Headlight Conversion

[Yampa Bob]

My only complaint about my Bachmann 2-8-0 Connies is the puny headlight.  Even at full throttle it's not very bright.  By comparison, when I barely advance the throttle for my Roundhouse locos, the light comes on bright and stays that way.  I like "flamethrowers", don't care if it's proto or not.

All my locos are DCC equipped.  I found schematics for making a constant brightness ciruit using an LED, but don't want to mess with that.

Several questions.

1. Has anyone changed the headlight to an LED, if so what were the results?  I want as near pure white as possible.

2. Other than adding an appropriate resistor, and modifying the metal casting to accept the LED, are there any tips or cautions.

3.  The Roundhouse apparently uses an LED, is the constant lighting achieved by the light board circuitry, or is it simply due to the low threshold of the LED?  If the latter, then the LED needs to withstand the voltage variance from 1 volt to 12 volts.  The Roundhouse light appears to remain constant over the entire throttle range, but may be going from bright to super bright, I can't tell.

4. Finally, why don't any or all decoders have the constant bright feature?

Thanks

[Jim Banner]

Several answers:

1. I put LED headlights in several of these.  The results were great.  You can have your choice between blue white, pure white and warm white.  I like the warm white because they look like incandescent lights, just like the real ones. 

2. If memory serves, the original gow lamp sits behind the headlight and the light is carried forward by a lucite "light pipe."  A 3 mm LED fits right in place of the original lamp.  The resistor can be anywhere.  1000 ohms, 1/4 watt is a good place to start if using DCC.

3. I don't know what Roundhouse does, but I do know the characteristics of LEDs.  They are linear, current driven devices.  This means that if you double the current, you double the absolute brightness.  (It also means that if you double the current, the voltage across the LED changes only very slightly.)   But remember that your eye needs a ten fold increase in absolute brightness for an apparent doubling of perceived brightness.  So lets look at some numbers.  With a 500 ballast resistor and a 3 volt drop across the LED, at 4 volts the current will be 2 milliamp.  At 16 volts, the current will be 26 milliamps.  The light output will increase by a factor of 13 between crawling and rocketing but it will appear only a little over twice as bright.  Compare to an incandescent lamp, which is a non-linear voltage driven device.  increasing the voltage across it from 4 volts to 16 volts is a factor of 4.  But the light output increases as the ninth power of the voltage ratio.  So at 16 volts, it light output is 262,144 times as much as at 4 volts.  Your eye will perceive it as about 43 times as bright.  Note that these changes are a property of the components, LED or incandescent lamp.

4.  All DCC decoders have constant brightness by the very nature of DCC's design.  When you turn a lighting function on, it applies rectified track voltage to the light.  With DCC, track voltage is constant, therefore so is the light.  However, if a decoder is installed so that the light is connected across the motor, the light will brighten and dim as more or less voltage is applied to the motor to speed it up or slow it down.  But this is not the way DCC was designed to work.  All DCC decoders and many non-DCC decoders have outputs for front and rear locomotive lights, and by and large, these are designed for constant brightness.  Some of the more advanced decoders allow you to select the level of constant brightness, for example to implement rule 17 lighting where a stopped locomotive must dim its headlight.  But again, those levels are constant in that they do not change with motor speed.  Still other decoders allow you to program in LED current without the need of a resistor.  These allow you to choose the level(s) of constant brightness that you want.

Bottom line, by all means replace the headlight in your 2-8-0 with an LED.  If you want a simple installation with the headlight connected across the motor, use about 560 ohms, 1/4 watt in series with the LED.  If you want a constant brightness headlight, run the wires all the way back to the tender and connect them to the lighting output of your decoder.  If it is a wired decoder, these will be the blue and white wires.  Use about a 1000 ohms, 1/4 watt resistor in series.  If the LED lights when the locomotive runs in reverse, then reverse its leads.

[Yampa Bob]

On my connies, the headlight wires already go all the way back to the decoder. But the light varies with the throttle, from puny up to mediocre. I get the same effect whether on DC or DCC power.  Perhaps the light is not wired correctly on the light board.

I'll just have to get some LEDs and try it. I rechecked my Roundhouse by running with the room lights off.  Just before the loco moves, the light comes on very bright, but at full throttle it was definitely much brighter.  My wife said "Wow, that looks like a real locomotive".   By the way, this particular Roundhouse loco is DCC ready, running on DC, I haven't installed the decoder yet. 

The present bulb is held sandwiched between the two castings, and there is a lucite rod leading to the headlight bezel.  I may have to either ream out the casting, or set the LED in front of the casting and shorten the lucite rod. The resistor will go in the tender.

The local Radio Shack has warm white 3mm LEDs. (here we go again) This time I know as I saw them in the drawer.  If I take them to the counter, and she says they don't stock them......(censored) 

[hotrainlover]

Bob,
What bulbs do you use?  Is there a certain type, or brand that fits best...

Thanks,
Lee

[Yampa Bob]

An LED is a "Light emitting diode", not really a "bulb" as we think of them.  A bulb has a resistive element, produces mostly heat and little light.  An LED produces a lot of light with very little heat. They usually require a resistor in series.  LEDs don't illuminate much, but have an intense visual presence. That's the extent of my knowledge about them.

I don't know about brands, the popular sizes are 3mm and 5mm.  They are normally mounted behind or inside a clear or colored lens, but the yellow-glo and warm white seem to be the choice for exposed lighting on locomotives.  The blue-white might be more suitable for a diesel loco.

[hotrainlover]

Bob,
I meant LED type, size.and/or brand that worked best....
Sorry for the confusion

Lee

[Yampa Bob]

That's ok, I live in the "State of Confusion"

Here's some good information on LEDs, I downloaded the PDF file:

http://www.tonystrains.com/technews/install-leds-decoders.htm

This is all new to me, I'll probably pick up some warm-white at Radio Shack and experiment with them.