by Bill Denton

Almost nothing on a model railroad grabs your attention more than seeing a working Mars light on a locomotive as it comes down the track. Well, except for maybe several locomotives with Mars lights beaming, on adjacent yard tracks ready to head out on the mainline. In any case, it really is an impressive sight. It’s also one of the questions I get asked most frequently at train shows. “What do you use for the Mars light?”

E5 with LED Mars light

CB&Q E5 with LED Mars light

Click here to watch E5 with LED MARS light video

Actually there are several answers to this question. It’s quite a simple matter to use the lighting functions available on most decoders to set up a Mars light and other effects. The only real question is whether to use lamps or LED’s. Initially I was using lamps. However more recently, I have switched over and started using LED’s. Here are a few items to consider:

1) In my opinion, lamps create a better range of light, more smoothly ramping brightness to darkness, for simulating MARS lights.

2) Lamp life is short compared to LED’s. Using the correct resistor and turning off lights when the engine is idle will greatly help extend lamp life, but be warned that you will need to replace the lamps at some point in time.

3) Another problem with lamps is that voltages vary from one DCC system to another. In other words, lamps that look fine on one DCC system may burn brighter or dimmer on another system. This is not a problem if you only run locomotives on one DCC system.

4) Lamps and resistors can produce more heat so use caution or the engine shell can be damaged.

As you can see, LED’s longer life and better consistency across multiple DCC systems has lead me to use them more in my new installations. In practice, since I still like the look of the MARS effect with lamps, I have found it useful to use a combination of both methods when I run back to back cab units (for example a set of A-B-A F-7’s). I install lamps in one A unit and LED’s in the other A unit. This way, I’m covered if a lamp burns out at a train show or I run on another DCC system and the lamps are too dim or too bright. I’ll show how I do both installations for those interested. Let’s start with lamps.

Lamps in cab units

Various cab units with lamps installed.  Kato F unit, Intermountain F-7 and Life-Like C-Liner.

Click here to see video of MARS lights using Miniatronics lamps

Working With Lamps

The lamps I recommend are Miniatronics 1.5 volt 15mA 1.2mm bulbs. Circuitron makes similar lamps but they are slightly larger (1.4mm vs. 1.2mm) and cost a little more. I buy the 10 piece package item # 18-001-10 since it’s more economical and I like to have some spare lamps around.

Next on the agenda is choosing the appropriate resistors. According to my calculations, the correct resistance on my Digitrax system should be 350 ohms. However the closest resistor Radio Shack stocks are 330 ohms. This is close enough and you might have to go up or down in value to adjust the bulb brightness to your liking. The next lower and higher value Radio Shack stocks are 270 ohm and 460 ohm resistors. A better option is to order on-line from Mouser as they have a better selection of resistor values. My calculations also say to use 1/2 watt resistors with the Minaitronics bulbs and that’s what I recommend as the 1/4 watt resistors will get hot. Having said that I have gotten away with using 1/4 watt resistors in the past because it was what I had available at the time and I didn’t know any better. It can also be difficult to squeeze two of the 1/2 watt resistors inside the loco.

Lamps connected to DZ123 in Kato F unit

Lamps connected to a Digitrax DZ123 decoder and installed in a Kato F unit.

Wiring diagram for lamps to decoder

Connecting the lamps to a wired decoder is pretty straight forward.

Wiring the lamps to solder in decoders is pretty straight forward. A resistor needs to be connected to each of the white and yellow wires. One lead of the lamp is then connected to the resistor while the other lead is wired to the blue wire, which is the common for both lamps. (see diagram and photo) Note: Not all decoder wires are shown in the diagram.

Drop in decoders are a little different but still require resistors with the Miniatronics lamps since the ones on the board are the wrong value. For example, on the Digitrax DN163I1c decoder for Intermountain F3 and F7 units, instead of soldering the resistor lead to FOF+ and FOR+, it must be soldered behind the resistor and not to the pad in front of it.

Lamps installed with a drop in decoder. DN163I1C in Intermountain F unit.

One last step before inserting the lamps into the headlight housings is to install a bezel. A bezel is a small white metal casting that acts as a light reflector. They can be obtained from Richmond Controls. The bezels are glued inside the headlight housings on the shell and hold the lamps in place. Remember to leave some excess wire on the lamp leads to make it easier to insert the lamps into the bezels. Once the lamps are in place, the mechanism can be fully placed back into the shell. The lamps may have to be pushed back into the bezel slightly after the shell is installed so they don’t protrude too far out of the shell. They should stay put without any adhesive especially since the shell should remain removable in case of any mechanical trouble or to easily replace a burned out lamp. Note that on Kato F units I had to grind away some of the frame that was directly behind the lower headlight to allow enough clearance for the lamp to be adjusted once it was inside the bezel.

Richmond Controls Bezels

Richmond Controls bezels installed in the headlight housings.

The only missing piece of the MARS light puzzle is to now program the decoder CV’s to use the MARS function. This will vary depending on the brand of decoder you use and which housing you want for the MARS light and which is the headlight. I also like to set up the bottom headlight for Rule 17 dimming. Programming these effects on all the different brands of decoders is beyond the scope of this article. I would highly recommend using a program such as JMRI Decoder Pro to set up the lighting functions. Decoder Pro not only makes it easy to set the correct CV’s for the lighting functions you want but it also allows you to experiment and try different effects almost effortlessly.

Working With LED’s

TCS decoder in Life-Like mechanism

TCS decoder in Life-Like mechanism. LED's attached with magnet wires. Note:Resistors are covered in heat shrink tubing and glued to motor side.

I think most people are going to want to install LED’s for reasons discussed previously. A good place to begin is to visit the Ngineering web site. I purchase most of my supplies from them. They also have some great video showing off their products and lighting effects. The LED’s I use are their N1022 Super-incandescent 2 x3 LED’s and also N1018 Micro Super-incandescent LED. You’ll also need some magnet wire to solder to the LED’s. This will be the hardest part of the installation. Prewired LED’s can be purchased from Richmond Controls if you aren’t comfortable soldering the magnet wire to the small LED’s.

LED's inside E5 shell

LED's inside E5 shell

Once you have LED’s with the leads attached, they can be connected to the decoder. I prefer to use TCS decoders because they seem to work best when using LED’s for MARS lights and for LED dimming. Finally, resistors are not needed if you use a drop in decoder. However remember to use resistors if you use one of the solder in decoders. The LED’s are pretty bright and I haven’t found it necessary to install the bezels that I use with the lamps. Instead I have been gluing the LED’s directly to the back of the headlight lens. This works well but requires painting all sides of the LED’s with some black paint to prevent light leakage into the cab. It also requires longer wire leads to the LED’s so that the shell can easily be removed. This isn’t much of a problem since the magnet wires are very thin and can be tucked into the shell.

TCS Drop in decoder with LED's.

TCS drop in decoder board with LED's. No external resistors are required. Note that the LED's can be mounted on the mechanism but they must be exactly behind the shell headlight openings.

There you have it. I think that about covers the topic to the best of my ability. I fully believe that adding the lighting effects puts the icing on the cake and will definitely set apart your cab units from the rest of the crowd. You’ll also find out quickly at how well they tend to draw a crowd!

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