LED tail lights

[lionelsoni]

Jim Duda asked me to recommend a way of adding two red LED tail lights to a lighted passenger car. I thought I would answer him here, so that anyone else who can benefit will.

Radio Shack sells a package of two 3-millimeter red LEDs for $1.29:
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F006%5F002%5F000&product%5Fid=276%2D026

These are about 1/8 inch in diameter, which is 6 inches in 1/48 scale. I would drill the holes undersized and ream them until I could force the LEDs in from the back. It is possible to saw or file off the hemispherical tip of the LED and polish it (with toothpaste) to make a more realistic light shape, if you want.

Wire the two LEDs in parallel, with the short lead of each one connected to the long lead of the other. Then put the pair in series with a resistor. Jim says that his track voltage is always between 10 and 16 volts. That works out to an average voltage of about 5 on each LED, which means that the resistance should be no less than 180 ohms for the maximum 28 milliampere rating of the LED. However, that is probably much brighter than you want. I would start with 1000 ohms and see how I liked it. Calculate the resistor's power rating by dividing 256 (the square of the voltage) by the resistance, then double or triple it.

[Jim Duda]

THANK YOU, Bob! And while it may not be prototypical, I don't really care...I just think seeing the rear of that car illuminated will look nicer when the ambient lighting is dim or dark. I like the idea of not needing a diode because of the way you suggested wiring them together. (Note that a single LED WOULD require a diode!)

Anyway, here is the pic I sent to Bob so you can see what I plan to do...

(click it)

[image]http://img.villagephotos.com/p/2004-11/888231/MadisonRearView1BN.JPG[/image]

[Jim Duda]

I showed this to Bob and his comment was the actual value of the resistor might not end up being exactly 1K ohm and that I should try different values to get the particular brilliance I prefer at my operating voltages...but I'll start with the 1K ohm and go from there...

(click it)
[image]http://img.villagephotos.com/p/2004-11/888231/MadisonLEDschematic.JPG[/image]

[Anonymous]

I have a question for Mr. Duda. If you get LEDs that operate in the voltage range that you run your trains with do you still need a resistor? I don't know that much about the electronics end of this stuff. Could you please help here?

Thanks.

[Jim Duda]

Dunno...Maybe Bob, Roy, or other EEs will educate us further... my guess is you could delete the resistor...Anyone?

[FJ and G]

thanks, all. Very useful info. Eventually want to do this as well as add # boards to loco

am saving this post and diagram!

[Anonymous]

LED's (Light Emmiting Diodes) are diodes just like rectifier diodes. This is why they normally run off DC. For the circuit above each LED operates off of one half of the AC cycle. If you used only one LED it could be damaged when AC polarity reverses but in this circuit the second LED protects the first. The resistor is necessary to prevent the current from exceeding the rating of the LED's. All LED's are like this, those listed as having a higher voltage or operating off AC have a resistor and/or a diode added inside the case. If you took the LED out of the case you would find them.

For those those asked that is the simple electronics behind LED's. It can get more complex if you wish.

[FJ and G]

Has anyone thought of putting headlights and taillights in cars and trucks, wired up thru chassis to power pak below the layout?

[lionelsoni]

Warren, no, you can't do that. An LED, like any semiconductor diode, when it is conducting, enforces a more-or-less constant voltage drop across itself. The real purpose of the resistor is to act as a "ballast", which is a circuit element that reconciles the difference between the supply voltage and the LED's voltage. Without the ballast, the LED would go dark when the supply dropped just below the LED's voltage or draw an enormous current that would burn it out when the supply climbed above that voltage or, only if the supply were set exquisitely accurately at the LED's voltage, light up.

Since the LED voltage is only a couple of volts, you would have to wire several of them in series to get up to typical toy-train voltages. Nevertheless, it is not at all practical to try to match the LEDs to the supply voltage, even if one is using dc. With ac it is clearly impossible, since the voltage varies continuously, going from -22.6 to +22.6 volts in 1/120th of a second in the case of Jim's 16-volt transformer.

[SPFan]

You can get 2mm LEDs which will fit in a marker lamp housing. These http://www.lumex.com/product.asp?id=1002012 are the ones K-Line is using in their locomotives. They don't have built in resistors so you have to add one.
You can get them from www.digikey.com

Pete

[lionelsoni]

That's good to know, Pete.

I suggested to Jim yesterday that, if 3 millimeters was more than he wanted, he could turn the tip of the LED down to any desired diameter. The plastic car wall could easily be drilled to the same diameter and then countersunk to 3 millimeters on the inside surface to accommodate the bulk of the LED, if necessary.

[FJ and G]

Bob,

There's a nice article in a recent MR (??? which one), that features putting smaller than 3 mm LEDs in HO locomotives. They use some sort of fiber optical device.

Also, the 3mm LEDs can be carved down to a flat face. They can be installed in an assembly w/superglue as that glue is translucsent. Wish I had the article w/me but it was w/in last few months.

[SPFan]

Another alternative is to use 1.5 volt grain of rice bulbs. These are actually thinner than 2mm (~.080") in fact I think they measure .055". I have used them in HO marker lamp housings and headlights. You can make them constant brightness by placing two diodes wired in series across the lamp leads. In the case of AC power you would need two pair of series diodes wire back to back. In the HO loco application these diodes would be run in series with the motor. For end of train application you would need some sort of load. Interior incandescent lamps would suffice as the load.

Pete

[FJ and G]

Why do toy train makers use incandescent bulbs when LEDs last longer for headlights?

Also, I understand that some of the newer LEDs actually light the area like a bulb. These would be good for passenger car interiors

[lionelsoni]

LEDs have two disadvantages as headlights, color and flicker.

Until recently, the only white LEDs available have been very bluish, because they are actually blue LEDs with some phosphor to add the other colors. Only lately have you been able to buy LEDs that mimic the color of prototype incancescent headlights: http://www.richmondcontrols.com/

Unless you operate the LED on dc and put a substantial capacitor across it, it will wink with every slight interruption in its supply voltage. This may not seem like it would be too much of a problem; but it really looks very peculiar in practice.

I think that we will see a transition to LEDs as new designs come out.

[FJ and G]

Bob,

Thus far, the only thing I've done for headlights and taillights is place a dab of Tidy liquid detergent on the car headlights.

I then dab red flourescent paint on taillights (and yellow parking lights--flourescent).

When the lights are out and the black light is on, these are very convincing.

[Anonymous]

FJ and G
I have and run Ogauge trains on my home layout, but I belong to the LOWER ANTHRACITE MODEL R.R.CLUB The layout is all done in HO (in Model Railroader mag a few years back) but we used fiber optics to light up the ?1/87? ho scale car head & tail lights and they look very good.
We have a open house every year the month of Dec. and we got tons of compliments of the realistic look of the lights.
Jim D.
a.k.a. SOCKO