Wiring White & Red SMD LEDs

[ISW]

Having solved my problem with 'dimming' LEDs I'm now struggling to understand why I cannot get the white / red loco lighting to work.

 

What I'm trying to do is use F0 to control the lighting as follows:

1. Blue/yellow circuit - 2x red rear lights + 1x front light
2. Blue/white circuit - 2x red front lights + 1x rear light

In other words, when the loco moves it lights the headcode box at the front, while lighting the red lights at the rear, And this is to work no matter which direction the loco is moving.

 

My circuit layout is below:

 

When I try this, only the (correct) red LEDs light up. So what have I got wrong?

 

With a bit of swapping around of the wires I can get the following to light on a single (white or yellow circuit):

• 2x red
• 4x red
• 2x white

But I can't get 2x red + 1x white!

 

In all above cases, the LEDs are in parallel.

 

Is there a 'difference' between red & white LEDs that I'm not taking into consideration?

 

Ian

[Dagworth]

In all above cases, the LEDs are in parallel.

 

Is there a 'difference' between red & white LEDs that I'm not taking into consideration?

 

Ian

Yes, the white LED needs a higher voltage.

 

The simple answer is one resistor PER LED.

 

(You'll also find you need a higher value resistor for the white - around 10k in my experience - than the 1.5k I know you have, for the red)

 

Andi

Edited March 9, 2018 by Dagworth

[Gordon H]

 

Is there a 'difference' between red & white LEDs that I'm not taking into consideration?

 

 

The Forward Voltage drop (Vf) of white LEDs is much higher than that of red LEDs, so a red LED in parallel with a white will almost certainly prevent the white one from lighting.

You would be better off either with the red and white LEDs in series, or having separate resistors for the white and red LEDs.

[ISW]

Yes, the white LED needs a higher voltage.

 

The simple answer is one resistor PER LED.

 

(You'll also find you need a higher value resistor for the white - around 10k in my experience - than the 1.5k I know you have, for the red)

 

Andi

 

 

The Forward Voltage drop (Vf) of white LEDs is much higher than that of red LEDs, so a red LED in parallel with a white will almost certainly prevent the white one from lighting.

You would be better off either with the red and white LEDs in series, or having separate resistors for the white and red LEDs.

 

Gents,

 

Many thanks for the responses. I had a feeling that it was something to do with the voltages being different, but my Google searches didn't throw up anything about it.

 

If I'm understanding correctly, I have 2 choices:

 

1. Series connection:
   1. 1x red + 1x red + 1x white + 1x 1k5 resistor (to give a total load of ~12v)
2. Parallel connections:
   1. 1x red + 1k5
   2. 1x red + 1k5
   3. 1x white + 10k resistor

Is this correct?

 

The bit I don't understand is that a white LED needs a higher voltage, meaning is has a higher internal resistance. So why does it need a larger resistor to work?

 

Ian

[Dagworth]

The reason for putting a higher value resistor on the white is just that they are MUCH brighter than the red, so the higher value tames it down

 

Andi

[ISW]

The reason for putting a higher value resistor on the white is just that they are MUCH brighter than the red, so the higher value tames it down

 

Andi

Andi,

 

That sort of implies that my 'series' solution will not work as the white LED will be too bright. Is that correct?

 

If so, what's the best way to wire up the white LED and 2x red LEDs to give reasonable brightness to all 3 LEDs?

 

(sorry to be such a pain)

 

Ian

[Dagworth]

Andi,

 

That sort of implies that my 'series' solution will not work as the white LED will be too bright. Is that correct?

 

If so, what's the best way to wire up the white LED and 2x red LEDs to give reasonable brightness to all 3 LEDs?

 

(sorry to be such a pain)

 

Ian

No reason why you can't have both reds at one end in parallel (or series) but with a single resistor shared between them. If the LEDs are the same they should both light. (check photos from your time period too, many locos could not have both red lights lit at one end, they were either one or the other, being able to light both together came in much later)

 

What I would do is to cut the vero track running to the white LED at each end and bridge that cut with a 10k resistor, and cut the track running to the red LEDs at each end and bridge that cut with a 1.5k. (Or cut the white track to each end six times and bridge each cut with a 1.5k so all the white resistors are in series, that would add up to 9k)

 

The other thing that I would seriously do if you can is to wire the red LEDs to the green and the purple function wires rather than white and yellow, then you can switch them off when the loco is hauling a train. You will never see a loco with tail lights on while it has a train behind it in real life. You can then control the tail lights from f1 and f2 of your DCC system.

 

Andi

[DCB]

Andi,

 

That sort of implies that my 'series' solution will not work as the white LED will be too bright. Is that correct?

 

If so, what's the best way to wire up the white LED and 2x red LEDs to give reasonable brightness to all 3 LEDs?

 

(sorry to be such a pain)

 

 

I think the problem is that A standard Red LED will light on around 1.8 volts.   A standard White Led will not light on 2 X AAA batteries around 3.2 to 3.6 volts needing nearer 4 volts.

This is why White LED torches need 3 X AAA cells So you will need much higher value resistors for the reds. Quite literally the reds will burn out before the whites come on.

so you need very different resistors for the reds and Whites if they are simply in parallel. I can't give values because I use 3 volt and 5 volt supplies for my LEDs not the 12 volt nominal 19 volt actual most people seem to use

[Gordon H]

I think the problem is that A standard Red LED will light on around 1.8 volts.   A standard White Led will not light on 2 X AAA batteries around 3.2 to 3.6 volts needing nearer 4 volts.

This is why White LED torches need 3 X AAA cells So you will need much higher value resistors for the reds. Quite literally the reds will burn out before the whites come on.

so you need very different resistors for the reds and Whites if they are simply in parallel. I can't give values because I use 3 volt and 5 volt supplies for my LEDs not the 12 volt nominal 19 volt actual most people seem to use

Dear me, not again...

Can we please stop referring to standard LEDs lighting from a particular voltage?

It has been explained on this and other forums time and time again that this topic needs to be considered in a different manner.

A controlled current is what lights an LED, and that current flow results in a voltage drop across the LED, the value of which varies with LED colour and internal chemistry.

Clearly, the supply voltage needs to be at least that which the LED is going to drop when lit, but any more voltage beyond that is then dropped by a further device, used to perform the current control - usually a resistor.

[Dagworth]

I think the problem is that A standard Red LED will light on around 1.8 volts.   A standard White Led will not light on 2 X AAA batteries around 3.2 to 3.6 volts needing nearer 4 volts.

This is why White LED torches need 3 X AAA cells So you will need much higher value resistors for the reds. Quite literally the reds will burn out before the whites come on.

so you need very different resistors for the reds and Whites if they are simply in parallel. I can't give values because I use 3 volt and 5 volt supplies for my LEDs not the 12 volt nominal 19 volt actual most people seem to use

You don't need higher value resistors for the reds, they are less bright than the whites by design so need a LOWER value resistor. White LEDs tend to be very bright and need a higher value resistor to dim them down.

 

Andi

[RAF96]

The best thing i ever bought was a prototyping breadboard, that I use to prove my circuits. Using smds on it means attaching wires to them first, although it is often simpler to build a typical loco light board and plug that in. Normal leds simply plug in.

 

My ‘universal’ circuit for diesel loco lights is 3 x white and 2 x red at each end. All the anodes (white and red) are commoned at each end with a single 1K ohm resistor feeding back to the decoder socket blue pin. All the white cathodes are commoned at each end with a single 1K ohm resistor feeding back to the decoder socket white or yellow pin (see later). All the red cathodes are commoned at each end with a single 1K resistor feeding back to the decoder socket yellow or white pin (see later). That makes 6 x resistors on a small board (see later).

 

Obviously to get the whites at one to light with the reds at the other end requires the front whites and the back reds to connect to the socket white pin and the back whites and front reds to connect to the socket yellow pin.

 

The mounting methodology is to put the resistors on a small veroboard with sets of red, white and blue wires running to the loco front end, back end and decoder socket. At each end the leds are mounted to a bit of veroboard using link wires to connect the leds as above, then the resistor board wires are connected to these boards. Finally after proving the installation with a battery, the socket wires are connected.

 

 

This is basic directional lighting with none of the extra switching required for day/night or tails off running that many folk may consider essential.

 

It works for me.

Rob

[ISW]

The best thing i ever bought was a prototyping breadboard, that I use to prove my circuits. Using smds on it means attaching wires to them first, although it is often simpler to build a typical loco light board and plug that in. Normal leds simply plug in.

 

My ‘universal’ circuit for diesel loco lights is 3 x white and 2 x red at each end. All the anodes (white and red) are commoned at each end with a single 1K ohm resistor feeding back to the decoder socket blue pin. All the white cathodes are commoned at each end with a single 1K ohm resistor feeding back to the decoder socket white or yellow pin (see later). All the red cathodes are commoned at each end with a single 1K resistor feeding back to the decoder socket yellow or white pin (see later). That makes 6 x resistors on a small board (see later).

 

Obviously to get the whites at one to light with the reds at the other end requires the front whites and the back reds to connect to the socket white pin and the back whites and front reds to connect to the socket yellow pin.

 

The mounting methodology is to put the resistors on a small veroboard with sets of red, white and blue wires running to the loco front end, back end and decoder socket. At each end the leds are mounted to a bit of veroboard using link wires to connect the leds as above, then the resistor board wires are connected to these boards. Finally after proving the installation with a battery, the socket wires are connected.

 

 

This is basic directional lighting with none of the extra switching required for day/night or tails off running that many folk may consider essential.

 

It works for me.

Rob

Rob,

 

I was thinking that a breadboard might be a good investment, along with a bag of various resistors (I have plenty of LEDs already ...).

 

I note that you keep the red and white LEDs on separate circuits and can understand your reasons. However, I was 'hoping' to be able to get the front white and rear red LEDs to work automatically on F0 the same as many of the current RTR locos.

 

I'm hopeful that with a breadboard, and a good look at the circuits inside a Bachmann Class 44 I have, I can duplicate the effect. Hopeful being the operative word!

 

Ian

[ISW]

No reason why you can't have both reds at one end in parallel (or series) but with a single resistor shared between them. If the LEDs are the same they should both light. (check photos from your time period too, many locos could not have both red lights lit at one end, they were either one or the other, being able to light both together came in much later)

 

What I would do is to cut the vero track running to the white LED at each end and bridge that cut with a 10k resistor, and cut the track running to the red LEDs at each end and bridge that cut with a 1.5k. (Or cut the white track to each end six times and bridge each cut with a 1.5k so all the white resistors are in series, that would add up to 9k)

 

The other thing that I would seriously do if you can is to wire the red LEDs to the green and the purple function wires rather than white and yellow, then you can switch them off when the loco is hauling a train. You will never see a loco with tail lights on while it has a train behind it in real life. You can then control the tail lights from f1 and f2 of your DCC system.

 

Andi

Andi,

 

Cutting the tracks and inserting the 10k resistor seems unlikely to help as the white is not lighting now (as it is commoned with 2x red LEDs at the other end). Re-wiring to use the green and/or purple wires and control using f1 / f2 is an option that I know will work, but I was really hoping to be able to use f0 for both red & white lights.

 

I'm planning to get a breadboard so that I can 'try out' different options and see where that gets me.

 

I know the DCC chip kicks out 12v on the blue wire, but what is the current it supplies? The reason for asking is that I'll probably use a 12v battery with the breadboard.

 

Ian

[melmerby]

Be warned

As leds are current driven devices, to use two (or more) in parallel they need to be matched, voltage drop wise, to a close figure else most of the current will go through one led leaving one very bright one and one dim one.

Their characteristics are much like a zener diode with a voltage drop that is fairly constant over a fairly wide current range, unlike a filament lamp.

 

Keith

[Dagworth]

Andi,

 

Cutting the tracks and inserting the 10k resistor seems unlikely to help as the white is not lighting now (as it is commoned with 2x red LEDs at the other end). Re-wiring to use the green and/or purple wires and control using f1 / f2 is an option that I know will work, but I was really hoping to be able to use f0 for both red & white lights.

 

I'm planning to get a breadboard so that I can 'try out' different options and see where that gets me.

 

I know the DCC chip kicks out 12v on the blue wire, but what is the current it supplies? The reason for asking is that I'll probably use a 12v battery with the breadboard.

 

Ian

As long as you have a resistor in series with the red led and a separate resistor in series with the white then both will light.

 

Try it for yourself.

 

Andi

[Suzie]

Modern LEDs are much more consistent than they were in the olden days so you can usually get away with wiring identical LEDs (and they must be from the same batch and everything) in parallel - just look at Express Models lighting kits to see it work in practice.

[tractor_37260]

Modern LEDs are much more consistent than they were in the olden days so you can usually get away with wiring identical LEDs (and they must be from the same batch and everything) in parallel - just look at Express Models lighting kits to see it work in practice.

 

The majority of Express Models Kits use Red and Yellow LED's, these work fine with parallel wiring, However once you substitute White Led's for the Yellow ones, unless split feeds are used with separate resistors, a basic linked directional circuit with White at the front  - Red at the rear does not work, although it works with Yellow Led's.

[Suzie]

The majority of Express Models Kits use Red and Yellow LED's, these work fine with parallel wiring, However once you substitute White Led's for the Yellow ones, unless split feeds are used with separate resistors, a basic linked directional circuit with White at the front  - Red at the rear does not work, although it works with Yellow Led's.

I would not recommend paralleling different coloured LEDs.

[melmerby]

I would not recommend paralleling different coloured LEDs.

Or different brightness leds of the same colour.

 

Keith

[RAF96]

ISW

The decoder spec sheet will give you the output current limits. These vary decoder to decoder just like the motor current capability. I use a 9volt brick battery to power my breadboard with a 1K resistor in the positive lead as I stupidly tested some leds in a loco body apart from the loco pcb and they blew.

 

Getting the front whites and rear reds to light together is as simple as joining the cathode wires together, If you can imagine my wiring as a Tee shape. Each horizontal leg of the Tee has a white, red and blue wire and the vertical leg has a white, yellow and blue wire.

 

The horizontal (front and rear end leds) blues go through resistors and join to go to the socket as blue. The front white and rear red wires from the leds go through resistors and join to go to the socket as white. The rear white and front red wires from the leds go through resistors and join to go to the socket as yellow.

 

When F0 is selected on going fwds you get the front whites and rear reds and vice-versa in reverse.

I hope that makes sense as I don’t have a drawng handy to post.

Rob

[John ks]

This layout gives each LED its own resistor

 

The difference between the 2 diagrams is the mounting hole positions (black circles) 

 

I have extended the Rail A & B vero tracks to the full length of the board (just a short wire from the vero board to the bogie pick up) 

 

 

The resistor values can be determined by trial & error 

I usually use 1K resistors 

With high brightness white LEDs you may have to go to 4K7 or even 10K to tame the light

 

If you need to use the green function the board will require some rearrangement.

 

Hope this helps 

 

John

[ISW]

This layout gives each LED its own resistor

 

The difference between the 2 diagrams is the mounting hole positions (black circles) 

 

I have extended the Rail A & B vero tracks to the full length of the board (just a short wire from the vero board to the bogie pick up) 

 

led wiring.png

 

The resistor values can be determined by trial & error 

I usually use 1K resistors 

With high brightness white LEDs you may have to go to 4K7 or even 10K to tame the light

 

If you need to use the green function the board will require some rearrangement.

 

Hope this helps 

 

John

John,

 

Many thanks for putting the effort in to explain in a 'picture' what others have 'described'. I'll be putting your arrangement into practice on a breadboard (recently purchased) before committing to, another, veroboard (I'll have none left at this rate ...).

 

My real problem seems to be convincing the white & red LEDs to play along at the same time. Hopefully, some 'fiddling' with resistor values on a breadboard will help.

 

Ian

[RAF96]

 

At last I have a diagram for you...

 

Rob

Edited March 13, 2018 by RAFHAAA96

[ISW]

DCC-Directional-Lighting.PNG

 

At last I have a diagram for you...

 

Rob

Rob,

 

Many thanks. Will have to break-out the breadboard and give it go ... Might take me a while (first time use and all that) as I'll have to cut / create a bunch of multivarious jumpers to use on the breadboard.

 

Ian

[John ks]

If you want to modify an existing Vero board the following should do the trick

 

You will need to make 4 cuts in the Vero tracks & bridge these 4 gaps with 4 resistors, 1K or there about should be fine for proof of concept

 

You can remove & bridge out the 2 existing resistors or I suspect it will work ok with them in circuit (the LEDs will be a bit dimmer but this may be acceptable)

 

 

 

 

a 5-10 minute job once the board is out of the loco

 

My previous Vero board layout was IMHO a better layout but modifying your existing board will only cost 4 resistors, a bit of time & use an existing board.

 

John