Constant-brightness headlights

Discussion in 'FAQs' started by XavierJ123, Apr 19, 2005.

  1. XavierJ123

    XavierJ123 Member

    I have an old steam locomotive with a headlight that brightens with the speed of the engine. How does one install constant-brightness headlights like John Allen had-----and----a glowing red bulb in the firebox??? :confused:
  2. ezdays

    ezdays Out AZ way

    As long as you are using rail power to light an incandecent lamp, this will be the results. You might try using a white LED and a series resistor. The light is more consistant over changes in rail voltage, but still won't light at real low voltages. You can also use a red LED for the firebox glow providing you can find room there.

    I haven't done either of these things, but they are my first thoughts. Could be that someone else has done this and has a better idea....;)
  3. nachoman

    nachoman Guest

    this is actually pretty simple. Here are some ways to do it:

    You are going to nead a constant voltage to your bulb in order to get constant brightness. A resistor won't do the job because the output voltage is proportional to the input voltage, which varies as the throttle setting is moved. This would only make the headlight dimmer, but it would still be brightest at higher speeds and dim at low speeds. The easiest way is to use an arrangemets of diodes. Diodes step down a voltage by about 0.7v no matter what the input voltage is. pair two diodes in series and you now have a voltage drop of 1.4v - perfect for a 1.5 volt lamp.

    you can buy diode constant lighting circuits with instructions at most hobby stores, and that might be easiest if you have little electronics knowledge. Or, you can buy the components yourself. You will need someting called a "bridge rectifier" (an arrangement of 4 diodes in one casing) and a 1.5v bulb. First, replace the headlamp with a 1.5 volt bulb. next, bend (+) and the (-) leads of the bridge rectifier together and solder. you will have the two free leads marked (~). This effectively creates a circuit that steps down voltage 1.4v no matter what the polarity of electricity is applied. In your locomotive, you need to create a circuit similar to this:

    xxxxxxxxxxxxxxxxxxxxxxxxx _____lamp______
    wheel contact____motor___|__bridge rectifier__|___wheel contact

    I hope the crude diagram shows up. Ignore the x's, I had to use them as spacers to make the diagram show up. The way it works is the rectifier creates a voltage drop of 1.4 volts from one lead to the other, and keeps the lamp power at that voltage. It will also reduce the voltage to the motor by 1.4v and cause it to run a teeny bit slower, which is usually a bonus rather than a problem.

  4. ezdays

    ezdays Out AZ way


    I agree with you on the series resistor, but I was suggesting using an LED rather than an incandecent bulb where the lighting is more constant over a larger range of voltage. For example, it is difficult to distiguish the difference in LED intensity between 10 and 30 mA. It's not the ultimate solution.

    Your idea of using a bridge rectifier as series diodes is Ok too, just I see no value in tying the plus and minus together, plus now we are getting into a larger component to find a place for. If you are looking for a constant voltage source, I'd recommend using a simple regulator. They come in all flavors including 1.5 volts and TO92 case size (small transistor size). A zener diode, or your suggestion of two diodes in series, is good, but will still require a series voltage dropping resistor.

    Regardless of what the solution is, the lamp or LED will not light very brightly at very low voltages, and something will get a bit warm when the voltage is higher since all that excess voltage has to be dissapated somewhere.
  5. CN1

    CN1 Active Member

    nachoman and Don

    Thanks for the "how to". I'll print it and keep it for future reference
  6. Matthyro

    Matthyro Will always be re-membered

    Another good reason to switch to DCC if you can. Headlights are just as bright when the train is stopped as when they are running.
  7. jim currie

    jim currie Active Member

    how high tech do you want to go there are circuits that use ni cad batteries that recharge while loco is running. the one that i have used in several switchers that i wanted to have lights on while sitting for extended time is a led for light with two batteries in tender using a magnetic reed switch to turn on and off.
  8. XavierJ123

    XavierJ123 Member

    Thanks guys for all the input. I too will print this for future reference. It's on my list of things to "accomplish."
  9. johnnyb1216

    johnnyb1216 New Member

    i was wondering if i can use a 3mm ultra bright led with a 1k resistor in placeof the bulb
  10. pgandw

    pgandw Active Member

    You won't need the resistor - the bridge rectifier is set up for a 1.5 volt light bulb. The question will be whether the LED will light adequately on 1.5 volts.

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