Replying to a reply because a missed the first post.

In article <4l45t3$> (Mark Achtemichuk) 

>   The basic idea is that a dual colored LED (red and green) lights green when it
>   receives a 12V lead.  When that 12volt lead disappears, the LED must turn red.
>   If I remember correctly the LED's work by switching their polarity for the
>   different colors.  This is where I run into trouble.

>   The circut will be used to test fuses in my car.  When a fuse blows, the LED
>   will turn from green to red.  The circut will have a constant 12 volt supply as
>   well as the 12 volt probe.  The LED's are rated at 3 volts so I assume it would
>   be easiest to put a current limiting resistor on them vs. building a power
>   supply.

>   Can anyone help me with this?  I would appreciate anything anyone has to offer.

If the led really changes color by switching its polarity, what about this 

    |                            |
    |                            |
    |                12V         |
    |     R1       + | |  -      |
    |                | |         |
    |                            |
    |                     6V     |
    |    R2       LED   + | -    |

When the fuse is OK, R1 and the 12V battery can be ignored, and 
the LED will be conducting (and, hopefully, green).  Adjust R2 
to limit the current into the LED to a proper value.

When the fuse is blown, the 12V battery forms a serie circuit 
with the 6V battery and the total voltage makes the LED reverse
biased (and, hopefully, red).  You can make R1 very large 
because only a little current is needed when the LED and 
reverse biased; and a large R1 will reduce the power consumption 
of R1 when the fuse is OK.

Note that it is the first time I heard about LED changing color 
with polarity (I am a newbie). I assumed that this type of LED 
operates like normal LEDs.  Maybe this assumption is not true: 
it seems strange to me that the very little current in the 
reversed biased LED can light it.

Any comments/question/critics would be appreciated.

Louis-Antoine Blais-Morin
Electrical Engineering         Ecole Polytechnique de Montreal             University of British Columbia

Date: Sat, 20 Apr 1996 21:15:57 -0700

Original Subject: Re: HELP designing a simple switch circut!

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