If you'd like to add indicators to your control panel and/or layout, this page is for you. Below are circuits for each popular type of switch motor:

• 3-wire twin-coil (Atlas, PECO, Rix, etc.)
• 2-wire twin-coil (Kato)
• stall motor (Tortoise, Switchmaster, etc.)

Stall motor circuits are inherently simple -- there may be just a toggle switch with the LEDs are wired in series with the motor. But before we get to the circuits themselves, let me offer a brief discussion on LEDs (Light- Emitting Diodes). If you're well-versed in LED stuff, just skip past everything in blue.

LEDs are semiconductor devices which are produced in extremely high volume and in a wide variety of sizes and styles. These circuits use LEDs (rather than incandescent lamps) because LEDs (a) last much, much longer, (b) usually cost less than miniature lamps, and (c) come with clear or diffused lenses in red, green, yellow, blue and even white. For these circuits, we'll stick with red and green diffused. You can choose your own size: most LED types come in both T-1 (3mm = .12" diameter body) and T-1¾ (5mm = .20" dia. body); larger sizes aren't typically used on model RRs, but the "miniature" types can be very useful for N-scale signals. The chart below shows part numbers for commonly-used LEDs from Radio Shack, Jameco and Miniatronics; hopefully, this will give you plenty to choose from.



Red and green LEDs generally require between 10 and 20 milliamps at 2.2 volts for full brightness; hence, when operating from a 12 volt supply, a "dropping" or "ballast" resistor is required to drop the extra voltage and limit the current thru the LED (unless we have a stall motor performing the same function) -- too much voltage or current will fry the little beasties. Ballast resistor values are shown in each of the circuits. If the LED brightness isn't quite adequate for you, try reducing the ballast resistor value by 20% (use the next higher-value resistor; eg, if the circuit calls for a 1 Kohm unit, 0.8×1K = 800 ohms, so try an 820 ohm unit); if the brightness is acceptable, stick with the resistor shown!

Unlike miniature and sub-miniature incandescent lamps, LEDs will typically outlast you, and they produce almost no heat (cool light...literally). Two-color (called "bi-color") LEDs are common, and the red/green combo is the most popular. Confusingly, there are three types of bi-color LEDs:

• BI-POLAR -- Only two leads; when current flows in one direction, you get red; when it flows in the other direction, you get green; neat, huh? These are the most popular type of bi-colors; use depends on application.
• COMMON CATHODE -- Three leads here. Remember that diodes have two electrodes (the "cathode" and the "anode"), and that current flows from anode to cathode. Three-leaded LEDs that allow you to illuminate neither, either one, or both at one time. As the name implies, the cathode of the red LED and the cathode of the green LED are connected together inside the package. These are the second most-common types; use depends on application.
• COMMON ANODE -- Again, three leads. Only this time, it's the anodes that are connected together. Least-common type; use depends on application. If you need common-anode LEDs, don't go to Radio Shack; instead, I suggest you go to www.google.com and type something like buy LED common anode into the search window. You'll get about 9000 matches, so just start with the first one and look for what you need at a manageable price; it won't take long.

The drawings below show all three types in schematic form. In the following circuits, we'll use both bi-polars and common-cathodes, depending on which best fits the particular circuit. Obviously, you can substitute individual units for any of the bi-colors. I use the bi-colors (mostly bi-polars, but a few common-cathode units) on my layout; you may choose to use separate red and green units, but beware of "LED clutter."

Use caution when soldering to LEDs. A smaller iron (20-35 watts) is suggested, and don't apply heat to the leads for more then 3 seconds. A "heat sink" is a good idea when soldering directly to any semiconductor device; there are lots of commercially-available heat sinks -- I use a variety of them as well as several hemostats; even an alligator clip will absorb some heat. Clip the sink between the solder junction and the LED body. If you simply can't get a sink connected, then solder quickly!

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THE CIRCUITS

Two optional elements are shown in several circuits -- the FUSE and the FILTER CAP -- but they're applicable to every one of these designs. If you're using an rather upscale power source with short-circuit protection and good regulation (such as that looted from an old PC), you won't need either of these; however, if you're using a wall-wart or something of unclear heritage, I'd go with both. The fuse will protect your supply if you cross the wrong wires -- try a 1 amp slo-blo fuse in an in-line holder (Radio Shack and everyone else has such things). The filter cap provides a "charge reservoir" to ensure snappier switch throws; try a 2200uf (or larger) unit with a 25 volt or higher rating (again, widely available).

MORE CIRCUIT HINTS & CONSTRUCTION TIPS [ PLEASE READ ] I STRONGLY SUGGEST that you always use a "decoupling capacitor" on ICs. It helps to keep electrical noise out of the IC's innards, and can help avoid a myriad of ills. Use 0.01uf/50V ceramic caps -- readily available at Radio Shack, Jameco, etc. Attach the cap between the Vcc and ground leads (in the case of the CD4011 used here, pins 14 and 7, respectively); mount on the back side of the IC socket, and keep the leads as short as possible. PLEASE NOTE that even if I don't show the power and ground connections on the schematic, you still have to connect them. On the CD4011, pin 14 always goes to Vcc (+12 volts in these circuits) and pin 7 goes to ground/common. "Wire dress" can be important. When wiring a circuit board, always keep your wires as short as possible. Avoid running wires parallel to one another for more than a fractional inch -- the less the better. Wires that cross at 90° are much preferred to those that cross at 15°. Power supplies salvaged from old PCs can seem like a great source of regulated DC for all kinds of projects. However, you should know that most of these critters use "switching regulators" (vs. the "linear regulators") shown on my Power Supplies page). Like most regulators, these things use a "closed loop" control to maintain regulation; but unlike their linear brethren, switchers require that all outputs be loaded to at least 10% of their rated output current...or they can become unstable and oscillate (very hard on your circuits). I use resistors to provide the load, and Ohm's Law still applies here. Be sure to use a resistor capable of dissipating the necessary power (Ohm's Law again). If you want to use such a supply, you may find my article "Using Olde PC Power Supplies" useful; it's in the PRACTICAL ELECTRONICS section on the navigation bar at left. If, after reading this mini-treatise, you're still uneasy about calculating the right resistor values, EMAIL ME the specs on the supply (ie, output voltage & current ratings), and I'll help you through it. When using twin-coil switch motor circuits, it's best to locate the "snubber diodes" as close as you can to the switch motors. We want to squash that inductive kick as close to the source as possible. (The "snubbers" are those 1N400x-series diodes shown connected across each coil.)

SIMPLE CIRCUIT FOR 3-WIRE TWIN-COILS

If you use Atlas, PECO, Rix, or any other 3-wire twin-coil motor, and you want the simplest possible indicator circuit...this is it! Each turnout requires one DPDT toggle switch (I use the miniatures -- sub-minis are too hard to work with and full-size take up too much room on my control panels), one Normally-Open SPST pushbutton, a common-cathode bi-color LED (or individual red and green units), and a 1K ohm resistor. Keep in mind that with the "simple" circuits, you have to first set the toggle to the desired alignment of the turnout, and then push the button -- don't be confused by the LEDs, as they'll show the new alignment as soon as you throw the toggle; the turnout won't know anything about this until the button is depressed. Don't use "momentary" toggles or "center-off" toggles with this circuit; the DPDT toggle is a simple 2-position critter.



If you want to have two sets of indicators (eg, one for each route on the control panel, OR control panel plus signal head), just add a second bi-color LED (or whatever) in parallel with those shown. You must use a separate resistor for each additional set of LEDs -- no sharing allowed here! Optionally, you could use separate red and green LEDs in series with the anodes of those shown, and change the single resistor to 820 ohms, 1/2 watt. For a discussion on the FUSE and FILTER CAP components, refer to "The Circuits" section (a bit further back in this article -- just after the LED writeup in blue).

SIMPLE 3-WIRE TWIN-INDICATOR CIRCUIT -- ALTERNATE VERSION
SIMPLE 3-WIRE TWIN-INDICATOR CIRCUIT w/ 2 COMMON-CATHODE LEDs
SIMPLE 3-WIRE TWIN-INDICATOR CIRCUIT w/ 2 COMMON-CATHODE LEDs (ALTERNATE)

DELUXE CIRCUIT FOR 3-WIRE TWIN-COILS

Atlas and PECO users who are afraid they'll forget to push the button and risk a dreaded "cornfield meet" will want to consider the "deluxe" circuits. They'll take a bit more work and a bit more spare change, but all you'll need to do is move the switch -- everything else just happens. What we've added are two diodes (1N4002s or equivalent) and one Integrated Circuit (or IC). It's a common CD4011 quad 2-input NAND gate which in which two gates are wired to form a "latch;" the latch remembers the direction in which you pushed the toggle and drives the LEDs accordingly. Since there are 4 gates in the package, we get two indicator circuits from each IC; the pin numbers for circuit #2 are shown in parentheses (no parentheses for circuit #1). If you only use half the gates in the IC, then you should connect the unused inputs (not outputs) together and tie that point to +12 volts thru a 10K ohm resistor (4 inputs share one resistor).


For the switch labeled "SW," we're using either an SPDT toggle switch (a momentary type with a "center-OFF," having a spring-return from either the up or down position), OR the Atlas slide/push control switch that came with your Atlas turnout. Either a common-cathode (CC) bi-color LED, and/or separate red and green LEDs are used, but this time adding extra LEDs can be a bit trickier: you'll need to add individual LEDs in series with the common-cathode unit (between the latch and the CC unit, red with red, green with green), and change the resistors to 470 ohms. This is often done by folks wanting both panel indicators and a railside signal at the turnout itself. If adding more LEDs is what you want to do but are unsure about the wiring, just send me an EMAIL and I'll be glad to elaborate. REMEMBER -- like LEDs, ICs are heat sensitive. Use the same precautions as outlined previously, AND be careful with static electricity -- I recommend you NOT try soldering directly to the IC -- use a 14-pin IC socket (available widely). Solder all wires to the socket BEFORE gently inserting the IC. Speaking of sensitivity, the two "snubber" diodes across the motor coils are absolutely essential...trust me.

DELUXE 3-WIRE INDICATOR CIRCUIT USING BIPOLAR/BICOLOR LED INDICATORS
DELUXE 3-WIRE TWIN-INDICATOR CIRCUIT
PICTORIAL WIRING DIAGRAM for DELUXE 3-WIRE TWIN-INDICATOR CIRCUIT
DELUXE 3-WIRE CIRCUIT w/ MULTIPLE COMMON-CATHODE LED INDICATORS
DELUXE 3-WIRE CIRCUIT w/ MULTIPLE COMMON-ANODE LED INDICATORS
DELUXE 3-WIRE INDICATOR CIRCUIT USING INCANDESCENT BULBS
DELUXE 3-WIRE INDICATOR CIRCUIT WITH MEMORY (TWO SUPPLIES)
DELUXE 3-WIRE INDICATOR CIRCUIT WITH MEMORY (SNAP RELAY)
DELUXE 3-WIRE INDICATOR CIRCUIT WITH MEMORY (SWITCHED LEDs)

Finally, it is possible to mix LEDs and incandescents -- you might have LED indicators on the control panel, and want a railside signal using grain-of-wheat (or some other grain) incandescent bulbs; if you just exclaimed, "Yeah -- that's what I want!", then you should probably Email me and tell me about what you want to do and about your signal. We'll need to add a pair of "driver" transistors and resistors, but it'll all work great! [TIP: For a discussion on the FUSE and FILTER CAP components, refer to "The Circuits" section (a bit further back in this article -- just after the LED writeup in blue).]

CIRCUIT FOR 3-WIRE TWIN-COILS
USING CAPACITIVE-DISCHARGE SUPPLY

If you truly want the best of all worlds using twin-coils, here it is! In my opinion, the best-possible way to throw a twin-coil machine is with a Capacitive Discharge (CD) power supply. If you've never tried one of these, you'll be amazed at the decisive and reassuring SNAP that occurs when you throw a turnout with a CD supply -- this is truly the Rolls- Royce of turnout control. Luckily, a design for such a supply is located on this very web site; it's just back on the CIRCUITS page -- you'll see it. Lots of folks have built this one and been very happy with it.


The IC and LED discussion for the 3-Wire Deluxe circuit above bears repeating here... but I'll spare you; just space up and read as needed. OK...I have to admit there is one small complication to this circuit: as you'll see above, the power supply for the turnouts is completely separate from the 12 volt DC power supply which runs the indicator circuits. The only component in common between the CD system and the indicators is the humble momentary-OFF-momentary toggle switch. However, the 12V supply will run lots of indicators. When you think about it, it's really a small price to pay for railroad Utopia. Think I'm overstating my case? Try it and see...then let me know what YOU think. [TIP: For a discussion on the FILTER CAP component and possible addition of a FUSE, refer to "The Circuits" section (a bit further back in this article -- just after the LED writeup in blue).]

3-WIRE CKT w/ MULTIPLE COMMON-ANODE LED INDICATORS for CD SUPPLY
3-WIRE CKT w/ MULTIPLE BIPOLAR/BICOLOR LED INDICATORS for CD SUPPLY

SIMPLE CIRCUIT FOR 2-WIRE TWIN-COILS

I'm not aware of any 2-wire twin-coil motors other than Kato...but I'm sure there are some out there. 2-wire critters depends require reversing the direction of current flow thru the coil to change the turnout alignment, so the circuits are a bit different that for the 3-wire types. The pushbutton is still there, but the toggle switch has grown to a DPDT type...and, we get to use my favorite bi-polar LEDs. Figure 5 below shown two similar circuits -- the first has only one bi-polar LED, the lower circuit has two; take your pick. Yes, you could even have three LEDs; just add a third in series with the first two, and reduce the resistor value to 470 ohms.



The comments regarding the FUSE and FILTER CAP also apply here (and everywhere else); use as needed. Likewise, remember to be gentle when soldering to LEDs.

DELUXE CIRCUIT FOR 2-WIRE TWIN-COILS

Now we're gettin' real uptown! In Figure 6, to avoid more exotic toggle switches (ie, 3PDT Mom- Off-Mom), I've used dual power supplies; ideally, they'd match, but it isn't necessary so long as they output the same voltage, have similar current specs and have low ripple. Don't let the dual supplies alarm you -- one can easily use two matching "wall-warts" (a/k/a "AC adapters")or jump in and build a complementary supply such as shown on the "Power Supplies" page on this very web site. We're using common-cathode LEDs again, and adding more of 'em is described above.

STALL-MOTOR CIRCUIT w/ SINGLE POWER SUPPLY

Could it be any simpler? A toggle switch and one or more bi-polar/bi-color LEDs -- that's it! Figure 7 shows two bi-polar LEDs; if you want just a single indicator, simply omit the top one (or the bottom one); if you want three, add another one in series. NOTE that with three indicators, you'll probably want to increase the supply voltage to 14 volts (likewise if you're using two and the motor doesn't quite move fast enough for realistic operation). In the case of stall-motors, the toggle switch isn't the Mom-Off-Mom type, but just a plain ol' DPDT.

TIP    Do you have signals on your layout? If so, and you're using a signal to indicate the turnout position, the auxiliary contacts on a Tortoise (or added to a SwitchMaster) might simplify the signal wiring. Many signal are wired "common-cathode" or "common-anode," and so cannot be wired in series with the motor itself; however, they can be readily controlled with one set of the aux double-throw contacts, removing the need to run extra wires back to the control panel. Just think of the aux contacts as a toggle switch.

STALL-MOTOR CIRCUIT w/ DUAL POWER SUPPLIES

If you just happen to have -- or would like to have -- dual power supplies for your turnout motors, this circuit utilizes the twin supplies and simplifies the toggle switch to a mere SPDT unit. Everything else is the same as with a single supply. This is what I use on my main layout; I built complementary (equal positive and negative voltages) supplies and routed them to my control panels. This allowed me to route only one #14 solid Romex "house wire" around the layout for COMMON (the single-supply version requires routing two wires to each motor). I think the savings in toggle switch cost, wire cost and wiring complexity can argue in favor of dual supplies. Think about these issues in the context of your own layout before deciding which circuit is best for you.

RELAY-BASED CIRCUITS FOR THE CHRONICALLY "SEMICONDUCTOR-AVERSE"

If the thought of dealing with ICs, transistors, or anything semiconductor causes you to break out in a cold sweat, or if you just happen to have a boxful of old relays in the garage you're just itching to use, I've done semiconductor-free circuits for both 2-wire and 3-wire twin-coils. These circuits needs dual power supplies and require a relay with a 12 volt DC coil (hopefully in the 200 ohm or greater range); either SPDT or DPDT contacts will suffice -- the LED types can be adjusted to fit the contact configuration. Two such circuits are available just below; however, if this is NOT what you need, then EMAIL ME with your requirements and relay info, and I'll strive to come up with a usable circuit for you.

3-WIRE RELAY-BASED INDICATOR CIRCUIT for CAPACITIVE-DISCHARGE SUPPLY
3-WIRE RELAY-BASED INDICATOR CIRCUIT for CAPACITIVE-DISCHARGE SUPPLY (MULTIPLE TURNOUTS)

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I hope you've found something useful here. I seem to average about two Emails per week asking for one or the other of these circuits, so I'm guessing there is a need. Let me know what you think about this page, and/or if there are other topics you'd like to see covered in web-accessible form.

THANKS!

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Updated 12-8-08