Directional Lighting - just not how we normally do it!

[Scottish Modeller]

Hi all,

 

For another project I have on the go I need to find a way to do the following.

 

Directional Lighting - Leading end bright LED, Rear end - dim LED.

 

Any thoughts/ideas about how this could be done?

 

If it was DCC - then it would be fairly simple using the CV settings - but it's DC....

 

Thanks

[Scottish Modeller]

I assume the brightness needs to change with direction as well?

 

First, create a positive with 2 diodes to either wheel side. Anode on the positive, then the cathode of the LED gets 2 resistors, which differ in value. The low value goes to the left rail, the high value connects to the right rail. Start with values of 860 and 1k8 Ohms. If you need a diagram I'll draw you one

Thanks for that....

 

Yes - brightness to change with direction.

 

Correct - a diagram would be a big help.

 

Knew someone would know how to do it!

 

Thanks

[Crosland]

The problem with DC is that if you try to fix the brightness you will notice the variation as the speed (i.e. supply voltage) varies. The bright one will get dimmer as the loco goes slower, but the dim one will get very dim.

 

I would use a zener diode to give a fixed voltage just above the LEDs forward voltage (approx 2.1V for red) and then connect the LED to this via current limit resistor. The LED brightness will remain stable until the track supply falls below the zener voltage.

 

Andrew

[cliff park]

Don't forget for very low zener voltages like this you can achieve the same effect with any old diodes in series. Check, because germanium and silicon have different forward volt drops, but you are very unlikely to get hold of germanium diodes anyway. Try 3 diodes in series with a suitable resistor, say 1k to start with and see how it goes.