[sdiy] Bi-colour LED

[Jaroslaw Ziembicki]

----- 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).

Nice Sunday for everyone
Jarek