BDL168 & BD4 False Detection/Noise

Background Information

The Digitrax BD4 contains 4 current based occupancy detectors.  

The Digitrax BDL168 contains 16 current based occupancy detectors on a single board and report status over Loconet.  

Both boards share the same analog detection circuits on the front end.  

The Detection method is done by monitoring the voltage drop across two back to back diodes.    The detection diodes are implement using high current standard full bridge rectifiers.  The BD168  and BD4 are designed to be sensitive enough to detect a 10K resistance on the rails.  Assuming a DCC track voltage of 12V, that translates to nominal "trip current" of 1.2mA (0.0012 Amps). 

BDL168 only: An "option switch" sets the sensitivity which is applied globally to all 16 detectors divided into 4 groups (Zones) of 4 detectors in each Zone.  10K is the default setting but the board also supports a 20K setting..   Assuming a DCC track voltage of 12V, the 20K setting translates to nominal "trip current" of 0.6mA (0.0006 Amps).


THIS IS A CRITICAL DIAGNOSTIC TOOL: The LT-5 is a tool that gives you REAL TIME local feedback of the detection states and zone power via 5 LED's.  It plugs into one of four connectors, one for each zone.   4 LEDs show the occupancy state of the 4 detectors with the 5th LED showing the Zone power status. 

The Two Problems:
In a real world large scale application, there are noise and others sensitivities that can cause these board to give false occupancy information or unstable occupancy information such that it prevents reliable operation.  To be clear, there are two fully independent detection problems that one can experience.  They are:

1) False Occupancy Detection.  The sign of this problem is one or more DS outputs have false or unstable detection as shown on the LT-5 LED tester.  If you have the whole zone acting funny, see problem #2.

2) False Loss of Zone Power.  The sign of this problem is shown on the LT-5 LED tester.  The 5th or top-center LED, that indicates zone power, will flash on and off randomly.  Loss of zone power will happen if you lost the booster ground connection to the booster that powers that zone.  Random Loss of zone power can happen if the home ground wire is to long or far away from the booster the powers the zone and/or the wire is shared with other boosters.  it is best to run a dedicated home ground wire directly to the booster.

It is possible to have BOTH problems at the same time.  In term of priority,  solve #1 first.  Make sure that the occupancy detection itself is reliable and then address the Zone power issue.

The following discussion identifies the conditions that cause the problem and the solutions for them.


The BD4 and BDL-168 design allows it to be highly sensitivity to low trip currents, it has an unfortunate side effect of being sensitive to current flow created by parasitic leakage currents.

Leakage Currents?  Yes current flow is created by:

1) Wiring capacitance.  Never a problem with old DC power, it has a big impact on DCC in terms of detection.   Longer wires runs and how they are bundled together control the capacitance between the wires.   To learn more about wiring capacitance, go here: Track & Wire Properties
2) Ballast/Track.  Resistance paths created by conductive impurities found in dirt/dust, metal filings and graphite that find there way in the ballast or throwbar area of a turnout.  Multiply this by the length of the track block and toss in humidity and you got a recipe for current conduction.  This problem is true for DCC as it still true for DC.

Simplistically, the problem is there is no way to ADJUST the sensitivity below 10K.   

As it turns out, the real problem is a lot more complex that just lowering the sensitivity and no amount of software configuration or setting adjustments will address the problem.  Simply put, these limited adjustments are two far back in the chain of detection circuitry to address the problem at the source.  To learn a lot more about what is really going on, go here:Technical Discussion on BDL168/BD4

Simple Solution: 

Buried in the BDL-168 manual (2004 and 2013 edition) is a solution to address the false occupancy problem AT THE SOURCE and is fully Adjustable.  Unfortunately there are so many errors in the manual that one may never find it.  The answer is in this diagram:

The diagram shows two resistors installed between the Zone Common and Zone A outputs DS1 and DS2.

The BD4 manual also says the same thing in words but does not give a drawing of where to connect them.

This resistor option can be added to ANY detection outputs as needed.

Resistors are connected between the detection output and the common Input.  The resistor is shunting current AROUND the detector or bypassing the detector.

The value of the resistor will determine the circuits de-sensitivity which is saying the same thing as saying it will increasing the current threshold required for detection.  

A 1K (1000) Ohm resistor will reduce the sensitivity by 1/2. 
A 100 Ohm resistor will reduce sensitivity by a factor of 10.
A 47 Ohm resistor will reduced sensitivity even more.

Start with 1K ohms.  If it still a problem, try the next value down will work but do so by cutting the value in half each time..

Using a resistor substitution box is one method that can be used to determine the resistance value you need.  However the wire leads MUST be kept under a few inches. I have seen that when one used wire leads that are two long it can influence the results.  

If you do not have a resistor substitution box, use a 1K potentiometer configured as a reostat is a good way to experiment to determine the right value. Read the ohm value from the Pot and drop the ohm value to the nearest standard resistance value to give you some margin.  Dropping the next lower ohm value beyond the first one will give you more margin in the long run that can address changes in the properties as the layout ages. 

When you have identified the final value, any 1/2W or 1/4W resistor will do.  The power rating is not an issue, only ease of installation is important.


Here is a commentary by Dr Chubb on detectors vs price and features.

To C/MRI Users:

There has been considerable discusson on the Layout Design SIG Group about Staging Yard Detection.  In reply to some of the discussion I posted the following message that I thought might be of interest to this group so I'm repeating it here for your use as well:

Dear Peter and Andy:

Looking at the BDL16 is fine but checking it out against other DCC compatible detectors the price quoted of $8 per block is somewhat on the high side for what you get, or better yet for what you don't get.  Many think it's great so consider that input as well.  Some of the important factors I recommend looking for in detectors are discussed below:

High sensitivity is critical to providing maximum protection of single cars, true prevention of throwing switchpoints under any part of a train, reliable OS occupation lights where track sections are short and single car detection is critical to prototypical performance, etc.

A good test is if you can lay your finger across a section of track and the detector shows detection you have good sensitivity.  Or put a 70K ohm resistor across the track and if it shows detection have pretty good detection.  Many DCC detectors on the market require 10K or less across the track to acquire detection.

Having a built in turn-on and turn-off delay is equally important to minimize dirty wheel problems and even some level of dirty track problems causing lack of detection. A good turn-off delay should be 2 seconds or more.

Having the sensitivity adjustable is absolutely paramount to achieving maximum performance out of any detector.  The leakage resistance between the two rails varies all over the map from layout to layout, from block to block, is a function of humidity, type of ballast, type of glue, number of turnouts in block and everything else under the sun.  Having sensitivity factory preset at some nominal value is an extremely poor substitute for having a built-in potentiometer for optimally setting the sensitivity separately for each detector.

Having a built-in LED before the above noted time delays is essential to being able to quickly and effectively set sensitivity to its maximum value per block without generating false detection.  Typically for example, OS sections are short with typically lower leakage resistance between rails thus you can set its detector to higher sensitivity which is exactly what you need for short OS sections where one or two cars occupy the complete OS section.

Avoiding detectors that rely on diode voltage drop for detection is also advantageous for DCC layouts. These cause changes in train speed when go from undetected to detected trackage, unless you add series diodes in all non-detected track section feeders.

The series diodes are pretty well required for DC railroads but there is no excuse for using that approach for DCC where you have an 8KHz power-signal that can easily be sensed with a pulse transformer as part of the detector circuit

Modularity is also an important factor. Having one plug-in detector per block is great when it comes to detecting where shorts exist.  During an operating session, even with series problems, fault isolation is rapid and corrective action is just as quick. Also suspect a detector problem change it out in a matter of seconds.

I could go on and on with the list of features required for good detection, but I don't want to make this posting too long.  Taking a serious look at the BDL16, and many other detectors as well, you will not find many of the above features included.

However, several detectors on the market do offer some of the above important features.  The sad part is that most users purchasing detectors aren't aware of the important features and which detectors include which features.  It's also important to realize that the better line of detectors don't cost much, if any more, than the less capable detectors.  In fact some of the less capable detectors are the highest in cost.

In summary, don't take picking the correct detector lightly as it's an important parameter in optimized layout design.  Fundamentally, it really pays to shop around for the optimized detector that provides the above listed features at the best price.

Bruce Chubb, MMR