DCC lights in Heljan diesels

Discussion in 'Techniques' started by FiftyFourA, 3 November 2016.

  1. FiftyFourA

    FiftyFourA Western Thunderer

    I wonder if any luminary out there can advise me on this -

    Looking at a chip I have just put into a modified Heljan 37 (Loksound XL v4.0), it occurred to me that I might be able to wire the red lights to spare aux ports (an extra one now the fan is disconnected for good) and programme them to two function keys so that I can switch them on/off depending on whether the loco is light engine or pulling a train.

    Is it that easy or am I missing something that will go bang!

    Will the lights need a resistor and how to wire it - bearing in mind I can just about wire a DPDT switch.

    Dog Star likes this.
  2. richard carr

    richard carr Western Thunderer


    Yes you can do that but its not as obvious as you might think to get all the wiring right.
    Personally I put the head code lights on the forward and reverse light options of the decoder and then put the tail lights on aux 1 and aux 2 so they can switched independently if you want to, although in practice I rarely put the tail lights on.
    You can do the same with cab lights if you want to.
    On some of the early models you need resistors but the more recents have them built in.

  3. Yorkshire Dave

    Yorkshire Dave Western Thunderer

    Hi Peter

    Firstly I'd go with what Richard has said about the headcode boxes on the normal forward/reverse lighting function and the tail lights on aux 1 and 2.

    Wiring resistors - this diagram is from the ESU Loksound 4 manual. Although for LEDs it is no different for bulbs and I use either 470 Ohm or 630 Ohm resistors.

    For the lighting in my locos I now replace bulbs with 12v white/yellow (or red) LEDs with either 470 Ohm or 630 Ohm resistors. I also reduce the brightness and switch the lighting outputs on the decoder to LED mode. I had no option with the latter where I used a 12v micro bulb with a resistor for a rotary beacon.

    And finally... If you have access to or know someone locally with either an ESU Lokprogrammer or SPROG programmer this makes the programming the lighting and other functions a lot easier.
  4. Softvark

    Softvark Active Member

    Have a look at my post here which explains what I do. With the addition of a relay you can make the tail lamps direction dependent without using any other decoder functions.
  5. Nigel Cliffe

    Nigel Cliffe Western Thunderer

    If the wiring is done as per Peter and Dave's answer, then how it is controlled is all down to CV settings in the decoder. I'd be using a larger resistor value than those quoted, they are about the minimum safe values for any LED, and a larger value will get a dimmer LED at the outset, rather than risking a degree of flickering from turning down the brightness with CV settings.

    Just about any function key control options are possible - particular F-button to turn all off, direction dependent behaviour for lights, etc.. etc.. But, setting them up manually by CV changes is a long complex process. Which is where the computer interfaces come in, anything which connects a computer to the DCC system (or a standalone programming device such as Sprog) will help with these changes.

    Julian's solution seems to save one function output. Useful if running out of function outputs, but if there are enough spare outputs on the decoder I don't think its worth the complexity. The function-key behaviours are all possible with CV changes.

    - Nigel
  6. FiftyFourA

    FiftyFourA Western Thunderer

    Thanks everyone for the advice - its pretty much what I was thinking myself.

    The problemfor me is the re-programming. I have a Sprog & bits but I have an major aversion to programming and doing much with computers (apart from the basics required in this day & age) after years of being involved with them, latterly as a university lecturer.

    I do find that the 'logic' of some of the people involved in designing these systems rather puzzling at times - made worse by being translated (often badly) from the original German or, worse still, written in 'American' by some spotty urchin. I am not, as you may have guessed, terribly politically correct :headbang:.

    Still, it is maybe something to while away the winter nights when not at the pub.

  7. Yorkshire Dave

    Yorkshire Dave Western Thunderer


    What size resistor do you normally use with 12v LEDS - 1K or higher? Or higher still with lower voltage LEDs?
  8. michl080

    michl080 Western Thunderer

    Peter, believe me, as a native German speaker, it is not the translation, it is the logic behind. Actually, the English translation seems to be superior because of its simpler wording.

    I think it is just the way software people think :)

  9. Nigel Cliffe

    Nigel Cliffe Western Thunderer

    Lets be clearer about "12vLED". There are LEDs sold as "12v" which are a LED+Resistor in a single packaged unit. Personally I don't use them, but if I were, I might add another 500 ohm to 1000 ohm and conceivably vastly more.

    Most LEDs just come as an LED. I buy mine from electronics sources where a spec sheet is available. But, even without a spec sheet, a rule-of thumb calculation can be done by assuming 3V drop over the LED. So, the voltage drop over the series resistor is "function output volts - LED volts", which is going to be approx. 15-3v=12v (assuming a 16V track voltage).
    An assumption on max current through the LED would be 0.025A max, perhaps 0.010 being more reasonable max figure.
    Using Ohms Law (V=IR), and knowing V and I, we get a value of R=1200ohm. So that's the minimum I'd use.

    For a lot of loco lighting, prior to the mid 1980's and the arrival of high intensity lights, the real-world lights were about half a glow worm. They are fairly dim even in the dark, and pretty hard to see if lit at all in normal daylight. So, if trying to achieve that, select appropriate resistors which give something fractionally brighter than "appropriate". It might be as much as 10Kohm (yes ten, could be more, I think I've used 20K in one case). So, buy a few common sizes (say 1K, 2K, 3.3K, 5K, 7.2K and 10K) and experiment.

    Having got the lights a tiny bit brighter than wanted, turn down the brightness a little more with the brightness control in the decoder.