How Do Bi-Colour LEDs Work?

[Sir TophamHatt]

Can anyone tell me how bi-colour LEDs work?

 

I have an old Lima model but it only has space for one tower type LED.

 

Wondering if I should go down the route of bi-colour LEDs but don't know how they work - how does the loco know which colour to show?

It seems some have three wires coming out the bottom, but others only two still.

Edited November 17, 2020 by Sir TophamHatt

[Edwin_m]

There are three types, all of which have two normal LEDs of different colours in the same package.  

 

One type has two wires, and the two LEDs are connected in inverse polarity.  If the current is reversed the LEDs change colour.  That might be useful for DC but difficult to do with DCC.  

 

The second and third types have three wires, and the two LEDs are connected common cathode and common anode respectively.  Common anode is the best type for DCC as the common terminal can connect to the decoder common (blue wire) and the other two to function wires (such as yellow and white).  

 

Always remembering to use appropriate resistors in each case.  

[Dagworth]

3 hours ago, Edwin_m said:

That might be useful for DC but difficult to do with DCC

Easy enough really, just that the DCC function wires go in a slightly different place.

 

 

 

Andi

Edited November 17, 2020 by Dagworth

[Sir TophamHatt]

2 hours ago, Dagworth said:

Easy enough really, just that the DCC function wires go in a slightly different place.

 

 

 

Andi

 

Would you be able to dumb that diagram down a little?

Can see the LEDs, I guess the R3/R4 are resistors (1K is the standard I use for lights) but unsure of the other bits and how they'd fit into a track pickup circuit.

[Dagworth]

30 minutes ago, Sir TophamHatt said:

 

Would you be able to dumb that diagram down a little?

Can see the LEDs, I guess the R3/R4 are resistors (1K is the standard I use for lights) but unsure of the other bits and how they'd fit into a track pickup circuit.

Sorry, yes of course.

 

The resistors (R3 and R4) are whatever value gives you a brightness of the LEDs to suit your eye, I tend to use either 1k or 10k.

 

Hopefully the bicolour LEDs are self explanatory, F0f and F0r refer to function outputs from the decoder, Function 0 forward and Function 0 reverse (or White and Yellow wires)

 

If both functions are "off" then no current flows. If F0f is on the current flows through both resistors - that through R4 does nothing except warm the resistor up - but that flowing through R3 also flows downwards through the LEDs lighting one side (colour) of them. Likewise of F0r is on then  current again flows through both resistors but this time R3 just gets while that flowing through R4  flows upwards through the LEDs lighting the other side (colour) of them.

If both function outputs are on then both resistors get warm and the LEDs do nothing, but that shouldn't be able to happen if you have directional operation selected when programming the decoder.

 

Andi

Edited November 17, 2020 by Dagworth

[Sir TophamHatt]

9 hours ago, Dagworth said:

Sorry, yes of course.

 

Brill!

 

Although I begrudge paying £4 postage, will get some LEDs and give this a go.

 

Thank you sir! 

[RAF96]

Just in case you are still confused by the diagram the F0f and F0r are the white and yellow switched negative wires from your decoder, via the wiring to the LEDs.

 

As an aside...

With bi-colour 3-legged LEDs say red-green you can power both legs at once and get amber-yellow but you need to balance the resistor values against the prime colours characteristics to get the best shade. A simple way is to sub one resistor on the bench with a variable potentiometer, twiddle to get the right shade and measure the pot value. I use this method of indication at paired points so that if one doesn't fire then I get a yellow light instead of the set of reds or greens.

[Sir TophamHatt]

It works!

Thanks very much 

[Sir TophamHatt]

However...

 

I have a loco that I've tried doing this at both ends but somewhere along the way, I have confused some wiring.

 

In the FWD position, the lights show white at one end, dimmed white at the other.  In the REV position, the lights show full white at one end and red at the other :S