LED tail lamps

[Ray H]

I'm about to fit tail lights to my lightly aged second-hand Bachmann Class 37/0 - the type with the split headcode windows.

 

I've bought what I see are termed as 1.7mm axial LEDs for the purpose.

 

There is probably room under the bonnet to fit dedicated resistors to each LED but that will ultimately require 4 resistors and the associated wires at each end of the loco by the time I've illuminated the headcode windows as well.

 

I recognise that wiring two LEDs and a single resistor in series means that if one fails the other ceases to be illuminated but I was wondering how much the brightness will be reduced by wiring them in series.

[neilrh]

You could wire the LEDs in parallel, then in series with a resistor - 1 resistor at each end for this set up.

[Ray H]

That's an idea I hadn't thought of.

 

I've just looked more closely at the LEDs and the packet they came in.

 

The instructions say that the leg with no marking connects to the +ve, the marked leg - there's a black line on the back of the LED - is the -ve. Sounds similar to electrolytic capacitors.

 

However, the marked (or -ve) leg is the longest. I always thought that the longest leg on a LED was the +ve side.

 

Have I missed something?

[cliff park]

Wiring them in series will not reduce the brightness, providing you have an appropriate resistor. The brightness is determined by the current through them. The current is determined by the series resistor. Remember that each LED will have a forward volt drop, and if you have too many the supply voltage may not overcome this (zener) effect.

[smokebox]

Handy LED calculator here.

[Ray H]

The idea was to have the two red LEDs with a series resistor at each end. Only one pair will be illuminated at any time.

 

Likewise, the headcode illumination LEDs - one in each box at the same end again linked by the series resistor.

 

The headcode LED pair at one end will be wired in parallel with the tail lamp pair at the other. So there will be a maximum of four LEDs illuminated at any one time, connected as two parallel pairs with a series resistor for each pair.

[Crosland]

If they are plain LEDs (not 5V or 12V types) then there is plenty of voltage available to wire two in series with a resistor.

 

Two such strings can then be wired in parallel.

 

Andrew

[Ray H]

Thanks.

 

The package says not to exceed 3v without resistor & to use a 1k resistor when the supply is 12v so I'll guess they're not 5v/12v LEDs.

 

The legs seem quite frail. I can understand how one broke but the leg on a second seemed to fall off just because it saw the soldering iron.

[Grovenor]

 

then there is plenty of voltage available to wire two in series with a resistor.

Bearing in mind that this is "non-dcc" using two in series will increase the speed at which the lights come on. You really need a constant current regulator rather than a simple resistor to avoid the light getting brighter and brighter as track voltage is increased.

Keith

[Ray H]

I know that I posted under non-DCC because I didn't consider it was really a DCC related matter. However, it is indeed a DCC fitted loco. Apologies if I misled anyone.

[Crosland]

I know that I posted under non-DCC because I didn't consider it was really a DCC related matter. However, it is indeed a DCC fitted loco. Apologies if I misled anyone.

 

I must be a mind reader

[Ray H]

SWMBO reckons that I am as well, sometimes!

[Grovenor]

I know that I posted under non-DCC because I didn't consider it was really a DCC related matter. However, it is indeed a DCC fitted loco. Apologies if I misled anyone.

Apology accepted, but using the decoder accessory outputs is very much a DCC matter, on Dc the setup has to be very different.

Perhaps the mods could move the topic.

Regards

Keith

[Ray H]

I was unaware of this.

 

Excuse me asking but how does the DCC setup vary from a DC setup?

 

I'm about to fit the LokSound v4 decoder and connect the lights to it.

 

I was intending to connect the headlights at one end with the tail lights at the other together and link one set to the headlight connection on the decoder and the other set to the tail light connection.

 

I recognise that this means that the tail lights are on even when the loco is working a train which I would like to avoid.

 

I fitted a (kit) set of lights to a Class 08 and had to use four different functions to illuminate the head & tail light at each end individually. That does allow both the tail & marker light to be on simultaneously, not something I need to allow for.

 

Is it possible to use one function to switch the lights on and off (F0?) and a (single) separate function to isolate the tail lights? I assume that all four sets of lights need to be wired to different decoder contacts to do this.

 

Apologies once again for originally posting under the wrong heading and thanks for arranging to have the topic moved.

[Nigelcliffe]

Excuse me asking but how does the DCC setup vary from a DC setup?

 

I'm about to fit the LokSound v4 decoder and connect the lights to it.

 

I was intending to connect the headlights at one end with the tail lights at the other together and link one set to the headlight connection on the decoder and the other set to the tail light connection.

 

I recognise that this means that the tail lights are on even when the loco is working a train which I would like to avoid.

 

I fitted a (kit) set of lights to a Class 08 and had to use four different functions to illuminate the head & tail light at each end individually. That does allow both the tail & marker light to be on simultaneously, not something I need to allow for.

 

Is it possible to use one function to switch the lights on and off (F0?) and a (single) separate function to isolate the tail lights? I assume that all four sets of lights need to be wired to different decoder contacts to do this.

Yes you can wire independent lights.  For this you need four function outputs.  The LokSound V4 has sufficient outputs, so if hard-wiring a loco, why not do it.

 

With a LokSound V4, you can set any combinations of function keys, motion and direction on the loco to control any combination of outputs. So, whatever control combination you want can be achieved if the lights are wired independently.  So, setting F0 to be "lights" key, and having it select which two lights based on direction of travel, and then adding another Function Key as "turn off tail lamp" is possible.    Working out the CV settings to achieve this can be complicated.  The values required depends on the specific sound project loaded into the decoder (though its possible in most cases to just "do it").  It is easier if you have one of:  CV structure from the writer of the sound file in the decoder, a LokProgrammer or a computer interface and current copy of JMRI/DecoderPro.  

[Crosland]

I was unaware of this.

 

Excuse me asking but how does the DCC setup vary from a DC setup?

 

With DCC you have a permanant track voltage of (usually) at least 12v wichi allows for wiring more than one LED in series.

 

With DC the track voltage varies and your LEDs will not light until it is high enough, which depends on the colour of the LEDs. Wiring two LEDs in series requires double the voltage before they work.

 

Andrew

[Ray H]

Nigel

 

I hadn't thought of it that way. It could also solve another problem with routeing the wires from the body to the chassis. I'll investigate further unless someone reading this knows the answer. Alas I don't have a LokProgrammer or a computer interface and current copy of JMRI/DecoderPro.

 

Andrew

 

That bit I do understand but hadn't looked at Keith's post in the light. I thought he meant something else might need doing.