The On-Line Magazine of Rideable Model Railroading
  NUMBER 111


© June 25, 2008   

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A Simple Signal System



Written by Kenny Linton


Single Direction Block Signal Using Four Common DPDT  (Double Pole Double Throw) Relays

Universal Single Direction Block Signal

Relays have been used for railroads since the late 1800's. Today, they have been replaced with transistors. I like relays because they are easy to design circuits around, troubleshoot, and replace.

When the Ozark Live Steamers needed a signal system, I had several problems to overcome. I thought I could do it the way railroads had done for decades. I designed a relay system, insulated a section of track, drove a train through, then scratched my head wondering why the lights didn't come on, or, why was it indicating red when it should have given me a green?  When the track was wet, why were the lights on? Why isn't this damn thing working right?

Through trial and error I came to several conclusions. First, it can't be done like the big boys. On full size railroads the rails have a welded wire at every rail joint to eliminate poor electrical connections at the joint. Our rails didn't, so after about five rails or so, there was no more conductivity. Secondly, our rails were aluminum and our rail joiners were steel. That created a weak electrical current that fought my circuitry (dissimilar metals create electricity). This, and expansion and contraction, loosened rail joiners. Again, poor conductivity. So, if I couldn't electrify the rails for hundreds of feet there had to be another way.

A latching relay is a regular relay wired so when energized, the current that energized it keeps it energized using its contacts. It is the heart of this simple four relay system. The other key component to this system is the contact rails. A contact rail is an insulated rail from 12 inches to about 1/4 rail length long. Insulated rail joiners must be used to keep the rails from moving when a train is on them (see photo). It helps to put plastic under the rail and plastic washers on the rail screws to hold the rail to the ties. Rain water does not conduct, earth ground does. This extra step helps insulate the rail from earth ground, but is optional. The rail opposite the contact rail is the ground rail and needs no insulation at all, just the system ground wire connection. The wheel and axle complete a circuit between the ground rail and the contact rail.

WEST |:|> |:|> |:|> > EAST


12v in out in out in out in out in dead track

The diagram shows you the basic system. At the beginning of your block system you have to have a starting point, the approach block. The heart of the system is the universal block signal. Add as many universal block signals as you need for your track. At the end of your block system you need the end block. The universal block(s) will not work without the approach and end blocks.

That is a simplified flow diagram of this system. For a bi-directional system just add the same circuits going the other way.

Look at the electrical diagram for a detailed flow. Let's begin this description with the end block. It is the source of power to the entire block system. It does not supply power to the bulbs, just to the relays of the system.

The end block has two relays and three terminal strips. I'll be explaining the relays later, let's concentrate on the terminal strips. Input, for the end block, is 12 volts which powers the entire block system. It does not supply power to the bulbs of the signal heads, (they're independently powered at each signal head). The track terminal strip goes to each contact rail and the ground rail. The output terminal sends full-time 12 volts to the universal block as well as switched 12 volt track detection information.

The universal block has three relays that connect to the contact rails and the ground rail. The input terminal strip connects to the end block output or to the output of another universal block. The same thing applies to its output - it connects to the input of the next universal or start block. The bottom terminal strip goes to the signal head bulbs..

The approach block is the starting point of your track block signal. It illuminates the first universal signal or turns the last universal signal off if you're going eastbound.

Now here is how the system works. Assume your train will be going right to left (westbound). For now your train is at the station. As you leave the station your wheels connect with the rail contacts of the approach block. R1 is the eastbound "off" relay. It disconnects R2. Eventually, your last wheel will connect with R2 to latch it on. R2 now sends ahead to the first universal to illuminate its bulbs.

As you look ahead you see the universal signal come into view. It is shining green because there is no traffic on the other side of the signal. As you pass the signal it changes to yellow, but you as the engineer can't see it, so it stops illuminating the yellow bulb. The yellow detection is still there, but since there are no trains behind you to see it, why waste batteries illuminating an unseen signal?

The universal block signal has three relays. R1 sends a red indication to the preceding universal. It also acts as a reset to the preceding universal for westbound trains. R2 resets R1.  R3 has several functions.  First, it looks ahead at the next universal R1. If there are no trains, it is open to indicate green. When your train passes over R3 contact rail, then R3 closes to indicate yellow. It will stay yellow until you get to the next universal or if you back-up.

The signal head can be of your design. The one shown in the photo is made using schedule 40 gray plastic conduit and outlet boxes. Cut 1/4" clear plastic into a semi-oval shape for the target face, then cut three pipe pieces into lens covers and glue these to the clear plastic face. Drill the 2 screw holes to mount the face to the outlet box. Cut the heads off three flashlights and glue these to the inside the face so they are behind each lens cover (see photo).

Assemble the relay boards and battery pack and its ready to be connected to the universal signal terminal strip.

All relays shown in the wiring diagrams are shown upside down, contacts up. Two types of relays are used. On the approach, universal, and end blocks, have DPDT relays with one additional SPST on the universal board. The signal head uses three SPST relays. Don't confuse the coil pins with the contact pins.

Use perf board as a circuit board. It's best to use dip sockets. Push the pins through the holes in the perf board. Do all the soldering to the dip sockets then, when finished, push the relays into the sockets. NOTE: the dip sockets have 18 pins and the relays use 8. Lay a relay on its side and compare its leads to the socket pins. You don't have to use the sockets. If you trust your reading of a schematic and your soldering, then go ahead and solder the connection wires to the relay contacts.

Testing the universal block - Connect three wires from output off, red, and yg to input off, red, and yg. Connect 12 volt light bulbs (or your completed signal) to the signal terminals. Connect signal ground to either input or output ground. Using a test jumper wire with an alligator clip on both ends, clip one onto track ground. Connect 12 volts to input 12 and ground.

Green should now be lit. Touch the track alligator clip to C2 and green should go out while C2 is being touched. Now touch C1. Red will light. Touch C2 again, red goes out and green comes on. Touch C3, yellow lights, and touch C2 once more for the green to light up.

No model railroad signal system is perfect, including this one. For example, let's say you are between signals and decide to stop. The signal behind you is still indicating yellow, so if a train is coming up from behind, he sees the yellow and continues ahead until he rear ends your caboose. If you need to stop, go past the next signal, then backup to the place where you want to stop. This will give a red indication at the signal behind you. If trains follow too closely, this will cause indication problems, too. Obey the signal - yellow means slow down.

A good place to buy the electronic parts needed is . They have the cheapest relays at $1.25 (RLY-622), 16 pin socket (ICS-16) at 4 for a dollar, and perf board.

The main cost of any block signal system will be the connecting wire. Six conductor cables are needed from signal to signal. The distance from the approach block to the end block could be several thousand feet! Gasp! Then consider the cost of the relays and signal materials. The good thing about this universal system is that it's expandable. So, start with the approach and end blocks and one universal block. Later on, remove the end block back and replace it with a universal and move the end block further down the track.

Even if you don't build this system, it may give you an idea or two to devise your own way of detecting trains.

Written by Kenny Linton

Editors Note:  Information contained in this article is for reference only. 
We at and our authors are not responsible for any loss that might occur from the practical application of ideas presented here.





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