[sdiy] Bi-colour LED

[Roy J. Tellason]

On Saturday 03 July 2004 05:23 pm, Jaroslaw Ziembicki wrote:
> ----- Original Message -----
> From: "Theo" <t.hogers at home.nl>
> To: "Jaroslaw Ziembicki" <aon.912230836 at aon.at>; "Rob Keeble"
> <rob at emulatorarchive.com>
> Cc: "SDIY" <synth-diy at dropmix.xs4all.nl>
> Sent: Saturday, July 03, 2004 5:40 PM
> Subject: Re: [sdiy] Bi-colour LED
>
> > Just one opamp, one resistor, the duo LED and two diodes (1N4048) will
> > do. Opamp + input to ground
> > Resistor is the input and goes to the opamp - input.
> > Now the feedback loop:
> > The anodes of the dual LED go to the output and - input of the opamp.
> > To the common cathode of the LEDs you connect the anodes of the diodes.
> > Cathodes of the diodes again connect to output and - input of the opamp.
>
> So simple! Excellent!
> Congratulations, Theo! And I thought it was impossible to do it
> with only one opamp...
> Note that the light intensity is proportional to the input voltage here.
> If you want a constant light strength, do the following:
> - connect the opamp (-) input to the ground,
> - connect the input to the opamp (+) input through a 10k resistor,
> - connect the opamp output to the opamp (+) input through a 1M resistor.
> It's a comparator with a small hysteresis. Then:
> - connect the "LED/diode network" (described above) with one end to the
>   opamp output and with the other end to ground through a current-limiting
>   resistor (approx. 680 Ohm for VCC=15V and I=20mA).

This is the sort of clever thinking that I really like to see,  and it didn't 
take a specialized chip,  surface-mount parts,  a double-sided board,  or a 
CPU thrown at it!     :-)

Cool stuff,  both of you guys...