RES: [sdiy] Question about LEDs

[Scott Juskiw]

Simplest way is to use a 2 wire bicolour LED, not the 3 wire kind. Put  
the 2 wire LED in the feedback loop of an inverting op-amp (connected  
between output and inverting input). Use a single resistor connected  
to the inverting input and ground to set the LED brightness. Apply  
your LFO to the non-inverting input. See schematic on this page for an  
example:

http://www.tellun.com/motm/mods/oms410/oms410.html

>
> 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?