O-scale fix for DCCOD occupancy detectors


If you are in O scale, and you signal your layout using DCCOD train occupancy detectors offered by Dr. Bruce Chub, you might over time find one or two charred resistors on a board. An O-scale permanent fix is easy and cheap, only adding two tiny and inexpensive parts. Don’t panic!

The cause of burned resistors was excellently and thoroughly analyzed by Mark Gurries, moderator of the NCE Yahoo group. In his analysis Mark showed how the current transformer works and its relationship to the “burden resistor” wired across its output. Mark showed what was happening using both theory and actual measurements.

Much of Mark’s excellent analysis may sound like techno-babble for those who did not study Electrical Engineering. I appreciate Mark’s work because he dead nailed the analysis. The problem is far more likely to occur on an O-scale layout than N because our larger motors draw more current.

Some suggest increasing the wattage of the two resistors that fail. Unfortunately, for us in O-scale that would not be sufficient.

However, two tiny Schottky diodes can be added to the back (foil) side of the Rev E board. If your soldering skills and iron are fairly good, adding these should not be a problem. These diodes clamp excessive voltages across the secondary of the current transformer that can eventually burn up the two resistors. Just order two “Surface Mount Schottky Barrier Rectifiers”, Vishay part number SS26S, Mouser.com part number 625-SS26S-E3, for each DCCOD detector board. Solder them on the back of the board according to the photo below. Note that one diode faces to the left, and the other to the right. They are connected directly across the secondary winding of the current sense transformer. When the transformer produces high currents through its secondary winding, these diodes safely provide a path for the current to flow and thereby protect the burden resistor.

So why use this particular diode? There are lots of Schottky diodes that can handle the current, but these also fit perfectly between existing Rev E PC board traces.

The above photo is of a Rev E DCCOD board, built by Vince, and retrofitted after installation. Also note that Vince installed a male connector on the detector board. This made it easier to remotely mount in the layout.

We use axle detection resistors as high as 8.3K. None of the modified detectors have any trouble detecting a single resistor axle with only 4 wires through the center of the core of the current transformer. More turns are not required for robust detection. That said, we usually install two resistor axles on each car, one on each end. Doing so makes them easier to detect if only one end is where “it shouldn’t be.” Also, a resistor on each end of a car helps maintain detection while traveling through a short interlocking.


Mark Schutzer has since designed the cpOD available from Model Railroad Control Systems which is plug compatible with the DCCOD but does not have the power issue so it can be used with O Scale layouts as is. It costs $30 assembled and tested.


I have been aware of that product, but a bit confused.

The detector appears on the Community Designs page of the MRCS web site, under a banner that states “Model Railroad Control Systems serves as a hub for community-supported open-source hardware.” How do I get the open source documentation, including schematic and BOM?

Or is it a proprietary (closed-source) design? I notice that none of the components appear in the MRCS Master BOM.




Good question: in this case the product was sufficiently attractive from
a cost and performance perspective that we decided to offer it even
though the author, Mark Schutzer, chooses to keep the design
proprietary. Since it wasn’t “our” design we listed it under community
designs, hence the disclaimer. We’re committed to bringing the best
technology to model railroaders at the best price so we may offer
proprietary designs when the benefits to user justify it.

As you can see from the photos, cpOD uses surface mount technology to
optimize costs in high volume assembly, so it isn’t intended as a DIY
item. In the unlikely event that a cpOD should fail in the field, (none
has, to date) just send it back and we’ll repair or replace it.

The cpOD is a completely new design, not a merely a surface mount
repackaging of the DCCOD (which we wouldn’t offer without Dr. Chubb’s
blessing). It does have two new features which may prove useful to some
users: 1) high current operation as noted previously so O scalers can
use the cpOD “out of the box” and 2) 5 volt operation option, which is
useful if you’re doing a new implementation with all LED signals and
don’t need 12V.

We offer the cpOD (for distributed installations)/cpOD-M (Molex plug
compatible with DCCOD) assembled and tested $13 quantity 1-9, $12 at
10+. For larger orders or custom connectors etc, contact us for a

thanks for looking at us!



Seth -

Thanks for your prompt reply. Perhaps an edit of the web page would clarify the proprietary nature of the cpOD, that it is not open source as the current web page clearly proclaims.

I’m disappointed that I can not perform an independent analysis of the design to confirm its ability to carry O scale current without purchasing one and reverse-engineering the circuit.

Adding two inexpensive diodes to Bruce’s DCCOD makes it robust under all current loads. That’s how we nipped the problem in the bud on the A&O.

Personally as an electrical engineer I find soldering surface mount parts preferable to through-hole, but it is understandable that most model railroaders would not agree.

All the best.


While bringing up the CTC system, Vince and I observed that some of the DCCOD detectors started detecting with nothing on the rails. A lesson here is that it is not adequate to turn on the system and simply adjust the sensitivity potentiometer until the detection LED just goes out. Turn it a little farther, as the threshold seems to drift a bit according to… humidity? temperature? phases of the moon? day of the week? who is scheduled to speak on the local college campus? Sorry.

Vince originally set the potentiometers on his work bench to detect a 10K resistor; we typically use a 7.5K but some axles have 8.something K ohms and the precise value escapes me at the moment. During and post-install I “maladjusted” far too many detectors, so at some point after the mainline is up and running, a “sensitivity tune-up session” with a 10K resistor would be prudent. Come to think of it… No, I won’t go there. :laughing: