CAR LIGHTING CIRCUITS


Here's a simple circuit for Passenger Car (or even Caboose) interior lighting. We use the 12V bulb as a "ballast;" the ballast absorbs the voltage over and above what's required by the 1.5V bulb(s). The 1.5V bulbs begin to glow with with less than 4 volts on the track, put out good light with 5 volts, and maintain relatively constant brightness up to 14 volts on the track. Since there's nothing polarity-sensitive here, the lighting is on for both forward and reverse directions. The diode bridge is connected to provide 2 series diodes in each direction, limiting the voltage which can develop across the bulbs to approximately 1.4 volts, thereby prolonging the life of these delicate little critters; almost any bridge will work, so shop price.

For those of you enjoying DCC on their layouts (like me), this circuit works great! But...IF you run greater than 14 volts RMS on the track, you'll probably want to replace the suggested 12 volt ballast lamp with a 14 volt beastie, such as Miniatronics 18-014. If the illumination bulb aren't as bright as you'd like, try using two of the 14 volt ballast lamps in parallel to ensure plenty of current for the 1.5 volt bulbs.

Miniatronics Web Site -- Bulb Chart

If you're interested in seeing more car lighting circuits, take a look at those residing on Don Crano's Web Site. You'll see circuits with regulators...one featuring a battery for the ultimate in flicker-free operation.

Below is a circuit which is similar to the previous one, in that we're still using the "ballast lamp" -- only this time we're using LEDs to provide the illumination. I like this one best, as the LEDs begin to glow with only 2.5 volts on the track, and maintain full brightness from 5 to 14 volts. Note that no diode bridge is needed here (the LEDs are their own diodes). Yellow LEDs are much cheaper than white ones, and the yellow light is acceptable to me; if you insist on a whiter light, be prepared to pay more for the relatively new white LEDs; I haven't tried blue LEDs, but they're another possibility (if you've tried 'em in this application, please let me know your findings and I'll add 'em to this section). This circuit is best for DC cab layouts (although it can be used with DCC); for DCC layouts, try the circuit just a bit further down the page.



For the analyticals amongst us, the graph below shows the voltage across the bulbs or LEDs with respect to the DC track voltage for both the circuits above. Note how quickly the LEDs peak and flatten, indicating "earlier" and more uniform illumination in the car.



Below is a circuit which should work just dandy on DCC layouts. I think you'll get plenty of illumination with just the two LEDs, although you could "double-up" (as above) if you feel the need for more brightness. As before, IF you run greater than 14 volts RMS on the track, you may want to replace the suggested 12 volt ballast lamp with a 14 volt beastie, such as Miniatronics 18-014. If the illumination bulb aren't as bright as you'd like, you might try using two of the 14 volt ballast lamps in parallel to ensure plenty of current for the LEDs.



If you can stand a bit more complexity (and have space to conceal one AA or AAA battery), here's a circuit which will provide constant illumination even when the train is STOPPED. The battery provides power to the lamps at all times, and recharges whenever the train is moving forward (DC cab). No ballast lamp is needed here, but the resistors can get a bit warm...so make proper allowances. The battery should provide at least 20 hours of illumination between charges. If it should run down, I suggest setting the depleted car on a siding all by itself (or with others needed recharging...but no locomotives) overnight with full throttle voltage applied; the bulbs will glow, and the battery will recharge. Take care in connecting the diode (D) to the rail; it should connect to the rail which has a positive voltage on it when the locomotive moves forward. Otherwise, the battery will charge only when the loco is moving in reverse.

Note that DCC layouts gain no particular advantage from this circuit, as the DCC carrier voltage is on the track at all times; hence, the first circuit on this page is probably the best choice for such layouts.



Want a lighting circuit that will "charge" itself no matter which way the train is moving (as long as it IS moving)? The two circuits below will do it. The "Deluxe" circuit uses a Memory Back-Up Capacitor, a super-high capacitance (up to 1 Farad) little jewel commonly used to maintain dynamic memory when power is off. They're quite compact and reasonably priced (under $3 for 0.47F, and under $5 for 1.0F). Depending on the voltage on your track when operating, this circuit could keep a car illuminated for up to one minute when the train is stopped. Use the 18-001 15mA bulb unless you must have the slightly higher brightness of the 18-201 40mA bulb.

deluxe circuit


I'll readily admit that the above circuit has grown a bit complex. If you're the adventuresome sort and prefer the memory capacitor to batteries, you might want to try the variation below. Here, we've replaced the second (low-voltage) regulator and its associated components with a "ballast bulb," which is simply a 12 volt, 40mA Miniatronics bulb that serves to consume the voltage not needed by the 1.5 volt bulbs.

alternate deluxe circuit

super-deluxe circuit


The "Super-Deluxe" circuit above uses batteries to keep your cars lit for hours. Three Nickel-Metal Hydride (NiMH) cells work splendidly...if you have the room for AA or AAA cells; if not, consider using one of the many battery packs sold as replacements for cordless (NOT cellular) phones -- they're usually 3.6 volts, and more compact that the AA or AAAs (AAs could last up to 15 hours, AAAs up to 8 hours, and a phone battery pack 2-3 hours). To change the brightness of your lamps, simply change the value of the 39 ohm resistor shown in these circuits; the chart below suggests resistor values for more or less brightness. Remember that more brightness means less bulb life, and vice-versa; choose wisely.


resistor chart

Miniatronics Web Site -- Bulb Chart



Want directional marker lamps on your cabeese? British Columbia model railroader Heinz Heyer uses the simple scheme shown below. For HO and larger scales, small LEDs can be mounted directly on the caboose ends. If you're in N or Z scale, you may wish to use one of Heinz's alternate techniques -- "...on some modern cabooses, use a single LED & fiber optic to get the marker lights to the roof ends; drill 2 small holes (tight fit) in to the LED and use heat shrink tubing to darken the LED and hold the fiber optic in place; route them through the back wall and glue the fiber optic under the roof corners )3/32" past the ends); once the glue sets (with a soldering iron, close by) melt the fiber optic to form a mushroom."

On work & transfer cabeese, Heinz likes to disguise the caps; he mounts them outside the 'boose and paints them like tanks for water, oil, fuel or weed spray. 500uf caps can be used if space is greatly limited; likewise, caps with a 16 volt rating may be used if you're absolutely sure that the DC voltage on your tracks will never exceed 14 volts -- in any case, you may confidently substitue 25 volt caps in lieu of the 35V ones shown and save a bit of space. Shown below is Heinz's original drawing for this circuit; he does good work!






I Hope This Was "Illuminating!"

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Updated 2-6-04