DCC Lighting - Directional Cab Lighting and Selectable Red Running Lights using a basic 4 Function Decoder

[NFWEM57]

I have developed a circuit which provides, using a 4 function decoder, normal running lights with red selectable on/off and direction sensitive cab lights.  No diode arrays or relays are used. 

 

The circuit makes use of optocouplers to provide a switched supply to red running lights and cab lights.  The circuit for the red running lights is shown below, the circuit for the cab lights is similar but uses Aux 2.  The cost of the the components is under £1, a dual optocoupler typically costing £0.28 and resistors just a few pence each although buying from a well know on line merchant might well be more expensive.  The optocouplers will only handle small currents , circa 20-50mA, but this is more than adequate to drive a single LED.  The current transfer ratio between input and output is nominally 1:1 and I have used 1KΩ to limit the current to around 10~15mA depending on track voltage.  To limit the current still further it is only necessary to change the 1KΩ resistor providing power via the optocoupler to the LEDs; the right hand one in the diagram below.

 

With Lights and Aux 1 enabled and running forward the rear red LED will be illuminated and the front red LED when running in reverse.  With Aux 1 disabled, no power is available for the red LEDs.  So for normal running with rolling stock, only Lights need be selected. With a loco running on its own, Aux 1 is enabled to provide a red running light. 

 

 

 

I am just finishing converting a Mainline Class 42 to EM gauge with DCC control and the above lighting circuit and will upload a suitable video and images when complete but thought I would share this idea as it seems to be an issue judging by various historical threads.

 

Comments most welcome.

 

Kind Regards,

 

Patrick

Edited March 7, 2021 by NFWEM57
added additional info

[Suzie]

Patrick, I think it might be easier to do it this way. It sort of depends on how small your opto-isolator is compared to a couple of 1N4148 diodes.

 

Activating the green wire will turn off the rear lights, and using separate resistors will allow red lights at both ends for when you are parked (if this is a feature of your loco, and available on your decoder). Circuit will draw 12mA or thereabouts per LED whether turned on or not, but that probably will not eat too much in to the power budget.

If using a single resistor as in the original diagram you don't even need the diodes with most decoders.

[NFWEM57]

Hi Susie,

 

Thank you for your suggestion. 

 

The circuit I outlined above is intended to be used for (old) loco conversions to DCC and with cheap basic 4 function decoders.  The reason I used an opt-isolator is because the loco in question might be sold on with a dcc blanking plug.  Many still use DC and there is a modification that can be made to blanking plugs with diodes that provides directional lighting which works fine with filament lamps on old locos, but not LEDs; they burn out quite quickly because of excessive reverse voltage caused by  the combination of diodes.  In addition, there is no control over illumination.  So, I have also developed an 8 pin blanking plug for any loco which is DCC ready to allow the directional lighting to work, filament or LED, provide a reasonably constant voltage out over the input range and some adjustment of brilliance.   The circuit is in a separate thread. 

 

On the optocoupler size, 8 pin DIP, and as running lights are normally the preserve of diesels and the like there is usually, but not always, plenty of space so I think slight differences in volume I think are less important.  That said, I managed to fit my original DCC lighting circuit and an 8 pin DCC socket into an older, non DCC, Bachmann Class 45 where space is very much at a premium.   Steam is a different proposition..!

 

The image below is the above circuit in the process of being fitted to a Bachmann Warship being upgrade to EM Gauge & DCC to give you an idea of size.

 

 

Once again, thank you for your interest and suggestion.

 

Kind Regards,

 

Patrick

 

 

 

Edited March 7, 2021 by NFWEM57
typo, clarification and removed off topic to separate thread