Introducing the CKT-BD1 DCC Block Detector

I’d like to introduce you to ISE’s latest model railroad product – the CKT-BD1 single channel DCC block detector!

The CKT-BD1 – our brand new single channel DCC block detector

This little DCC current-based detector is designed to be highly sensitive while being resistant to false triggering, robust, and very easy to install.  All you need to do is pass one of the bus wires to the block to be detected through the current transformer and provide 5-18VDC to power up the detector.   It can run on as little as 5VDC at 15mA, so it’s perfect for connecting to digital logic such as Arduinos or C/MRI systems.   It has open drain outputs for both detecting and not detecting states, so it’s compatible with a wide range of other model railroad products such as the Modular Signaling System, C/MRI, input modules for systems like JMRI, standalone signal sytems, or even just seeing if there’s something in that hidden section of track on your layout.  It also has adjustable sensitivity, so you can tune it to ignore leakage current through your trackwork while still picking up minute currents from rolling stock.  Precision current measurement circuitry and a little digital microcontroller onboard helps filter the response so that you achieve maximum sensitivity without false triggers.

The CKT-BD1 can be used anywhere you’d use a number of the competitive products today, such as NCE’s BD20, Dr. Chubb’s DCCOD, etc.  So why should you choose ours?  The same reasons that hundreds of other folks have chosen our products – they’re reliable, they’re open hardware, they’re built in the USA, and we stand behind them 100% – we wouldn’t use anything less on our own layouts.

The C/MRI ODMB-compatible version of the BD1Normally the BD1 ships with screw terminal blocks, so that it’s easy to connect up wires.  However, since we know there are hundreds if not thousands of C/MRI installations out there, we’ve provided an option to add a connector so that they’re plug-and-play compatible with the standard C/MRI detector motherboards (ODMB).   If you’re using C/MRI detector motherboards, order them with that option and just plug them in!

Because the design is based on a transformer that couples to the alternating DCC waveform, it won’t work reliably with straight DC control.  If there’s enough interest, we may eventually pursue a straight DC detector, but it’ll be a completely different design.   In my experience, trying to build a single detector for DC and DCC leads to a sub-optimal detector for both applications.

Block Detectors 101

Fundamentally, there are two types of block detectors – optical and current-based.   Optical detectors work on either equipment reflecting or blocking light, and are great because they require no modification to existing equipment.  They’re quite useful if you need to sense something at a specific point in the track, such as approaching the frog end of the switch so that you can automatically throw the points.  ISE has offered our reliable, easy-to-install CKT-IRSENSE optical detector for several years now, and it’s been an outstanding success.  I can firmly say it’s one of our most solid, reliable products and I’m quite proud of it.

Optical detectors are not very good, however, at monitoring an entire block of track.  Because they only sense a train right at the sensor,  you would need a lot of them to cover a 10-20 foot block of track.    If something happens to stop between detectors, suddenly the block looks unoccupied.  If that something happens to be an errant car that came uncoupled, any following trains may come through a clear signal to find an unpleasant surprise!

That’s where current detectors come in.  Current-based detectors rely on the fact that our locomotives (and lighted or resistor-equipped cars) draw a small amount of current when they’re bridging the rails.  When well designed, current detectors can pick up very tiny loads such as 10k resistors, idle locomotives, or even a human finger.   As long as something is drawing current somewhere in the block, a current detector will continue to indicate something is present.   They’re the most reliable way to sense occupancy for applications like signalling.

About 12 months ago, after a number of revisions to get the circuit just right, we finally arrived at a design that meets our high standards for performance, simplicity, and reliability.

Meet the Whole Family

The CKT-BD1 is the smallest, newest, and most versatile of our block detector offerings, but it’s not the only one.  The same fundamental circuit has spawned a whole family, each with a specific application in mind.

MSS-CROSSOVER Modular Signal System DCC Block DetectorIf you’re planning to use the CKT-BD1 with a Modular Signal System implementation, we’re launching a series of MSS products within the next few months.  One of them will be the MSS-CROSSOVER, which provides MSS “Crossover” block detection functionality.  It’s the same design as the CKT-BD1, but with some additional features added, such as a built-in DC power supply and MSS bus connectors.

Shortly after we launch the MSS-CROSSOVER, we’ll be launching the MSS-CASCADE which integrates two block detectors for “cascade” (signal boundary) functionality.  After that, there will be the MSS-SWITCH, which has three integrated detectors for diverging routes in an MSS system.

MRB-BD4X 4 Channel DCC Block DetectorIf you need more detectors with logic level outputs, check out the MRB-BD4X.  The 4X is essentially four BD1s on the same board.  However, it differs in that it has true push-pull 5V logic level outputs, as opposed to the open drain outputs of the BD1.

MRB-BD42 4-Channel MRBus-Compatible DCC Block DetectorIf you’re into DIY model railroad electronics and want a set of four block detectors that can communicate over MRBus (an open model railroad control bus standard based on a modified version of RS485), there’s the MRB-BD42.  It’s the same reliable four channel detection of the MRB-BD4X, but with a MRBus interface attached.  Using MRBus, it’s then possible to drive signals through a MRB-GIO, display occupancy status on a panel using a MRB-GIM2, or even connect it into a computer using the MRB-CI2 and MRBFS.

Finally, if you don’t want to do current detection (because, say, you don’t want to resistor-equip all those wheelsets), there’s our tried and true optical detector, the CKT-IRSENSE.  It has a very tiny sensor, is immune to ambient lighting conditions, and doesn’t need any tuning or adjustment.  It provides two open collector (D and /D) outputs, just like the CKT-BD1, but it doesn’t require gapping track or equipping your rolling stock with resistor wheelsets.

The Long, Slow Road to Get Here

So how did we actually arrive a the current block detector design?  It’s actually a rather interesting story, pointing out a number of the things that can go wrong, and explores why we made some of the design decisions that we did.  I originally had it in here, but it made this post horrendously long.  If you’re interested, there’s a second blog post covering all the lurid details of block detector evolution.

Open Hardware

As with all Iowa Scaled products, our block detectors are completely open hardware.  You can download the schematics, PCB design files, and source code from our GitHub site.   You’re more than welcome to build your own, or improve on our design if you’d like.  If you do, we’d love to hear how you’ve adapted the design.




  1. I will be using a Current Detector on a reverse loop on my layout. I will have it controlled with an auto reverser. Do I need to put the current detector before or after the auto reverser?

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