Using Signals To Indicate Reverse Polarity : LUSENET : Wiring for DCC : One Thread

I have created a reverse loop on my DCC layout leading away from and back into a double crossover. When I wish to pass trains straight through the double crossover, I flip a DPDT switch controlling the double crossover and maintain normal polarity. However, when I wish to have trains crossover from one track to another, I flip the DPDT switch in the other direction and flip a second DPDT switch to reverse polarity before the engine leaves the reversing section. (Someday, I have to get with it and install auto-reverse modules on either side of the double crossover, but for now...).

My question is this: I would like to install a Tomar signal that is green when normal polarity occurs and red when reverse polarity occurs. The tech support guy at Tomar tells me that the LED will not light up under reverse polarity conditions. Is there an easy way to indicate reverse polarity with LED signals?

-- Rich Jurgovan (, January 31, 2005


Don Vollrath has been nice enough to produce some circuit drawings which are now on my website.

-- Allan Gartner (, February 17, 2005.

Yes. Remember that there is a form of AC voltage on DCC powered rails. Wire the LED AND a current limiting resistor (680 - 1,000 ohms, 1/8 w), AND a standard diode (for reverse voltage protection of the LED) in series. Then connect that series string of components across one of the rail gaps defining the boundary of your revering track section. (yes, on the same rail) Now when there is opposite polarity at that gap, the LED will be ON. If the track polarity is the same, (not reversed) the LED will be OFF. So use a red LED, and make sure the loco engineer doesn't pass it. You can even add a second LED, wired in series with and in the same direction as the first, to have the red signal show in both directions.


-- Don Vollrath (, January 31, 2005.

Gulp. Not being an electrical engineer, I had a feeling that I wouldn't fully understand the answer. Let me see if I understand it. Once the LED, resistor and diode are wired in a series, you solder the string of components right across the rail gap from the reversing section to the non-reversing section? Just on one rail?

Also, Don, you mention that if track polarity is the same, then the red LED will be off. In that case, how do I get the LED to turn green, or will that happen automatically?

Thanks for your help.

-- Rich Jurgovan (, January 31, 2005.

Use the resistor-LED-diode series connection (separate for each LED) and... Red LED - connect across the gap on the same rail. [will light when there is opposite voltage across the gap.] Green LED - connect diagonally across the rail gaps. [will light when 'normal' power is available from rail-rail, across the gaps between isolated sections.]

Actually if you are careful you can get by with only one resistor. Connect the resistor to one of the rails of the reversing section. Connect the cathodes of both LEDs (flat side or shorter lead) to the other end of the resistor. Connect the cathodes of the diodes (the banded end) to the other lead of the LEDs. Connect the other end of the diodes to opposite rails of the mainline track. you can double- up on the signals by adding another set of LEDs in series with the first.

-- Don Vollrath (, February 01, 2005.

I appreciate your help, and I will follow your advice.

I do have two other questions about wiring the Tomar signal. Although I only want to use the red and green portions of the LED, the signal comes equipped with a red-yellow green LED combination. Can I safely ignore or disable the yellow portion of the LED? Also, never having worked for a railroad, let me ask a dumb question: when would the yellow portion of the signal light be used?

-- Rich Jurgovan (, February 01, 2005.


When you mention using a "standard diode", are there any particular specifications that I should look for when shopping for the diode.



-- Rich Jurgovan (, February 01, 2005.

Moderation questions? read the FAQ