Aha - I see.  In addition to the potential swing allowed by the NMRA spec, I see I was starting wrong at 12V rather than peak to peak.  I checked my LED specs and while they recommend 16-18ma they can handle up to 35ma.so I will stick to the 2K (or closest I have: 2.2K) resister.

Thanks for the explanation.

-Ed

I have one more question.  The original simple circuit to test polarity at the beginning of the thread used an LED without an additional diode.  But the turnout direction detector uses the 1N415 diode in series with each LED.  I am wondering what the purpose/advantage of the additional diode for each of the LEDs is.  Is this another peak handling situation of some sort?

Thanks,

-Ed

By reading some other articles on the web plus reading the specs for the LED versus the specs for the 1N4148, I think I figured out why the extra diode for each LED.  It looks to me like the reverse breakdown voltage is far higher for the diode than for the LED.  As a result, the diode should limit current more effectively in the reverse direction ensuring we can't draw more reverse current through the LED than it can handle.  I have seen other circuits that put a diode in the reverse direction in parallel with the LED instead.  But I am guessing this series arrangement accomplishes the purpose.  

Am I understanding it correctly?

Thanks,

-Ed

You will note the circuit connects the two rails through back to back LEDS. In theory, a backward biased LED should block current through the other LED, but in practice, an LED can leak enough current to allow the other one to light. When I tested a version without the extra diodes, both LEDs would glow a bit when the A and B clips were attached. The extra diodes prevent that.

pqe

I was so focused on the path through the resistor, it hadn't even dawned on me that there were the back to back LEDs and nothing else between the two power buses if not for the diodes.  The LEDs I have appear to be rated for reverse voltage of only 5V.  Don;' know about the ones you used, but for mine that suggests to me that the voltage difference between the two power buses would exceed the brakdown voiltage for the LEDs causing current to flow.  So that might explain your experience.  The reverse voltage rating for the 1N415 series (or my 1N4148) is well above the peak to peak between the power buses.  

-Ed

Okay but where can I get software to run a stepper motor controlled turntable.

Arden

Dear Arden,

For "software" to be required, it implies you already have some form of "hardware" in mind, or indeed in-situ...

- For PCs, there are plenty of "servo/stepper drive" apps and interfaces available
(suggest looking at the IOIO interface board and Adroid supporting apps,
or, if you want Industrial/bulletproof/overkill solutions, try the USB connected animals from SeaLevel
http://www.sealevel.com/ )

- For TI Launchpads, and Arduinos, you're into specific coding languages
(see Dr Geoff Bunza's Arduino threads on this forum, The "Search" function at top right of this page is your friend...)

- For Rasberry Pi and similar "PC on a chip" systems, you're into Linux world.

- If the "stepper motor" you are referring to is actually a commercially-produced DCC-controlled turntable,
then the DCC system you are using and the Human Interface you wish to use comes into play...
(EG for NCE systems, the "MiniPanel" can make up into a useful little "control front-end UI" for most DCC-controlled items)

However, if you're actually wanting to follow the ethos of this thread,
and "build a simple electronic circuit" solution to driving a stepper motor,
(IE no specific "software" involved or needed)

then I'd suggest checking out another of the circuits on Rob Paisley's excellent "Model Rairoads and Misc Electronics" website, namely

http://home.cogeco.ca/~rpaisley4/Stepper1200.html

Happy Modelling,
Aim to Improve,
Prof Klyzlr

​PS the question "How to create a control system for a stepper-motor-driven turntable?" really does IMHO deserve a thread unto itself... I'm sure the rank-n-file here onlist would have some useful suggestions...

I use Arduino cards for stepper control, since the code is simple and only one driver chip is required. You don't need LINUX, as Arduinos can be programmed on a Mac or PC. However, as the Prof points out, that's not the kind of thing I had in mind for these tutorials.

I do have an idea in the back of my head for a circuit that uses magnetic sensors to stop a motor at a particular point-- maybe that could be an alternative way to control a turntable.

pqe

I am planning to put SPST toggles on all of the hidden staging tracks and every storage track off the turntable.  I never thought about using LEDs to show if the track is powered or off.

I'm another one that built Heathkits and also Knight Kits and built my first ham transmitter from junk TV's a half century ago.

Ken

Pelsea:

I've enjoyed these threads and have a request for a new "simple"(?) project. Without resorting to an Arduino, is there a simple latching circuit that I can use with a magnet & reed switch to turn on an off my battery in a loco or car when not in use?

Thanks in advance!

Neil