RES: [sdiy] Question about LEDs

[Jason Proctor]

>On a related note, I have bicolor LEDs for modules that produce both
>positive and negative outputs - my LFOs, for example.
>In order to activate those accordingly (RED when the output is positive,
>GREEN when it goes negative), I had to use a number of components in the LED
>drive circuitry. Positive is fine: resistor + NPN transistor + LED + current
>limiting resistor. Now, for the negative side I had to first invert the
>input via one op. amp at unitary gain, then it goes diode (so only the
>negative semicicle is taken in account) + resistor + transistor + LED +
>resistor. The LED is a common-cathode (3 wire) LED.
>
>Is there a simpler way I could have done it?

yes, you can just use a bicolour LED, which is effectively two LEDs 
in one package with opposite orientations. if the juice runs one way, 
you get colour A, and if it runs the other way, you get colour B.