I have a question about the Atlas signal system. I’m designing a system for my layout, and I’m in a situation where I really need to use two block detectors instead of one. I’m using the NCE BD20 detectors with the Atlas signal boards. I can’t find the answer to this anywhere on the internet, but can I connect two NCE BD20 detectors to one Atlas signal board so that if neither detector is detecting anything the system will be green and if one or both detect something it will drop to red? Will I encounter any electrical problems wiring it up this way?
Bobby,
First of all - are you using DC or DCC to run your layout? IIRC, the Atlas signal boards are for DC layouts. Normally, I would expect that either detector will trigger the signal board. The only issue would be if when both detectors are ‘on’, maybe the overall resistance will be too low and too much current will flow through the detector(letting the ‘magic smoke’ out). I think you are going to need to sit down with the NCE and Atlas manual as do some serious reading.
Jim Bernier
That should work just fine. The output from the BD20 is what is known as an “open-collector output.” Without going into the technical details of it, it operates similar to a switch that connects the output to ground. You should be able to wire these in parallel without any problems, and if either one or both are on you should get the “occupied” signal, and “clear” only when both are off.
Indeed this is correct. The BD20 uses open collector outputs so it’s perfectly safe to connect 2 (or more) to the same control pin on the signal controller. Any one detector tripped will activate the signal, and both of them active at the same time won’t hurt anything.
–Randy
O.k., good! Thanks for your replies. I thought I could do this, but I wasn’t 100% certain as the manuals to the NCE and Atlas signal components don’t address this, unless some how I’ve overlooked it. I knew you could use one block detector for two signal boards, but I didn’t know if you could do it the other way around. Thanks for your replies.
Also, one other question. When I connect the two detectors to the Atlas signal board, do I simply connect the 2 NCE logic connectors to the single DIN signal board connector (as usual), or is special wiring required to accomplish this (such as resistors, etc…)?
Just connect them together to the same pin. No extra components like resistors are needed. It likely wouldn;t be explained in either devices’ manuals as Atlas doesn’t expect you do be using NCE detectors, and NCE isn’t expecting you to connet their detector to the Atlas system. Plus the concept of 2 or more devices on the same control pin is more an issue of how an open collector circuit works more than anything. In a nutshell, open collector is like a switch that connects the output to ground when turned on. Clearly you could have a dozen switches connecting one input to ground and flip one or all of them and it would be the same - like a whole bunch of parallel wires. It gets a bit more complicated because there are transistors used, not toggle switches, but the design is such that multiple open collector circuits in parallel won’t feed back through one another and cause damage. Operating the other way, where the transistor turns the power source on and off rather than connecting the output to ground has the problem that if you hook multiple circuits in parallel they will interfere with one another. You have no choice but to add additional circuits to handle multiple inputs. But nothing to worry about with the open collector design of the BOD20.
–Randy
I’m glad to hear that it is that simple with the BD-20. I sort of understand the logic behind open collector output, but I’ll keep trying to figure it out. I’m the type of person where I always try to figure things out; it’s hard for me to accept that something just works. Of course, I am using DCC and NCE BD20 detectors, but for those who use the Atlas block detectors, could they wire 2 Atlas detectors to 1 signal board or does the Atlas detector not allow this?
I just had a thought. Is a non-Open collector circuit similar to the effect that happens when the power goes off and an individual illegally hooks up a generator and back feeds to the power company’s line, and is an Open collector circuit similar to the automatic main shut-off that prevents back feeding when the power goes off and a generator is started?
That is a good analogy. The Wikipedia entry on open colelctor explains it pretty well: http://en.wikipedia.org/wiki/Open_collector
Note that the function outputs on most every DCC decoders are open collector outputs - si it’s quite ok to connect a single light or LED to more than one function - so that it will light up if either one or both are on.
I don’t know about the design of the Atlas sensors, if they are open collector then the same type of wiring could be used of course, but I haven’t seen any details on it.
–Randy