Wayne Roderick, 3rd Division, PNR, NMRA (life)

06/05/98 rev 01/16/07

Since the Teton Short Line has made a presence on the Web, we've heard from a couple sources that they intend to use our ancient signal module, as published in MR, June 1972, on new club layouts. At first I was surprised that it would even be considered 'til one correspondent stated:

"Regarding your surprise that our club would consider your old signal logic design; we like it because of its simplicity -- we really like simple stuff, 'cause we understand it!"

That says it pretty plain, and so I figured it might be a courtesy to my fellow hobbyists to put a little update on it. The text following is scanned from my original manuscript sent to MR, with a little editing to bring it up to date. My sketchs have been re-done and are also updated. I dropped MR a note asking if they had any problem with posting this and got no response so here goes.

BIG surprise! Jeff Scherb adopted the TSL circuit and symbols for his excellant article "Simple Circuits for Automatic Block Signals" in Model Railroader, March 2001. I'm flattered- for anything electronic to last 30 years is indeed a surprise.VBG He uses LEDs of course and gives you a circuit board layout too.


Wayne Roderick


rev 01/16/07

Introduction paragraph is eliminated- way out of date

A working signal system adds a great deal of life to the model scene. The TSL system has operated successfully for several years and during this time additions have been made using the basic design that has stood the test of time and operation. The most recent addition is two signal bridges controlling traffic leaving Malfunction Junction. One of these bridges has four heads, each of which has three lights. Very colorful and active as they are controlled by route alignment as well as block occupancy. This is also a great assist to visiting engineers I would like to share with you the techniques developed here in the TSL Signal Dept.

When I designed this system, I was able to meet many requirements which are not totally satisfied with any other of which I am aware.

The requirements are:

Three aspects or colors.

Usable for single or double-track.

A single power lead for all lamps to reduce wires on signal bridges

Small voltage drops and a few ohms of resistance in connecting wires shall not interfere with performance.

Easy to interlock with sidings, junctions, yards and manual controls where desired,

Resistant to damage from short circuits between power supply lines and interconnecting wires.

All functions are operated by merely grounding a control wire to a common line. This is usually the common rail in a common rail system.

Approach lighting similiar to UP practice should be optional

Should be inexpensive.

The resulting system can be divided in to three basic parts:


This can be any commercial or homemade three-lamp semaphore, block signal, bridge or what- ever you desire. The bulbs should be about 12 volt .050 amp similiar to the Kemtron X-410 rice bulb. Those on the TSL are homemade with wire, tin can metal, and brass tube at a cost of 80 cents each including the bulbs. (1998 update) I'm sure that you'll never find the bulbs, especially at that price, but when we get into the circuit, I'll suggest alternatives.


This is the heart or the system, It determines the priority, so that red will extinguish yellow and green, and yellow will extinguish green. The module uses two silicon PNP transistors and six resistors. (1998 update) The original system and the one published in MR used silicon NPN and a negative power suppy for reasons related to technology and cost at the time. Later, the polarity was changed, turning over diodes and using PNP 2N2907 transistors so the system could be integrated with others using Positive Logic.


This device Causes a current to flow to the ground or common rail when a train is occupying the block. There have been several detectors published including the famous Twin-Tee. The detectors on the TSL will sense ten-thousand ohms across the track under any possible setting of cab controls or throttles. A built in time delay eliminates lamp flicker due to dirty wheels or track when running slow.(1998 update)Many detector circuits are available including later generations used on the TSL. Note that the simple logic is traded off with high current requirements in the detector output. A lamp is extinguished by shorting it out leaving the 12 volts limited by 100 ohms for about 120 ma. A detector could be sinking as much as 6*120=720ma under the worst case scenario. The detector should use a relay or high current transistor in it's output. A better way is to use LED's running at 10-20ma with appropriately larger resistors.

Because several signals are available and the techniques for building them are simple, and because various detector circuits are available, I shall describe only the Logic Module here.

In figure 1 note that the three lamps are fed from a single twelve volt conductor. You can appreciate the advantage of this when you build a four headed signal bridge which requires 13 wires instead of 24. A ground placed on the "green enable" terminal will light the green lamp through the 100 ohm 2 watt resistor., Besides being important to the operation, this resistor limits the lamp current for realistic brightness and virtually infinite life, In over two years, I have not lost one of the about 70 lamps. (1998 update) We never lost a lamp over the twenty-five year life span. Normally the ground will come from the previous logic module. If an approach condition should occur (train in second block ahead) a ground will appear at the "yellow-enable" terminal from the logic module ahead, This will light the yellow lamp and forward bias the transistor Q1, While Q1 is conducting, it shorts out the green lamp thereby extinguishing it. If the block ahead is occupied or other hazard exists such as an open turnout, a ground is placed on the "red" terminal, lighting the red lamp. This forward biases transistors Q1 and Q2 extinguishing both green and yellow.,

There are no critical voltages or resistances in the circuit. The transistors are wired in the common-emitter mode which gives us voltage and current gain, This makes up for possible losses elsewhere. The module has outputs to light the yellow behind and the green ahead of the occupied block. Notice that the wires between the module and lamps or detector can be shorted either to ground or +12volts without damage to the module module or signal, This is a major advantage when you consider that model railroads are not usually wired to industrial standards,

Schematic, Old signal module

To simplify things I use the symbol shown in figures 1 and 2 to represent the logic module, Figure 2 shows the interconnections for single-track main-line with some of the options that can be used with the module. A diode and a switch has been added between westbound modules 2W and 3W, Closing the switch will cause 3W to indicate approach while the diode prevents setting 2W to red, This is useful when a turnout is aligned to the siding and reduced speed is in order, An open turnout in block 4 will set 4W and 4E red, also SW (not shown) and 3E would display yellow approach. If you desire a signal to be illuminated at all times merely cut the green enable wire and ground the terminal as shown for 5E. Using these principles, the system can be expanded to handle very complex junctions and interlockings.(1998 update) Using LED's instead of bulbs is the sensible way to do it today. The +12 volts is not critical either- Use a lower voltage if preferred. Divide your required LED resistor so enough of it is in series and very close to the signal to protect it. An accidental short of the connecting wire to ground should not blow the LED. The balance of the resistor has to carry the entire power when the lamp is extinguished. Compute its wattage with P=E^2/R. For the 270 ohms shown at 12volts, it comes out to be 12*12/270=.53 watts- It might be better to use a one-watt device. Caution- the value of 20ma for the LED is just an example- we run our new sub-miniature ones at 10 ma. About 1000 ohms in series.

Module Connection & Options

The signaling described is a beautiful addition to an operating railroad. Keep in mind that signaling does not have to be a monster, I suggest you try a couple or modules and locate the signals in a conspicuous place, Ground the "green enable" terminal at the first module in your system and forget about the unused output on the last. Then listen to the comments from your visitors. Soon you'll be building a big signal bridge for your division point

REMEMBER- This was written in 1971. The current signalling system is a bit more complex but I like it.

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