Circuit information please

[der5997]

I have two reverse loops powered through the same Atlas Twin. I would like to install polarity indicators (either lamps or bi-color LEDs) to show where the polarity mis-matches are. I had these circuits on my first "real" layout about 20 years ago, but can't remember how I built them [:I] Any pointers would be welcome. Thanks.

[cacole]

Bi-color (red-green) LEDs would probably be the easiest to wire, since they illuminate according to polarity; i.e., if the current flow is in one direction, the green lights, and if you reverse the polarity the red lights. One problem I foresee, however, even with incandescent bulbs, is that they go off if there is no power on the track, and their intensity depends on the voltage. You're going to need a constant voltage source to achieve good results, and only DCC will give you that without a lot of very complicated wiring and relays.

[der5997]

Thanks, cacole. I can live with the no power, no light thing, especially with LEDs since they'll come on very shortly after the trains begin to move.
What I need is the circuit. Any ideas?

[locomutt]

Might want to look at the circuit using the opto-isolaters.

[cacole]

The absolute simplest circuit you can get by with since you don't care about the LEDs going off when there is no power on the track, is to wire a bi-color LED through a 1K Ohm resistor directly across the rails. Changing the polarity of the track will change the color of the LED to either red or green. The resistor is needed only to limit the current that can pass through the LED and protect it.

[der5997]

That's what I thought, cacole. I tried it with aligator clips and hook up wire with a 1K resistor. I couldn't get the LED to change from red to green without moving the wires to different track. That's why I started the topic.

locomut, do you have an opto-isolator circuit diagram?

Thanks for the replies.

[cacole]

I forgot to mention that wiring the LED across the rails and the color changing when the track polarity changes will work only with a two lead LED. If you are trying to use LEDs with three leads, try to locate a two-lead one and try it.

All Electronics, for example, has both types. A T-1 with three leads is 35 cents, and a two lead one is 50 cents. This is still a lot cheaper than trying to connect opto-isolators, relays, or other circuits.

[der5997]

cacole: I'm using 2 lead LEDs. In some configurations the red light works, but not the green when the polarity is switched. In other configurations it's the other way around. In no case have I been able to get red on for one polarity, and green on for the other. Maybe I haven't tried all the possible combinations yet. I'm still puzzled as to why the LED lights in one polarity and doesn't in the other, since they are bi-color.
Circuit anyone?

[cacole]

Green LEDs need more current flow than red LEDs, so you may need to cut down the value of the resistor in increments until you find the right combination. As a first attempt, put two 1K Ohm resistors in parallel, which will cut the resistance to 500 Ohms, and see what happens. You can't do any more harm than burning out an LED, no matter what. It may take quite a bit of fiddling until you find the resistor value that works best. Another possibility would be to use a potentiometer as a variable resistor until you reach a point where both red and green portions of the LED light adequately, and then measure the pot's resistance to determine the value of resistance needed.

[der5997]

Better yet, I'll use the gizmo from Loy's Toys that the 1K resistor I mentioned above is housed in. It has 820, 680, 560, 510, 430, 360, 300, 270, 220, 180, and 120 Ohm resistors all in the one box. The one I have came as a kit and is really easy to put together, and a boon to own. Thanks for the tip about green LEDs. I had no idea there was a difference in resistance. Both the red and green sides of the bi-color light on 1K Ohms, however. Still, I'll keep working at this. What really bugs me is that I had the system up and running in the 80s, with 12 volt lamps in a track diagram. It was very useful, but how did I wire it up?????[banghead]

[BR60103]

DER: try connecting the LED+resistor across the insulated rail joiner at the end of your reverse loop -- probably in the outside rail. If it lights up backwards, reverse the connection. If you have all-rail frogs, you might get an interesting effect between the frog rail and the loop.

[der5997]

Thanks David,
Here's what happens. On the outside rail. At the insulated join where there is matching polarity, the bi-color LED does not light in either orientationof the leads. At the other end of the loop, with the bi-color LED connected across what I know is oposing polarity, the GREEN lights, but only when the train is going in the direction of travel that took it through the matching polarity join at the other end of the loop. If I change the direction of the train, the green side of the LED goes out. In no case does the RED light at all regardless of the orienation of the leads!!!! [banghead][banghead][banghead]
When I'm DCC, (which is yet to be purchased, but for which the layout is designed) the changes of direction will not be made through change of polarity on the track, so will not be an issue.
Further suggestions welcome.

[cacole]

Okay -- last ditch effort -- use two different LEDs of RED and GREEN instead of a bi-color one. Then the resistor values can be different for each, and they should work OK. If a LED doesn't light, then reverse the polarity. Individual LEDs are easier to work with because you can easily determine their polarity by looking at the leads -- the positive lead is longer than the negative. Only one should light for each polarity. Are you absolutely sure that you have power on the track when you are reversing polarity? Only one color of the LED lighting suggests that perhaps something is not wired correctly and you don't have power on the track in one direction.

[der5997]

cacole: I'm beginning to think that the bi-colour LED I have (last of a bunch I bought for block occupancy detection, where they work just fine) is a dud. Like the littlest Who's christmas tree bulb, it won't light on one side ![}:)] I'm going to try the individual LED approach, get that working, and then convert to bi-color LEDs when I can next get to the electronics store (LES?)
Thanks for all for your concern.

[der5997]

I think we can close this off now. jwmurrayjr has solved the problem in another way in the topic dcc revesing loops. Thanks for all your contrbutions and suggestions.[:D][:D]