Re: NCE BD20 block occupancy detector

I just bought my first Lenz LB100 detectors and discovered that they are the voltage drop type. Most disappointing.

Scott,

There is no delay when the block is clear. The BD20 is simply a block detector and leaves the complexity of what you are doing beyond that to you.

There are a number of simple timer circuits based on the 555 timer.

Take a look at Rob Paisely's page for some:

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The inductor method may work for DCC but you then have the problem that there is a minimum output from the inductor due to the capicatance of the wiring and track to deal with. This can easily swamp the load placed on the circuit by a car. If you are really worried about voltage drop, use Schottky diodes as they give a low voltage for the drop across them. I'll note that the drop of a diode can be compensated for by running a slightly higher voltage if you wish to run your trains at 100mph plus speeds.

-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works evevery time it is tried!

Excellent, this looks like there may be some "adaptable" circuits I can use here. Thanks for the link!

Scott

Bob, thanks for the advice. I'm just experimenting with different BODs now. Looks like I can definitely build the diode voltage type with no problems, but I really don't have a good way to make the circuit boards. I'm attracted to the BD20s from NCE because not only do they not affect the track voltage, there are (relatively) inexpensive.

I'll definitely play around with them in a variety of tests, including a few long blocks to test if they can be adjusted adequately.

Thanks, Scott

Rick, thanks for the feedback. Could you elaborate a little bit more on how the BD-20 "doesn't work as well with the ISS signal logic...". Are you getting false readings? Is it an actual problem with the detector, or with the signal circuitry (I haven't decided yet whether to purchase or build my own signal circuitry)

Thanks >

Sorry to say this but the NCE block detector will not work at all on a DC layout. The NCE uses a current transformer. Transformers only work on a changing current, ie AC.

As such it does drop a modest amount of AC voltage but this is particularly unimportant for DCC circuits.

Now if you impose a 5-20Khz signal on your DC rails you will not only get the NCC detector to work but will also have the option of constant lighting. You should probably look at some very old MRs for details on constant lighting circuits.

Best Regards,

Ken Harstine

[This followup was posted to rec.models.railroad and a c> Hi, was just curious if anyone has experience using the NCE bd20 block

Everyone, I wasn't very clear but I definitely am using DCC on my (yet to be built) layout, so the NCE block detector should work for me.

That being said, it definitely appears that the NCE does not have any sort of "debouncing" circuitry built in. I'm still trying to find a LM555 circuit (suggested earlier) that works well. If I come up with anything, I'll definitely post it. Also will definitely let everyone know what I find with regards to current "leakage" from the ballast and glue, etc. making it difficult to detect a true signal from the background noise.

Please let me know if anyone has an already designed circuit that would do the following:

1) give a several second delay after last detection event before saying the block is unoccupied

2) if it receives intermittant "occupied" signals (i.e. dirty track), would essentially "reset" the timer to start counting down again.

Thanks! Scott

Scott,

I don't know what your long term plans are but, if they include computer control, Railroad & Co. layout operating software allows you to set block detection, train ID and signal timers at the software level.

Regards,

Britt Harrington Miami, Florida

There was an article in an older MR, something like ODDCC or similar. It was a redo of a Dr. Chubb circuit using current transformers. Had a mother board on which a dozen mounted. I have these 12 and the mother board. They do have some delay but I don't remember the details. You might try the magazine index search and see if anything pops. I think I saved the site but can't find it at the moment.

I am using the DCCOD to detect my layout. I am in the process of installation now. 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

Dr. Chubb said it all . As I mentioned before I have some other detectors for testing but have installed the DCCOD on the new layout!

Well, after weighing my options I think I'm going to go back to looking at the DCCOD instead of the NCE bd20.

I can purchase the kit form (includes all components) for $14, when the NCE bd20 runs for about $11 - $12.

The tradeoffs I weighed were:

Pros of DCCOD:

- includes delays to filter dirty track problems (definitely want this)

- easy to adjust sensitivity due to built in LED and potentiometer

- seems to have quite a few users, plenty of resources to get help

Cons of DCCOD:

- more expensive ($14 block instead of $11-$12 a block, but after I added "debounce" circuitry to NCE bd20 would probably cost about the same

- have to assemble from kit myself (would cost $20 each to have them pre-built. Will try to put 1 or 2 together and then if they are really tough may pay the $20)

In any case thanks everone for the feedback! Scott