Re: Anybody Seen a Simple LED "Fail-Over" Circuit ?

On 11/26/24 3:40 AM, rbowman wrote:

On Tue, 26 Nov 2024 02:24:12 -0500, 186282@ud0s4.net wrote:
 

Critical Redundancy - One LED fails, another takes over ?

 Complicated question. For a complete catastrophic failure where the LED is
either open or shorted  a couple of transistors might do it. Even that
would be difficult if a PWM dimmer is used.

   I can kinda think of a few multi-transistor solutions, but
   was hoping for a one-transistor or no-transistor fix. The
   more trans/diodes you have to go thru the more the voltage
   drop.

Even worse the degradation may be light output and/or color rather than a
simple go / no go.
 Then you get into the human part of the equation:
 https://learn.sparkfun.com/tutorials/light/visible-light
 I'd actually been playing with PWM output in a Pico with C. The Uno piles
a lot of sugar on it with analogWrite rather than the Pico SDK
hardware_pwn
 https://cec-code-lab.aps.edu/robotics/resources/pico-c-api/
group__hardware__pwm.html
 I've got to play with it a little more but simply incrementing the duty
cycle isn't too smooth perceptually.

   Ah, I know that one ... mostly due to internal capacitance,
   esp in 'consumer-grade' LEDS, the devices may not 'saturate'
   from very brief pulses. So, your 5% PWM pulse doesn't look
   like 5% to the device while your 95% pulse does. The
   advertised color assumes 100% saturation at the spec voltage
   and current levels.
   Two and a half "easy" fixes :
   Decrease your PWM base frequency - maybe down to 100hz or
   even 60hz. This gives even the short pulses a chance to
   saturate the device.
   Or ... bring the device ALMOST up to emission threshold.
   Alas this takes a voltage-divider and maybe a diode. The
   idea is that you put static DC potential on the thing -
   like 2.0 volts for a 2.3 volt device. The amps do not have
   to be very much at all. Thus 'primed' they will be more
   responsive.
   LEDs used for like fiber-optic links are engineered to
   have lower capacitance plus a few other tweaks to make
   them respond faster. That they're generally driven
   binary at 0% or 100% for such uses also helps.
   Another ugly trick can be to just intentionally add
   more capacitance - a tiny cap - so your PWM does not
   really directly drive the LED but can instead be seen
   as just charging the cap. The leftover voltage in the
   cap will kinda keep the LED near threshold, as mentioned
   above (gotta match the cap to the PWM freq).