Signals for your layout?

There are a number of commercially available signal systems. You can also by off the shelf components and cobble together a system. Or you can get really ambitious and build your own, which is the route I am going. Why build my own? Well, the big reason is I don’t want a system dependent on a computer with all the complicated interfaces involved. And I want something simple enough for me to figure out what the problem is if it stops working. This last point was driven home when a friend om mine’s system stopped working. Unfortunately the fellow who installed it for him is no longer with us and no one knows how it works, much less how to fix it. So the dead signals just sit there and the lights in the control panel are all dark.

When it comes to signals, there are a couple (and probably more I don’t know about) ways to light up your lights. If you have plenty of electricity you can keep the lights on 24/7. However a lot of railroads operate their signals off battery power. Keeping the lights on all the time can be a drain on the batteries so what other option is there? Approach lighting is the answer.

With approach lighting systems, signal lights are only turned on when a train is in a block. That way the engineer of the train in the block has a green light for the unoccupied block ahead, a red light for his block, and a yellow light for the previous block to warn approaching trains. As trains proceed down the tracks the lights go off in the preceding blocks to save battery power.

This sounds like it would be complicated to implement on a model railroad, but actually there is a neat circuit designed for it. In the March 2001 Model Railroader, Jeff Scherb had an article describing this circuit and providing diagrams for those wanting to make their own boards. His design was based on one that Wayne Roderick published in MR back in June 1972, so this circuit has some legs! So how does it work?

Jeff’s version was designed to work with an open collector transistor detector like Bruce Chubb’s, and most of the current crop of occupancy detectors work that way. Basically this is a transistorized switch that is on when a block is occupied, and that turns on the signal logic circuit. I am using NCE BD-20 detectors, although they did require some special wiring. I am planning an article later this year in my column on this setup.

The great thing about Scherb’s circuit is it requires only 2 transistors, 6 resistors, and a zener diode to work, so instead or spending a bunch of time and money making circuit boards I just soldered them together on a piece of perfboard. For my entire layout I only need 22 making it an easy option. It operates off 5, 12, or 15 volts DC and is compatible with inexpensive signals like those from Oregon Rail Supply. Wiring is minimal, requiring only two wires between each board, a detector input, power(+12v), and 3 wires to the signal LEDs. For those of you with a copy of my wiring book the circuit is shown on page 91 in figures 11-13.

The most important feature for me is I can install a three aspect signal system without the added complications of using a computer. Of course I won’t be able to provide my dispatcher with a touch sensitive computer control panel but it is also easy to build, install, and maintain, so it is likely it will operate in my lifetime. And if they don’t work reliably they will come out and we’ll switch to complete dispatcher control because the only thing worse than no signals is ones that don’t work reliably.

8 comments

  • Hey Larry,

    Quick question would the LED outputs on a Digitrax BDL168 be strong enough to drive Scherb’s circuit? I’m thinking about how to implement simple signaling while preparing for more complex computer control.

  • Thanks This post peeked my interest. I had a crude 2 aspect signal system on a layout I built in the 90’s when I lived in Tennessee that I built from plans in a book put out by Kalmbach in 1979 written by Peter Thorne. It sounds very similar to the one you are using. There was also a 3 aspect system in the book. I used reed switches and magnets to activate the signals. It works great and was fairly easy to build. Now I have a double track layout and wonder how to go about adding a signal system with bidirectional running. But I guess I could do it. Need to do some homework.
    Thanks for the post.

  • The same scenario happened in my operations group. The fellow was a computer genius and wrote the software himself. Now it doesn’t work anymore because he is in bad health and can’t remember how he designed the program. He has Alzheimers now and sits with us when we operate. A real gentleman, and a good friend. So having a more simpler way of having signals is a good idea. I will be following along with your progress. Thanks Larry.

  • Way back, before I discovered CMRI, I designed and built my own 3-color ABS system using 7400 Quad NAND Gate ICs. It worked, was simple , and parts were cheap (although prone to noise as it was pre-DCC ) Once I read the article on CMRI, I made the change and have never regretted it. Using JMRI made it work with other components. Just my $.02 Great article

  • Hi Larry,

    Quick question. If you’re using DCC compatible occupancy detectors doesn’t that mean that you effectively have DCC detection in parallel with your simplified signal operation? So that way if you wanted you could explore opportunities with computers, DCC and digital control panels all while your signals keep operating? Or am I missing something?

    Thanks,
    -R

    • Yes, and NCE sells an interface for use with their system. There also is Bruce Chubb’s CMRI and Dick Bronson’s RRCir-Kits designs. Azatrax also offers a computer interface for their system.

  • This Signal system is great for three aspect signals, but I don’t see how to make it work with
    single aspect 1950s targets using red/green LEDs. Any suggestions you may have discovered
    in your research that look promising for older signaling which need a yellow created? I looked
    at computer options and found it complicated to get working without an installer with experience.

    • Joel—the traditional method is to use a circuit which allows you to turn on both the green and red simultaneously giving a sort of yellow color. There probably is a variation of Scherb’ circuit floating around that does that but I don’t have one. For a commercial source check out Azatrax.com.

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