# Protecting a crossing



## lownote (Jan 3, 2008)

I made a post earlier about automating a "trolley" line in DCC. I think I will get a lenz decoder and try asymmetric DCC. The lenz decoders include cvs for speed and direction change when they encounter asymmetric DCC. Simple and cheap.

But my trolley line also crosses the mainline, and I want to be able to stop the trolley when a mainline train is in the crossing.

It would be relatively easy to do this with DC, but I want to try doing it with DCC. What I have in mind is not simple, or cheap, but it seems like it would be reliable and elegant in operation 


So I'm thinking of putting a "blockwatcher" on each of the mainline legs of the crossing. When a mainline train enters the crossing, it would send a signal to a Lenz BM2 module, one on each of the trolley legs of the crossing. The Lenz module allows for slowdown and stop, then start up again. That way, no matter what direction the trolley approached the crossing, it would slow and stop, then start again when the mainline train passed through. You can set the delay for the module, so it waits a reasonable time before starting.

Block watcher: http://www.tonystrains.com/products/type_signaling_1.htm 
Lenz BM2 (http://www.lenzusa.com/1newsite1/Manuals/bm2.pdf)


This seems like it would work. All my freight trains have a lighted caboose at the end; all my passenger trains have lighted cars, so it would be clear when the end of a train left the block. I already have a lenz decoder I can put n the trolley to test it out

QSI claims the new Titan "supports asymmetric DCC," but I have not been able to get a clear answer on what exactly this means. Doe sit include programmable CVs to control what happens when asymmetric DCC is encountered? 

Any thoughts? I'm about to order the components.

PS I realize this could probably be done more cheaply on conventional DC, but I don't want to run it that way and I dont' want to use relays to switch off power to the trolley line. With DCC, the trolley would continue to have lights and power for sound while it waited at the crossing, and would slow and start more prototypically.


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## East Broad Top (Dec 29, 2007)

Check out DCC BitSwitch's automation stuff. I know he's got a crossing protector. It's designed to work with magnets on the locos, but I can't see why you couldn't use an IR or other passive sensor that doesn't involve sticking things on your trains. From what you describe, the BitSwitch stuff does the same, or similar, thing. It senses the one train, and that triggers a stop command to the other track. He offers three types of "brake generators," one that uses the decoder's "Brake on DC" command, one that senses asymetric DCC, and one that sends a stop command. Take your pick as to which one you use. 

Later, 

K


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## krs (Feb 29, 2008)

So I'm thinking of putting a "blockwatcher" on each of the mainline legs of the crossing. When a mainline train enters the crossing, it would send a signal to a Lenz BM2 module, one on each of the trolley legs of the crossing. The Lenz module allows for slowdown and stop, then start up again. That way, no matter what direction the trolley approached the crossing, it would slow and stop, then start again when the mainline train passed through. You can set the delay for the module, so it waits a reasonable time before starting. 


Conceptually, this should work.
Do you really need a BM-2 module, would a BM-1 module not do?
All BM-2 adds I think is the option of a 'slow' speed rather than just go and stop of Bm-1 (which is just five diodes you can wire yourself - at least for test purposes.


Knut


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## lownote (Jan 3, 2008)

Posted By East Broad Top on 28 Dec 2011 06:02 PM 
Check out DCC BitSwitch's automation stuff. I know he's got a crossing protector. It's designed to work with magnets on the locos, but I can't see why you couldn't use an IR or other passive sensor that doesn't involve sticking things on your trains. From what you describe, the BitSwitch stuff does the same, or similar, thing. It senses the one train, and that triggers a stop command to the other track. He offers three types of "brake generators," one that uses the decoder's "Brake on DC" command, one that senses asymetric DCC, and one that sends a stop command. Take your pick as to which one you use. 

Later, 

K 


Kevin I did look there first. Because this is a back and forth "trolley" line, the crossing protector has to work in both directions, and the only appropriate crossing protector he has would actually be more complicated and possibly more expensive to implement--it would take eight reed switches, for example, and several other components. I'm skeptical, though I have to admit I haven't tried it, about reed switches out in the rain and snow and buried under debris. Plus as you mentioned the magnets stuck under all the locos and end of train cars.

The thing I'm thinking of trying would only have four wired components: two "block watchers' and two Lenz modules...but I'm still not 100% sure it would work! 

I did have a really well done DC-based solution designed by Dave Bodnar. The trolley autoreversed using an aristo reverser unit, and as trains entered the mainline crossing they were detected by an infrared emitter/receiver, which cut power to the trolley track at the crossing for a set time. A second infrared pair told the processor when the train had passed out of the crossing. I could run the mainline on DCC, and the "trolley" on DC. All praise to Dave, a smart, generous and friendly man. It worked very well for the most part. I used an aristo eggliner as a trolley, and it would come racing up to the crossing and stop dead just in time. Very clever, and kids loved it. But not very prototypical, and it failed if anything--grass, leaves, etc--blocked the infrared beam, and if the mainline train was moving too slowly, or was too long, the eggliner crashed into the train, then the whole thing had to be reset. As part of a general garden/Rail clean up, I took all the DC components out. I suppose rethinking it on DCC is a perfect example of "well I bought this really nice tie: now I need a really nice suit to go with it."


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## lownote (Jan 3, 2008)

@import url(/providers/htmleditorproviders/cehtmleditorprovider/dnngeneral.css); Posted By krs on 28 Dec 2011 06:42 PM 
So I'm thinking of putting a "blockwatcher" on each of the mainline legs of the crossing. When a mainline train enters the crossing, it would send a signal to a Lenz BM2 module, one on each of the trolley legs of the crossing. The Lenz module allows for slowdown and stop, then start up again. That way, no matter what direction the trolley approached the crossing, it would slow and stop, then start again when the mainline train passed through. You can set the delay for the module, so it waits a reasonable time before starting. 


Conceptually, this should work.
Do you really need a BM-2 module, would a BM-1 module not do?
All BM-2 adds I think is the option of a 'slow' speed rather than just go and stop of Bm-1 (which is just five diodes you can wire yourself - at least for test purposes.


Knut 




Knut, the BM 1 isn't triggered by a signal, as far as I know--it's just five diodes. So there would be no way to tie it to the block watcher. I believe you can program a delay into the stop, but there's no way to make the delay fit all possible trains in the crossing. As I understand it the thing the BM-2 adds is the ability to be triggered by a signal from the bloickwatcher.


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## krs (Feb 29, 2008)

Posted By lownote on 29 Dec 2011 08:58 AM 


Knut, the BM 1 isn't triggered by a signal, as far as I know--it's just five diodes. So there would be no way to tie it to the block watcher.

Yes, the BM-1 is just five diodes arranged this way:










It is normally triggered by the position of the signal.

With the signal set to "stop" the bypass switch is open, the diodes create the asymmetrical DCC signal which the Lenz decoder detects and the loco stops.

If the signal is set to 'go' (or green), the by-pass switch is closed, there is no asymmetrical DCC signal and the loco proceeds.


The whole wiring looks like this:










I assume you can tie in the "blockwatcher" and have it control the bypass switch either directly or via a small single contact relay.
Not sure what the output device of the "blockwatcher" actually is.

Or just use a different method to detect an approaching train on the main line and use that to open the bypass switch which will stop the trolley.

Knut


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## toddalin (Jan 4, 2008)

Posted By lownote on 28 Dec 2011 07:25 PM 

I did have a really well done DC-based solution designed by Dave Bodnar. The trolley autoreversed using an aristo reverser unit, and as trains entered the mainline crossing they were detected by an infrared emitter/receiver, which cut power to the trolley track at the crossing for a set time. A second infrared pair told the processor when the train had passed out of the crossing. I could run the mainline on DCC, and the "trolley" on DC. All praise to Dave, a smart, generous and friendly man. It worked very well for the most part. I used an aristo eggliner as a trolley, and it would come racing up to the crossing and stop dead just in time. Very clever, and kids loved it. But not very prototypical, and it failed if anything--grass, leaves, etc--blocked the infrared beam, and if the mainline train was moving too slowly, or was too long, the eggliner crashed into the train, then the whole thing had to be reset. As part of a general garden/Rail clean up, I took all the DC components out. I suppose rethinking it on DCC is a perfect example of "well I bought this really nice tie: now I need a really nice suit to go with it."


The Tortoise Bump Accident Sentinal System works similar except that is suffers from none of these anomolities. There are no reed switches, no light or IR beams to get blocked, speed and train length have no bearing on the system, and there is nothing to reset. And it is much cheaper.

But there is no gradual slow down, so it may not be for everyone.


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## Greg Elmassian (Jan 3, 2008)

So asymmetry of just 2.1 volts is what it takes for the Lenz to see this? Interesting, never knew the "assymetry specification"... 

I'll have to read the Titan manual... 

Greg


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## lownote (Jan 3, 2008)

Knut, I see your point now--thank you. It might have been worth it to use the blockwatcher to trigger a relay or switch the Lenz BM1 in/out. Would have been cheaper. Too late! I ordered the components. 

Dave Bodnar's systerm worked extremely well for the most part, for years.


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## toddalin (Jan 4, 2008)

Posted By toddalin on 29 Dec 2011 11:08 AM 
Posted By lownote on 28 Dec 2011 07:25 PM 

... and it would come racing up to the crossing and stop dead just in time. Very clever, and kids loved it.


If you are protecting a crossing from collision, slowing down may not be desireable or even feasible if you are dealing with tight areas and near misses.


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## krs (Feb 29, 2008)

Posted By Greg Elmassian on 29 Dec 2011 11:54 AM 
So asymmetry of just 2.1 volts is what it takes for the Lenz to see this? Interesting, never knew the "assymetry specification"... 

I'll have to read the Titan manual... 

Greg 


Greg,

The original concept was developed more than 10 years ago by Urs Meyer of Umelec, a Swiss company that alos makes DCC decoders.
They only used four diodes total or a 1.5 volt difference in the DCC amplitude.










Turned out that a 1.5 volt difference was not enough to detect the asymmetry reliably mostly because thre was no NMRA spec defining how symmetrical the DCC signal had to be.
That I believe has now been rectified and the amount of asymmetry allowed at the output of the Command Station or DCC booster is now defined.

I remember that years ago, this asymmetrical concept would not work with a few specific Command Stations simply because their DCC signal wasn't symmetrical enough to start with.

Some people in the industry are dead against that concept because according to them "It's not digital".
That is strictly speaking true, but I still think it's a great idea and serves it's purpose very well.

Knut


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## lownote (Jan 3, 2008)

Posted By toddalin on 29 Dec 2011 02:59 PM 


If you are protecting a crossing from collision, slowing down may not be desireable or even feasible if you are dealing with tight areas and near misses.


The lenz decoder has a feature called "constant stopping distance" which will stop the loco in the same space/distance no matter what speed it's going. I assuming--though I don't know for sure--that this will also work with other decoders that recognize asymmetric DCC


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## lownote (Jan 3, 2008)

I finally got this to work, after much struggle. The BlockWatcher works very well, but the Lez module is much harder to deal with. I had assumed--completely without evidence--that the block watcher could output enough voltage to drive the Lenz module, but that turned out not to be true. I had to build a 12 volt supply tied into the track to get enough voltage. But now that I have I just managed a hasty test and it all seems to work. I may have to cut out the "slow approach" feature and just go with a more abrupt stop, due to physical space issues. There probably would have been cheaper, simpler ways, although I like the "Block Watcher" a lot.



I got good tech support from Am hobby distributors. I'll test it over the next month or so and report back.


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## toddalin (Jan 4, 2008)

Posted By lownote on 10 Jan 2012 03:20 PM 
I finally got this to work, after much struggle. I may have to cut out the "slow approach" feature and just go with a more abrupt stop, due to physical space issues. There probably would have been cheaper, simpler ways, although I like the "Block Watcher" a lot.



Just sayin... 

Everything should be made as simple as possible, but not one bit simpler. Albert Einstein


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## krs (Feb 29, 2008)

Posted By lownote on 10 Jan 2012 03:20 PM 
I finally got this to work, after much struggle. The BlockWatcher works very well, but the Lez module is much harder to deal with. I had assumed--completely without evidence--that the block watcher could output enough voltage to drive the Lenz module, but that turned out not to be true. I had to build a 12 volt supply tied into the track to get enough voltage. But now that I have I just managed a hasty test and it all seems to work. I may have to cut out the "slow approach" feature and just go with a more abrupt stop, due to physical space issues. There probably would have been cheaper, simpler ways, although I like the "Block Watcher" a lot.



I got good tech support from Am hobby distributors. I'll test it over the next month or so and report back. 


Could you perhaps post a sketch how you have things hooked up?

I'm rather confused by your description.
The block watcher per the documentation doesn't output any voltage at all as you assumed, it simply provides a solid-state contact closure when the block is occupied.
Unfortunately the block watcher spec doesn't specify the on and off resistance of that SS switch which is a rather important parameter if one wants to interface the block watcher output to some other circuit.


The Lenz BM-2 module typically uses the voltage to drive the green lamp in a signal as the input to the 'clear' terminal. The spec there states that any voltage between 11 VDC and 24 VDC will generate a "go" signal and an open at the 'clear' terminal will generate the "Stop" signal.


So the logic provided by the block watcher has to be inverted to drive the BM-2 module.

I don't know what the input current requirement is for the BM-2 module but I would assume it's in the milliamp range.
For that I would have derived the power from the DCC track signal via diode bridge and use either a transistor or small reed relay to control the BM-2.

It would be nice if the block watcher could be programmed to provide reverse logic at the output, ie ss contact closure when the block is not occupied and an open at the ouitput when it is.

Knut


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## lownote (Jan 3, 2008)

The struggle, I should add, was because of my inexperience and lack of knowledge.

The block watcher's switched output can be programmed to reverse its logic--it's a straightforward CV adjustment. And yes you are right, it needed to be "reversed."


I set up the 12 volt power supply from the track, and sent the plus feed to one terminal of the switched output on the Block watcher, then from the other terminal of the switched output wired a lead to the Clear terminal of the Lenz. Then I ran a wire from the neg. output of the power supply to the Stop terminal on the Lenz. It seems to work well, but at this point, I'm not using the "slow approach" feature. I may try to implement that later, but first I want to sort the thing out in practice. There are a LOT of parameters to set--the decoder's momentum/deceleration, the Lenz "constant braking distance," and the "hold time" at the block, which can be programmed via CVs.

I don't have any software tools for drawing a good diagram, so I can only show you this silly one:

http://chnm.gmu.edu/courses/magic/bwdiagram.jpg 




Now the trolley line reverses at each end and stops at the crossing when there's a train in the way. I need to make a waterproof enclosure of some kind, and stick the whole business back under a station building


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## krs (Feb 29, 2008)

The diagram you posted is fine, but.... 
you should really reverse the two connections at the BM-2 module. 
The way it's shown in the diagram, you are switching the common side of the BM-2 module - "C" stands for common, not "clear". 
The inputs to the BM-2 are the two signal icons - the one on the right with a single arm indicates - go, full speed ahead - that's the one you want to use. 
The one on the left of the three-position screw terminal with the two arms is the signal indication - go ahead slow. 

Since you only use the go ahead full speed one, switching the common rather than the input lead doesn't matter, but if you want to use the go ahead slow input, you need to make "c" the common lead. 

BTW - don't know what you would use the go ahead slow option for - that's why I thought the BM-1 module would have been sufficient. 

Knut


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## lownote (Jan 3, 2008)

Thanks Knut--I hastily made that diagram from memory. I'll make sure it's switched as you suggest


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