Hi again, I would like to have a red/green indication of the position of a switch. I could use 2 separate LEDs or if feasible one of those dual color LEDs. (refer to attached diagram) In either case I would really appreciate if someone could draw out a basic circuit that I can use.

1. how to reduce the 12v dc to something the LEDs can handle
2. how to connect the LEDs so that one of them is always on depending on the position of the switch.

thanks very much

 

Here's one idea. Both LEDs would be on when the switch is closed.

 

Actually that would make one or both on at the same time and what I really want is to only have one on at one time.

 

This is the best I can think of at the moment. I'll bet there's a better way, but I can't see it.

[update] Oops, this won't work if the unseen circuit impedance is not >1K. If it's not, the left-most LED will blow just like a fuse. The diode-drop approach below is more versatile.

 

Just a few points of information that may or may not be useful:

1. You can get illuminated switches to do essentially that with no additional hardware.
2. Using a SPDT switch would simplify the design.
3. LEDs made with a built-in resistor for 12V are available.
4. Single color LEDs generally look better than dual color LEDs, especially in brighter ambient light conditions.

 

thanks all for the replies, I might just take the simple approach and use stdt switches... good advice on the bi-color leds too.

 

Here is another method. When the switch is closed, only the green LED lights because the forward voltage of the green LED is less than the forward voltage of the red LED + diodes. When the switch is open only the red LED can light.

 

that looks pretty good, I think I will try that, thanks

 

that looks pretty good, I think I will try that, thanks

Incidentally, that circuit came out of a book on power supplies where the switch was replaced with a fuse. As long as the fuse was OK the green LED lights and acts as a 'power on' indicator. If the fuse blows the red LED comes on to inform you of the problem!