cost so much, you would be just as well off to buy a normal block detector.

Lastly, this simple circuit cannot feedback into a DCC system.  To make it do so, would add cost.  This thing isn’t worth it for that.  Buy a normal block detector.


INFORMATION #10-12:  Equalization between Detected and Non-Detected Blocks.

Many block detectors use diodes in series with the DCC signal going to the track. This isn’t the ideal way, because it introduces a voltage drop to the track, but as the above cheapskate detector demonstrates, it is a cheap way of doing it.

I designed the cheapskate detector for a friend who didn’t need feedback to a computer system. I do want such feedback. I use a more sophisticated detector, but it still has a diode in series with the block.

I don’t detect my industrial sidings and yard tracks. So there is a voltage change when going from a detected block to a non-detected block or back. It’s not as bad as with the cheapskate detector, but it is there.

For other than the cheapskate detector, the difference is only about 0.7V. This might not be noticeable to you. Or you may not find it objectionable. I don’t. If you run lighted passenger trains, it might be easier to notice. You still might not care. If you don’t notice the difference or don’t care, do nothing — you are done! Stop here!

The following is for those who notice the difference and care.

This first approach is useful when you only have and want one track bus under any particular area of your layout. The diodes should be 3A, 25V minimum rating. Yes, my friend’s cheapskate detectors continue to work fine with 1A diodes. I still attribute this more to good luck and him running small trains. Large trains or multi-unit lash-ups would certainly take out the 1A diodes.

Use at least 3A, 25V diodes for this — a higher amperage rating if you are using a 5A booster. The diodes should be able to handle more than the booster can put out. This approach uses fewer diodes overall. You do have to run a second bus. In the end, this may not be cheaper, but not having to put together a bunch of diode pairs will make it faster.

You don’t actually have to use two different boosters. The diode pair shown on the left could be “t’ed” off the “A” terminal of the booster on the right.

Block Detection Made Easy

Railnet offers easy to install block detectors that are also easy to set up.

Setting Up a Digitrax DS-54 for Block Detection

How to set up a Digitrax DS-54 is discussed in the section on Turnout Control. You can either use the DS-54 for detecting 8 blocks or 4 blocks and 4 turnouts. The CV configuration is the same for either approach.

Using Twisted Bus Wires or End Terminators

See discussion in track wiring section. If you are using twisting or end terminators, you must put your block detectors after the twists and end terminators.

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