[sdiy] Signals leaking into the PSU?

[Neil Johnson]

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