From: Brent Brown <brent@eds.co.nz>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 7 Apr 2020 14:14:56 -0700
Subject: Re: [EE]: Bi-Colour status LED
Thread-Topic: [EE]: Bi-Colour status LED
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Yes, good point. Without the cap only shades of orange/yellow are possible.=
... it=20
would be impossible to hold either LED off for longer than one character ti=
me (start=20
and stop bits always illuminate one or other of the LEDs).

In the no capacitor case you can loose the resistor too... LED current reli=
es on the=20
RS232 driver current limiting (~10mA).

Here I only had discrete red and green LEDs, not an actual Bi-Colour LED. A=
iming=20
for fully Red, fully Green, and fully Off states I chose RC values to minim=
ise flicker=20
and show no light at all from the off LED(s).

Baud rate can be increased to decrease capacitor size/cost, but that means =
less=20
time between UART characters so CPU overhead increases... may or may not be=
=20
significant.

On 7 Apr 2020 at 7:17, David C Brown wrote:

> Or, without the capacitor you could have shades of yellow.
>
> On Tue, 7 Apr 2020 at 04:41, Brent Brown <brent@eds.co.nz> wrote:
>=20
> > Interest only, I must have too much time on my hands...
> >
> > Todays brain exercise was how to add a red/green status indicator to an
> > existing
> > product. There is an available RS232 port, and firmware mods are feasib=
le.
> > The
> > LED is required to be off when the power is off, green when functioning
> > correctly,
> > and flashing red when there is a fault.
> >
> > Attached circuit is I think about as simple as it can be. The LED is on
> > solid Red
> > when the power is on but no data is being transmitted, i.e. during boot=
up,
> > due to the
> > idle state of TxD being -ve. Could flip the LED and make the default Gr=
een
> > if that
> > were desired. LED can be set to red/green/off simply by changing the da=
ta
> > being
> > sent. Flashing shouldn't be too difficult. For extra marks dimming is
> > possible.
--=20
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