[sdiy] Signals leaking into the PSU?

[Scott Bernardi]

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

On Thu, Feb 23, 2023 at 3:50 PM Neil Johnson via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Hi Florian,
>
> > The problem is, that there is current draw switched on and off, which
> puts a sudden load on the supply. The trick is to keep the current
> constant, but redirect it over a transistor (excerpt from one of my
> schematics):
> >
> > The R19 should be fed by your U1C pin 8. If you have only 12V as supply,
> you should use a 3k3 or 4k7 resistor for R24.
>
> Except that's not constant current.  Yes it's better than just
> switching the LED on or off, but the current still flips between
> almost 5 mA and around 3.3 mA, so that's a 2.7 mA peak-peak
> alternating current with sharp edges coming from +5V and going into
> ground.
>
> A constant current source good enough for this application is a single
> transistor, a couple of diodes and a couple of resistors.  Across 24V
> you should be able to string together about 10 LEDs.
>
> Neil
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