Circuits for Signalling and Control

So who needs High Tech?

There has to be a better way
to grow in the hobby without
spending a fortune

Circuits suitable not only for model railroads, but most any hobby control application. After over forty years as an "industrial" electrician, automation electronicist, instrument mechanic, and computer field engineer, I have learned that "high tech" solutions aren't necessarily "high reliability". Fully integrated systems have far too many things that can go wrong, leaving the entire device as fodder for the parts box. You don't trouble shoot an integrated circuit. You replace it, when you can get one......

Yup!; I'm allergic to the things. When they do the job, they do it well. Just like the designer wants it. But not necessarily how you want it. By using discrete components to build modules, you can make the circuit do things exactly the way you want. ICs have their place, in controlled environments. How could you have found this page without a serious computer? But they aren't suited for hanging by the wire under the benchwork, where the family cat can get amongst the wires to play.

While I do market a line of circuit boards, this is my hobby of fifty years and I want to see as many modelers as possible do well in model railroading. So, try some of these simplified circuits and see if they fill your need. If you like the results, order the circuit boards or kits from me. Or, build 'em yourself, if it pleases you. You never know, you might learn something useful in the process. Though, it does take time away from running trains.

I present the circuits as individual "modules", basic functions that are then connected together to provide the results needed. There will occasionally be some redundancy, each circuit is stand alone. Most applications could be made more efficient by combining functions but then you couldn't learn as much from them or combine circuits to suit your own needs.

I'm not attempting to teach electronics here. If you weren't already curious and motivated you wouldn't be reading this. If you seriously want to learn electronics, try this as a starting point;

Naval Electrical Engineering Training(neets)

Probably the best theoretical training you will find without going to college. On this page, I am merely giving up some of my designs in a format that will give some insight into how "I" think about circuit design. Some of my circuits have been in continuous use on layouts for over twenty years. That should be reliable enough, ya think?

About components: I use 2N4401 and 2N4403 transistors. The circuits presented here will work with most any "small signal" transistor. For example, substitutes for the 4401 are 2N2222, 2N3904, NTE123AP, and those are just the ones I'm used to working with. The drawings use "standard" symbols; I include a table here because my "standard" is from instrumentation and may not be the same as the "standard". from an electronics school or a project book. Any resistor not marked is 2200 ohms, 1/4 watt. Any diode not marked is a 1N914.

I use "NPN open collector" outputs for most of my circuits. That means a transistor will act as a switch to ground. A relay contact will do the same job, or a push button, or even a loose wire. I try to avoid referencing anything to power unless it's really necessary.

Now, some basic "logic" circuits: First up is a memory circuit, to the left; a flip-flop. It has two inputs; operate one, "SET" and the output turns on. Operate the other, "RESET" and the output turns off. Either element may be used as the output, hence the LEDs on both sides. If both are used, the two outputs oppose each other.

Cute, but by itself not very practical. Although LEDs could be attached between the two outputs, showing direction. Such as a Bi-colour LED with two leads. One way is Red, the other Green. At the bottom line, this is all it takes to operate a "Tortoise" switch machine. Later, we'll make more interesting stuff out of the basic circuit. As used here, the LED turns off when the transistor is on.

      So, let's move on to driving DC motors. PerMag motors are so named because they have a permanant magnet as a field. The little curved bar above the armature in the illustration implies a magnet.   Reversing the current through the armature reverses the motor. All well and good, just the same thing your power pack does when you operate the reversing switch..... So why a special circuit for that? How about remote and/or automatic control......

       In the "bridge amplifier" or "H-bridge", that's all we're doing. Turning on transistors to reverse the current through the motor. But we do it by swapping the control leads, left and right. With one at ground and the other pulled up by (you guessed it) the pull-up resistor, the output is through the center leg of the "H". Swapping the control inputs reverses the control, so the opposite pair of transistors is now conducting. Reversing the motor in the process. In this circuit, the motor runs only as long as a button is held in.

       There is no current limit at these outputs, it is "across the line". High current will let the smoke out of the transistors. Be sure the semiconductors are rated high enough for the current you anticipate.

By itself, another interesting but not particularly useful circuit. But, let's combine it with the Flip Flop. With the F-F outputs connected to the control inputs of the bridge amp, we have a motor reversing circuit. Press a button and it drives in one direction. Press the other button, it drives the other way. Six transistors and four resistors; why do we need an IC for that?

So, what's it good for? Well, there are "stall motor" switch machines that usually have a toggle switch controlling them. So what happens when you're on the other side of the layout and need to operate a turnout. It's a long way around to the switch. Or, with this circuit, you press the "other" push button on your side of the layout and operate the turnout. Yup, just keep hanging buttons on the input to ground anywhere you need one.

Fate dictates that a wire will one day get crossed and the magic smoke gets out of a components Despite our best efforts, it will happen. The original user for whom this was designed has most of the 22 year old boards still in service. Failure is rare, but if it happens, this circuit is readily repaired, at negligable expense.

So maybe you have half a dozen of these stall motors at the throat of a yard and want to use one pushbutton per track to set up all the turnouts. That's called "route control" and is another of the reasons behind this circuit way back when. Study the drawing and you'll see that push buttons and matrix diodes are both connected to the same point, allowing individual manual control and automatic route selection.

This is about the most flexible method you'll find for stall motor switch machines.

At this point, you still don't need the "H" bridge. The Flip-Flop will handle a tenth of an amp with the right resistors. We'll come back to the circuit later and you'll see an addition that does require the bridge.

But for now, how about some detection? A simple, two transistor circuit that will take several photo sensitive resistors. Use them in series. With my circuit boards, I have tested up to four within a 10% range and they worked perfectly.

As a block signal sensor, this circuit will output so long as at least one photo cell is covered. It's useful for grade crossings as well, operating the cross-buck flasher and gate actuator. The LED is lit when the block is clear. What gate actuator, you ask? Well, the stall motor switch machine from the circuits above is a good start. We'll cover it later, when we modify the "H" bridge.

This gizmo is an oscillator. In this case we can call it a flasher because it runs so slow, one cycle in a second.and a half . The lower LED will stay lit whenever power is on. We get away with one resistor because the LEDs don't really operate in parallel. One side flashes, the other side is off. Then it swaps. With only one lit at any time, current will be the same for either. If you want crossbucks on both sides of the track, duplicate the LED circuit in parallel, or put the LEDs in series.

This oscillator is essentially the same circuit; the difference being the extra components to accomodate external control. When power is first applied, the flasher sort of stumbles for the first few flashes. The simplest solution is to keep the flasher running all the time. The 5K resistors keep the flasher "loaded", somewhere for the current to flow. The other solution was a much more complex circuit, something I try to avoid.

      Such a circuit is not inherently self starting. Thus the slightly different resistors, 30K vs 33K. The 10% difference is not noticable.   If you want to change the speed, adjust the two resistors up or down, in about 5K increments. For radical changes, the capacitors can be changed and then the resistance fine tuned.

      To use this circuit for strobes or such items as tower flashers, make one side slow, say 50K with a 100uF capacitor. The other side would have a shortened time, say 5K and 5uF. Use an LED on both sides until you get the flasher the way you want it, then remove the extra LED. Just be sure to leave a resistor to load that side.

      As a crossing flasher, the LEDs may be lit by connecting the "GROUND to Flash" point to the same output of the detection circuit as used by the LED. Or replace that LED, the flasher contains one constantly lit. That constant LED serves several purposes. It was initially installed in my display as a beacon on the gate arm. It turned out to be convenient as well for adjusting sensitivity of the detection circuit. One last thought..... An LED is a diode, it can serve double duty as an isolation diode in a circuit provided current is below the max rating of the LED. As will be seen in the next circuit:

Now, here as promised is a Tortoise(R) actuator circuit that requires the "H" bridge. Why? Look closely at the circuit; there's only one input. The NPN at the lower right functions as an inverter. When the input is open, this transistor is conducting, holding the "load", the base of the right side of the bridge low. The left side of the bridge is high, through the pull-up resistor. The motor drives one way and stalls. Ground the input, the transistor stops conducting and the right side of the bridge goes high from the "pull up". The left side is low of course, it's grounded. The motor drives the other way. Now, this is useful.....

      So, connect them all together and you have a crossing gate controller. The individual circuits may need a little fine tuning, to suit your project and models but are basicly functional. Each function is "diode isolated", as mentioned earlier. To prevent interference with the timing circuit of the flasher. Any pin numbers that got left in refer to LBO circuit board connections. This is one of my production circuits.

    Lighting signals is no more complex than the control circuits above. What is complex however, is when to light what lamp. That's your part of the project.

    A basic three aspect control is illustrated below. In its' simplest form, a three aspect controller can be devised with three diodes. Pete's circuit, simplified for discussion, is shown to the left. This circuit can be expanded, with additional diodes, to accomodate any number of aspects. The key is knowing when to light each LED. And that requires an understanding of what the signal aspects represent. The hard part.....

    Pete's circuit is about as simple as it gets. The "INPUT" can be accomplished with an open collector or a relay contact to gound. The difficulty lies with accessing both the top and the bottom of the LEDs. No problem if you're building your own signal models. But, the method isn't compatable with most commercial models because there is usually a "common" connection between the LEDs to reduce the number of conductors in the mast. Most models with three lamps will have four wires. The diode method requires all six be available.

To accomodate the commercial models with their common connections, the circuit on the right is more suitable. It is more complex, but I have yet to find a signal model it won't control. This is the "core" circuit of my SLC board.

In both cases, the circuit shown will control one signal head. For bi-directional signals, two iterations will be required for each block.

The detection circuit can be shared between the two; the input lines are diode isolated. A suitable detection circuit would be the photocel circuit above. Detection is a whole 'nuther subject. My suggestion would be to look at my professional site and download the SLC Manual. It covers detection in depth. But, is over 80 pages in all.

The magic of diodes:

    Diodes are the fundamental basis of solid state electronics. P-N junctions are at the core of the most complex micro-controllers. Even your computer is nothing more than a coupla bazillion P-N junctions, arranged just so. Our perception of technology is biased toward the latest and greatest super-duper gizmo. But it all comes back to diodes; or at least to P-N junctions.

    I noted before, I'm not trying to teach electronics here. But there are a few issues that aren't covered in most electronics schools. They tend to concentrate on more advanced applications. If you were to follow design of a DCC decoder, computer technology would be the subject to learn. But, there are many uses for the technology that aren't discussed outside of "creative" circles.

    Take a simple soldering iron..... When you work at the bench, your soldering iron spends a lot of time idling. It's hard on the tip, high temperatures accellerate the corrosion of the copper tip. But, to power it down means you have to wait for it to heat up every time you want to use it. So, arrange a box with a cord and receptical and a diode in series with the line. That cuts the power in half. Then, a switch to bypass the diode. The iron stays warm but not hot-hot. Heat up time is a matter of a minute or so, rather than the five from room temperature. This is, of course, assuming you have the sense to not use an "instant heat" soldering iron. They are next to useless for working with small electronics and the "induction" type will destroy a circuit board in short order. This will only work for "non-reactive" loads like heating elements and lamps. DO NOT try to use this on anything that has a coil or transformer.

    Another interesting characteristic of diode junctions is the voltage drop. At an engineering level, the phenomenon is called the "Fermi level". Named for the physicist that first observed the effect while developing the first junction. Some interesting history there.... For the common silicon rectifier, the drop is 0.7 volts. There are diodes with less, and more. Specialty stuff. Here, we will stay with common devices.

    An example would be the 1N4004. The 1N400x series are all 1 amp, the last digit indicates their "reverse voltage" rating. A 1N4004 is rated 500 volts; high enough for anything we would ever get into here. The forward voltage drop is the same for any device in the series.

    A resistor "drops" a voltage proportional to the current through it. (E=IxR) The more current, the more voltage drop. Resistors must be calculated for the anticipated current flow. A diode is a little different; if there is current through the diode, the drop is 0.7 volts. Period. Lots of 1.5 volt lamps on the market these days. Two diodes in series drops 1.4 volts. An excellent voltage regulator for those tiny lamps. There still needs to be a current limiter, but the absolute value isn't such an issue.

A good example of this is "constant lighting" for a locomotive. Two diodes in series will provide a 1.4 volt tap in the motor circuit for a low voltage (1.5v) headlight. But, it only works in one direction; reversed, the motor won't run. So, we add another diode in parallel, in the opposite direction. This has the added effect of providing half voltage when the loco is in reverse. A common practice in 12 inch / foot scale railroading..... I do recommend you use diodes suited to the size of the motor. 1N4004's won't handle the current of some larger scale locomotive motors.

The circuit does work with DCC systems. Sort of redundant though; most DCC decoders have a headlight function built in. The point of adding it to the drawing is to show how such a device will work on a DCC signal. The pulse train will show up across the diodes, making it a useful detection circuit. The drop will only show up when there is current through the diodes. Oh, and the directional function doesn't work. The lamp will light nearly full brightness all the time. DCC is like A-C in that respect. The concept is useful in many applications, not just signalling. Any time you want to respond to the presence of current in a circuit.

The technique isn't limited to constant lighting. By using a "ballast lamp" across the power supply and placing the diodes in series with the circuit, voltage can be regulated to operate a number of lamps such as shown here. Depending on the end use, the circuit may be a combination of series and parallel. This is quite useful for lighting a large number of lamps such as street lights and building lights in a city scene. The ballast lamp could serve double duty as a "foot light" below the layout. Depending on the number of diodes, the lamp may have no more than a dim glow.

An advantage of this method is that, unlike a normal series circuit, loss of one lamp will not affect the rest of the circuit. Loss of a diode would cause a problem but that would be rare. And troubleshooting would be easy. So long as the ballast lamp isn't shorted, a jumper anywhere in the circuit would do no more than blank some of the lamps. The number 93 lamp shown is an arbitrary selection, rated at 1 Amp (some 12-14 watts). Lamp choice would be a matter of power supply capacity. With 1N4004 diodes, up to eight series strings can be regulated.

Now the good part; let's "short circuit" proof your track circuit. Most of the lamps listed are automotive numbers and should be available at any well stocked auto parts supplier. The funny looking symbol on either side of the motor indicates the track to wheel connection.

The 1157 has two filaments. Both paralleled provides nearly 3 Amps capacity, pushing the limits of all but the largest of power supplies. Watch your power supply capacity with this one. For separate "districts", a lamp in each district would be the way to go.

Lamp filaments are "non-linear". When circuit current is low the lamp has low resistance, a very few ohms. As current rises and the filament starts to heat up, resistance starts to rise. Maximum current lights the lamp at its' normal brightness. So, used in a track circuit, worst case is a dead short across the rails. Just as in turning on a switch, the lamp lights. In normal H-O operation, the motor will limit current to some 300mA, well below the current required to light the lamp. But, drop a quarter across the rails (or derail in a turnout frog) and there is a short circuit. This method of current limiting is as old as model railroading.

I use a "DigiTrax® Zephyr" supply on the bench for circuit analysis. Rated at 1.5 Amps, the optimum protection would be an 1141 lamp at 1.4 Amps across the 1.5 Amp DigiTrax supply. Automobile electrical systems, although referred to as 12 Volt systems, are actually higher than 14 volts with the engine running. The lamps listed are rated for that voltage and are well suited to Model Railroad power supplies. These lamps have a "bayonet" base; sockets are also available as replacement parts. Or junkyard parts if you don't mind scrounging. 1157 is a tail/brake light, 1141 is a back-up light, 57 is common as an instrument lamp, and 67 as a secondary marker light.

Brute Force Universal Reverse Loop Control

       So, here we go again; ~~reinventing the wheel~~ devising reversing control. With the advent of DCC, the usual method of controlling a balloon track or WYE leg with steering diodes is no longer viable. The diodes intefere with the DCC signal, preventing its' proper operation.

       While the "Power District" controllers for DCC do provide a measure of control for reversing sections, they bring that control at a steep price. And often won't co-operate when you switch back to analogue power.

       When on a limited budget using mixed control, a reversing section is often removed from the layout or never used. Limiting the operational possibilities it was installed for in the beginning.

       Here, I present a universal "brute force" method that can usually be implemented out of the junk parts box. It is crude, has operational restrictions, and is generally disagreeable to use. But it's cheap, simple to install and works with either Analogue or DCC. Or both when you have mixed control.

       There are some limitations to consider. In most cases, the train must be stopped within the reversing section while operating the control. This applies primarily to Analogue control; in some cases with DCC , stopping is not required. This can be read as: "I've tried it without stopping and most of the time it worked."

       As with most methods for dealing with reversing sections, the circuit is allergic to metal wheels, sparking as each wheel crosses the gap. This is most noticable on lighted cars or ballasted detection cars where electrical pick-ups place a wheel on each side of the gap. The wheels will spark and cause momentary shorts as they cross the gaps.

       This problem exists on most controllers, including the current crop of automatic DCC devices. They have a relay on the controller board that has a definite time lag and often chatters. So, in this case it's merely an issue of manual operation or automatically controlling without stopping. A matter of how far are you are willing to go to save a few bucks.

       But it works, the whole point. And it's simple to wire. For a WYE, twenty feet of wire will suffice. For a balloon track, even less. It can be automated later with a relay from a signalling circuit; when you advance that far.

The illustration is self explanatory;

Determination of "Entry" and "Exit" are a matter of the direction of travel. Here, I bring a train in from the upper right, exiting to the lower center. The process works just as well in the opposite direction. Travel from the upper left in either direction requires no action.

Explanation of the switch innards is superfluous. Hook it up and run. If it doesn't work properly, rotate the switch 180 degrees in its' mounting hole. It doesn't get much easier than that.

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To post these graphics, I copy out my prints, extract the elementaries, and then convert them to JPeGs, all the while trying to keep them legible and sometimes adding colour. All in the midst of testing and developing even more specialty circuits.

So this is all there is for now; I'll be adding more as time allows. The end result will cover capturing and using the pulse train from DCC to show occupancy. And controlling RC Servos without all sorts of expensive hardware. And the fundamentals of how my signal systems work. And how to build them for yourself.......

And the inevitable tracking counter. If there isn't much interest,
I won't tie up space with this page. If you like it, use it....