Wiring up bipolar head/tail light circuit to a decoder - 3 options

[JanW]

Here is the problem:
Modern train models have their front and rear lights made in the form of LEDs fitted on a small printed circuit, complete with a common resistor (see figure 1).  The circuit has two connections that are wired to the power pick up from the wheels. When driving forward the white / yellow LED is conducting and emits light. The red tail light is blocking and is off. When power is reversed and the train/ loc reverses, the red light comes on and the white/yellow LED blocks. 

Most general purpose decoders come with three connections for the head and tail lights. A common blue power connection (+) and a white wire for the head light/LED (sinks to - connection when the head light should be on) and a yellow wire for the rear light/LED. 

How to connect the 2 bipolar wires to the 3 decoder wires?   Here I listed 3 options. There maybe more that I am not aware of. 

1) Modify the printed circuit board (PCB) that came with the model such that one of the LEDs is reversed  
2) Add some circuitry to achieve bipolar current through the PCB without bipolar decoder outputs
3) Use a decoder with bipolar LED outputs (I have not found these but they may exist) or the decoder motor output.

The first figure shows the circuit on the PCB as originally shipped with the model. 

Option 1:
This involves the modification of the circuit and connections on the original PCB. The figure 2 shows how the circuit should end up. I tried that once with mixed results. It is a very fiddly and nerve wrecking deep surgery undertaking that I prefer not to do again!  If you are very good at soldering of e.g. SMD devices you may give this a try.  

Option 2:
This is a trick I learned on a Facebook group on N-scale enthousiasts.  The trick is rather simple, see figure 3.  You add two 1K Ohm resistors. The decoder will draw current through the yellow or white wire. This causes a current to flow through one of the added resistors but also through the other resistor and the PCB.  
This works really well and only causes slight dimming of the LEDs (they have an added resistor in series). However that is not a big problem at the 14V from the decoder.
Another drawback is that you will need some space for the resistors. That could be a problem in some models. 
When you apply a high track voltage, the blocking voltage of the LEDs may be exceeded. In that case you need to add additional diodes in series with the resistors to protect the LEDs on the PCB. 

Option 3a:
Use decoders with a bipolar LED connections.  These may exist but I have not been able to source them. Suggestions are welcome!

Option 3b:
This is a another trick I learned on a Facebook group on N-scale enthousiasts.
See figure 4. 
This option uses the motor connections of a decoder. 
Drawback is that you will need to fix the motor power output such that the head/tail lights have the desired brightness at all times and control the motor with F0. 
I use the LaisDCC 860012 decoders (really cheap decoders but a bit bulky) for this purpose. That decoder has the advantage that it can be programmed to be a accessory motor decoder (for turntables, cranes etc) with a constant albeit programmable speed . You can program to have the 'motor' (i.e. the lights) to turn on/off with a function key (F0 in my case) and where the motor direction follows the direction command from the command station.  
So I give the LaisDCC decoder the same address as the real motorcar decoder and when that changes direction, the LaisDCC decoder changes polarity of its grey/orange wires as well. 

The photo shows the E351 'Super Azusa' front car fitted with a LaisDCC decoder. To camouflage the thing a bit I airbrushed the decoder and wiring. I need to machine a number of seats away to make space for the decoder. 

[chadbag]

I use Option-3a -- https://www.snjpn.com/ngdcc/df19/df19x4bpss.htm  and https://www.snjpn.com/ngdcc/df11/df11r6j.htm

 

Easy to order from (in english) through email.  Works reallyw ell.

 

Edited June 30, 2021 by chadbag

[gavino200]

I use option one. Generally it just involves a bit of tinkering and soldering. I usually go lo-tech unless I'm forced to go hi-tech. But I have to say, I like Chad's option. I may try it next time. 

[JanW]

1 hour ago, chadbag said:

I use Option-3a -- https://www.snjpn.com/ngdcc/df19/df19x4bpss.htm  and https://www.snjpn.com/ngdcc/df11/df11r6j.htm

 

Easy to order from (in english) through email.  Works reallyw ell.

 

I ordered decoders from this person before, but never realized he makes these function decoders as well!

 

The decoder only seems a bit large compared to others. I like the panto flash led connection though! 
 

Many thanks for this tip!

[JanW]

43 minutes ago, gavino200 said:

I use option one. Generally it just involves a bit of tinkering and soldering. I usually go lo-tech unless I'm forced to go hi-tech. But I have to say, I like Chad's option. I may try it next time. 

You might wish to try the second option. It is really simple .

[warnerj01]

Option 2 is interesting. It makes perfect sense. I'm going to try this soon.

[chadbag]

2 hours ago, JanW said:

I ordered decoders from this person before, but never realized he makes these function decoders as well!

 

The decoder only seems a bit large compared to others. I like the panto flash led connection though! 
 

Many thanks for this tip!

 

They are not really that large.  Probably smaller than the LaisDCC motor decoder (I don't have any motor decoder from LaisDCC -- just function decoders to run interior lights inexpensively)

[chadbag]

2 hours ago, gavino200 said:

I use option one. Generally it just involves a bit of tinkering and soldering. I usually go lo-tech unless I'm forced to go hi-tech. But I have to say, I like Chad's option. I may try it next time. 

 

Didn't you get some NGDCC decoders with me a few years back when I ordered a bunch?

 

[gavino200]

49 minutes ago, chadbag said:

 

Didn't you get some NGDCC decoders with me a few years back when I ordered a bunch?

 

 

Actually, I think I did. It's a long time since I've ran DCC on a real layout. I almost feel nostalgic about decoder installs now........Almost!