RGB leds, this one to challenge the Electronics gurus.

It would be easier if you got a common cathode version:

www.superbrightleds.com/TriColor%20LED.htm

If you had this then simply a resistor in series with each anode leg to driving light +, dash lighting +

Hope that sort of helps.

Adrian

Shouldn't be too hard as long as you have a discrete earth to your driving lights (ie not earthed via the case). I'm not sure how well I can describe it here but I'll give it a go.
B represents the blue cathode, G represents the green cathode.

------------------------------------ Power source (Live when power available)
| |
Z Resistor |
| O Driving lights relay
O LED |
G/ \ B
/ Z Resistor
/ |_________________|
/ / Switch for lights
Z Resistor |
__/______________________| Ground

So when there is no power available, nothing will be on.
When power becomes available, it will run through the Green LED to ground.
When the switch is activated, the relay will switch on, turning the lights on.
Also, there will be a ground path provided for the Blue LED to turn on.
The resistors need to be appropriately sized so the LED only gets 20mA (from spec sheet). Apparently forward voltage is 3.5V, so I *THINK* you need around a 470 Ohm (12V) or 680 Ohm (15V - remember voltage is higher when engine running) resistor, but my calculations could be wrong there. I would go with the 680 ohm.

Also, the circuit as above will have both Green and Blue on when the lights are on. If you only want one or the other, you need a double throw switch that, when off, completes the path from G to ground.

Hopefully that is slightly clearer than mud.

There are probably nicer ways to do this, but the first one that popped in to my mind involves a simple SPDT relay (ie; any standard automotive relay).

Connect the blue cathode to 87, the green to 87a, 86 & 30 to ground and 85 to the +'ve side of your driving lights. The anode on the LED goes to a +'ve voltage supply - whatever you want to reference as power available (battery, ignition etc).

When the relay is in the NC position, the LED will illuminate green, when the relay is switched on (with power from the driving lights), it the NO pole of the relay will be energised - causing the LED to switch blue.

Or something like that :)

Hi Fawlty

You have some good answers already - but I don't think the light output of that led is very good via its diffused housing. The seperate 3mm leds from say jaycar look more impressive.

I have used one of those leds and the 8 possible colour combinations to show the different voltage levels from a battery. But this is a little more complex.

Robin Miller

Ok, thanks chaps..... Adrian's solution is now to hand a common cathode LED from Jaycar. His solution requires the least skill and I am therefore awarding the prize to him. Adrian email me with your snail mail addy & I'll post the kit to you.
sydney-riley@hotmail.com
Once again gentlemen I thank you all for your efforts...

Bugga - i was gonna suggest a microprocessor and PWM fade between colours :-(

:-)

Well there is no such thing as a free lunch.... Now seriously guys Adrian read the question, I wanted to use the green & blue and if they are on together I'll still get blue. However Adrian is now stuck with a DS steam whistle kit based on the AE circuit which to any afficiando will assure you that there is now way this kit when assembled makes a noise that in any way resembles that of a steam whistle. So here is an even better challenge Design a circuit that does produce a steam whistle sound. Your answers suggest that there is any number of you who are skilled enough to do it....I now use a real steam whistle concealed behind the bumper of the Jack and powered by an Air Horn compressor with a small reservoir to give an appropriate ascent & decent of the note rather than sounding like a tug boat. So I do have some spare real brass steam whistles, I did have them on a manifold so that they could be played calioppe fashion but the neighbours had differing thoughts...
I tell ya it gets the message thru to PFT,s and other morons who do not slow down at railway xings.