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Review of the DCC Specialties Block Watcher

Block Watcher

by Don Fiehmann

I grew up living along the SP mainline right-of-way. Signaling was part of my railroad experience. When I started to build my layout, one thing that I had to have was working signals. This was way back in the 1970's when little was available in the way of detectors and signal circuits. In the first section of the layout I designed a detector using a reed-switch mounted in a coil of wire. By the time the second section of the layout was built an enhanced version of Linn Westcott’s Twin-Tee was used for detection. This all worked fine with DC, but when DCC was connected to the rails, the old reed-switch detectors would not work at all! The more sensitive Twin-Tee worked too well. They would turn on even without a train in the block.

Ways to detect a train

There are a couple of ways to detect the presence of a train. One is using a photocell under the track, another is to detect the current to the rails. The problem with photo cells is the light level in the room can’t be reduced without affecting the status of the detection circuit. I’ve even heard of a case where sunlight that leaked in to a room would cause block detectors to fail to detect a train. I prefer current sensing like the Block Watcher. Current sensing is the way the real railroads use to detect a train in a block.

Conversion to DCC

The older reed-switch detectors were replaced with NCE detectors that were modified to drive the relays used in this section of the layout. The Twin-Tee detectors were desensitized by adding resistors across the inputs. This modification was not entirely successful. Needless to say I have been looking for a replacement for the ageing Twin-Tees that would be more compatible with DCC.

When I had a chance to look over the Block Watcher (the intelligent detector) I found that it would handle my simple setup and also some very complex applications. Many of the features of the Block Watcher are programmable to match a layouts requirements. One of the features that makes it easy to install is that the Block Watcher is self powered an does not need extra power. The two DCC wires in and two out to the block are all that is needed. The Block Watcher is all solid state and does not have any relays. The detect (switched) output is optoisolated from the rest of the circuits and can switch up to 1 amp AC or DC and 30 volts maximum. This means you do not have to be concerned about the polarity of this output connection. The Block Watcher has a very low voltage drop with a resistance of only 0.060 ohms. It can handle currents up to 20 amps. With this hi current and hi sensitive detection the Block Watcher should work with any scale, Z to G.

One of the problems with the Twin-Tee decoders occurred in the winter with wet weather and wooden ties. The ties would absorb up some moisture and would then show the block as occupied. The Block Watcher avoids this by automatically adjusting to any leakage current up to 130 milliamps each time when it is powered up. The amount of leakage current can also be set by programming CV54.


The Block Watcher can also be turned on (occupied) or off by sending it an accessory command. The address can be programmed in the range of 1 to 2044. This is handy for testing out the signals or for a dispatcher to set a block as red to control train traffic.

There are some CVs that can be modified to further enhance operation. CV53 can be set for turn on/ turn off delay. Default is 32 milliseconds. CV54 is used to set the value for the leakage current if the automatic setting is not used. CV55 sets the amount of hysterisis, default is 1. This prevents the detector from fluttering when the sense current is near the trip level. CV63 can set all of the values back to factory settings.

On my layout I replaced the old Twin-Tee detectors with Block Watchers. The DCC block wiring was connected to the Block Watcher connector. The layout is common ground so only the two input connections were needed and a single wire on the output (pin 1) to feed the block power. The switched output was connected with one wire to the common ground connection and the other to the output line of the old detectors. The old detectors also required a 12 volt connection that was no longer needed as the Block Watcher is self powered. As I have been converting the layout to DCC I have found a lot of wire has been removed from the layout.

One important thing to do is mark the block name or number near the Block Watcher so it can be identified later. A felt tip pen works well to mark the location the Block Watcher is connected to.

When the layout was powered up the Block Watchers all performed correctly. Now my three position semaphores work the way the should. A single caboose with lights on was detected.

TrainMaster 4809 is coming down the grade
and about to pass a pair of semaphores.

Current Sensitivity Settings

To test the sensitivity I put a 350 ohm resistance on the rails and the BW turned on. Using Ohms law that would be a current of about 40 milliamps. This was not right, the sensitivity should be a lot better than this. I did a little checking and determined that I had set the jumper on the board to use the CV value and not the automatic leakage detecting. When the jumper was moved and retested the detection was greatly improved. The Block Watcher would detect with 5.1K Ohms. That would equate to a little over 2 milliamps. I have some freight cars with resistors on the wheels. The resistance is 20K Ohms for each axle. The Block Watcher would detect with just six axles on the rails. Passenger cars have the Miniatronics LED light bar and a single car would detect OK. When the layout is powered up and the Block Watcher is set for automatic it will set the level of detection to just slightly higher than the current flowing to the block. That residual current can be as high as 130 milliamps. If you have a locomotive or lighted car(s) setting in the block and the current is not very high (less than the 130 ma) the Block Watcher will set this current as the off setting and ignore to detect the equipment in the block. If you have blocks where equipment idle, then you can use CV54 setting to avoid this problem. To determine the right CV setting on my layout I removed all equipment from the rails and started with a value of 10 in CV 54. All blocks came up unoccupied. Next the value was lowered to zero, it was still working correctly and gave me the best sensitivity. This coming winter the current level may have to be raised if there are any false detections. Most of the blocks out on the mainline were set in the automatic mode as these blocks are normally unoccupied. Here’s a hint on setting a number of CV54s a one time. Power down, set the jumper to program on all the Block Watchers that you want to change. Power up and issue the command sequence to set CV54. This will program all the Block Watchers with the jumper set to program. Power down and move the jumper back to the run position.

To be able to test out my signals I programmed each Block Watchers with an address. (Be sure to pick a range of addresses that do not conflict with other devices on the layout.) This allowed me to set a block as occupied to test the signals by using accessory command from the hand-held cab.

When programming the Block Watcher you need to turn the layout power off each time you move the jumper. Here is a tip if you are using an NCE system. Layout power is turned off when you go into program track mode. You can set the program track mode with the cab and then change the jumper. This allows you to work at changing the setting without having to run back and forth to the power switch.

When programming with Digitrax systems you need to disconnect the Block Watcher when you power up. The Digitrax system broadcasts accessory commands to the rails as it powers up. Power up and after 10 seconds connect the power and then do the programming.

False Trigger Solved

I did have one very usually problem where one block was turning on intermittently when a train was in another block. There was no wiring connection between the two blocks. The puzzle was solved when I looked at the Block Watcher and noticed that the power wire feeding the block, that had the false indication, was routed just above the top of the small transformer on the BW. I moved the wire an inch or so and the problem went away.

Block Watcher with Relays

If you are connecting the Block Watcher to relays you need to add a diode, like a 1N4001 or 1N914, to the relay coil. This will prevent the voltage kick back that occurs when the relay is turned off from damaging the Block Watcher output device.

Block Watcher with PSX-AR, PSX-1

If you need detection with a block connected to a PSX-AR auto reverser/ breaker, the Block Watcher should be connected to the output of the PSX-AR. The PSX-AR will change the phase (polarity) of the DCC, but this will not affect the Block Watcher. Since the Switched Output is optoisolated there is no problem with the phase reversal. If you install the Block Watcher before the PSX-AR current used by the PSX-AR may look like an occupied block.

I have three tracks in a reversing loop. By having the Block Watcher connected to the loop I can select one of the tracks and “see” if it is occupied using the remote LED.


If you need to detect equipment in a block, regardless of scale, the Block Watcher is certainly the best choice presently available. The flexibility and reliability of the USA made Block Watcher has made it the choice of professional layout installers.

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