[sdiy] Signals leaking into the PSU?

[Mike Bryant]

If the original LED signal comes from a MCU and you have spare pins then another approach which uses less board area is to have two LEDs, one on the front panel and another hidden inside the module.  Run a resistor from VCC to the two LEDs, then feed the other ends of the LEDs to two pins on the same MCU port.  Writing to the port causes and instantaneous change from one LED to the other.  However because the LEDs are in different locations, 100nF from the LED/resistor joint to ground may be needed to smooth out the last wrinkles.
________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Neil Johnson via Synth-diy <synth-diy at synth-diy.org>
Sent: 24 February 2023 11:16
To: Scott Bernardi <scottbernardi55 at gmail.com>
Cc: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Subject: Re: [sdiy] Signals leaking into the PSU?

Hi Scott,

> Here's a way to keep a constant current driving a flashing LED using a differential pair.  It only takes one extra resistor and transistor compared to the standard transistor
> I whipped up a page for it:
> https://www.bernacomp.com/elec/og2/constant_current_led.html

Thanks - an interesting solution.  My only concern would be driving it
from 0V to 5V.  At 0V both bases are at the same voltage so the
current will split between the two transistors depending on their
matching.  If they were both perfectly matched then the current would
split equally between the two paths, so the LED would see half of the
"on" current.

Driving it from a bipolar signal is fine, for example, for a -5V  to
+5V input you get a sinusoidal-ish LED current (according to sims
anyway ;) ) for lovely smooth on-off transitions (probably makes the
tone warmer too, more analogue-y richness with a mellow undertone,
like melting butter oozing over hot crumpets.... errr..... nice.....).
And you could extend this to drive two LEDs (say, red and green) for
extra blinky goodness.  Or you get to choose the polarity of the drive
(inverting as you have drawn, or non-inverting if the LED is in the
other collector).

Cheers,
Neil
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