Lighted Turnout Indicators 

by David Barron

For better than 20 years, I have built or helped build several model railroad electrical control panels. At first, these ended up very simple; later ones ended up somewhat more complicated. During my latest venture, I wanted to install something with a little more zing, and yet foolproof, simple, and inexpensive. The thought of having illuminated turnout position indicators allows you to tell at a glance if you are lined up or not. The idea of having this illumination, rather than toggle switch position, appeals to everyone.

Several commercial illuminated switches have been offered, which at times were mechanical nightmares or complicated wiring nests. If anything ever went wrong, as it always eventually does, simplicity is a must.

Light emitting diodes (LEDS) seem to be the answer. The following circuit will work with any system that requires a constant power supply to the switch machine (i.e., Tortoise by Circuitron -- not solenoid or momentary contact circuits).

Figure 1 shows the minimum components required for this circuit; for each indication required they include 1 1K Watt Resistor and 2 colored LEDS, the colors are your choice. Optional pairs would include 1 plastic LED holder per LED in the control panel. All of these parts are readily available at all electronics stores, including Radio Shack, or your local hobby shop by Miniatronics. Miniatronics has an 18 pieces set of red, yellow, and green LEDS with resistors all in a single package (Model 12-050-18) for about $7.50. That is enough to do at least 5 position indicators and have some parts left over.

Figure 1. Components for circuit.

My requirement was to color code the LEDS so that Green would indicate the switch was set for the main line operations and Yellow would indicate the switch was set for the siding. A quick glance at the control panel was all that was needed to tell if all switches were properly aligned prior to main line or Green Light operations.

Here's a quick note or explanation as to the functions of these two simple components for those who are unfamiliar with them. Resistors come in many sizes and shapes but they all do the same basic thing, they limit current. They operate like a funnel and only allow so much current to flow through at a time. They are color coded with different bands of color to indicate their different restricting values. You can consult any electronic book for your favorite memory code jogger, or visit http://www.dannyg.com/examples/res2/resistor.htm for a Resistor Calculator.

Diodes also come in many sizes and shapes.  They also come under different names depending on their function, the ones that we want are known as Light-Emitting Diodes, LEDS.  These come in three basic colors: Red, Green, and Yellow.  An LED works, or lights, when a current passes through it in one direction only.  That is the way a diode works, it only allows current to pass thorough it in one direction and not the other way.  Kind of like a one way check valve.  By positioning the LED the way we want we can control its function much like a one way electrical valve and see the valve position. 

The reason we are using a resistor in this circuit is that we are using the 12 volt DC poser supply common with most power packs.  We want to drop the power down to a value that will not damage the LEDS.  The LEDS are used in opposing positions so that as we change the current polarity, by throwing the toggle, we reverse the current through the LEDS, so it cannot go through the lighted LED anymore but it now goes through the LED not previously illuminated. 

Although there are exceptions to every rule, I have never found an LED that didn't have one long leg and one short leg.  The proper names are the cathode lead and the anode lead, but I'll refer to them as the short and long legs.  There are a few things that you must remember in order to correctly get this circuit working. First NEVER attach the LED directly to power without the dropping resistor or it will go pop and like a match it won't work a second time.  The second thing is that these LEDS must operate opposite of each other or they will both light together or go out together, not what we are looking for.  Lastly, use heat sparingly as excess heat when soldering can damage the components. 

If you are using a double pole, double throw (DPDT) toggle switch, wired up as Linn Westcott indicates in his book, How to Wire Your Model Railroad, to reverse current, then this circuit can easily be added to your existing system.  Figure 3 shows how the back of the DPDT Toggle should be wired.

Now, Let's get started.  Figure 2 shows the basic circuit that we are about to put together.  First, locate where you can't the LEDS located on your control panel.  I put mine next to the respective toggle switches in a vertical line.  This allows me to take a quick glance down at them to make sure everything is lined up.  After drilling the appropriate holes in your control panel, (I found that a Masonite hard board is perfect for the job), you can either glue the LEDS directly in place or use the special plastic snap clips to mount them in.  I like the plastic clips in that it leaves a nice finished black ring around the LEDS when viewed from the top and it makes them easy to replace should you have to.  Now, solder either end of the 1K Watt Resistor lead to point A on the toggle and the other end the short leg on one LED and long leg on the other LED.  Next, solder one end of the wire to point B on the toggle and the other end to the remaining two LED legs.  Turn your switch machine power on and you should have one LED lit and the other one off.  Flip the Toggle and the lit LEDS should switch illuminated positions.  If the LEDS both go on or off, at the same time when you flip the toggle switch, you simply have one in backwards.  Unsolder one and turn it around and test it again. 

Figure 2. Double pole double throw toggle switch.

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