[sdiy] Signals leaking into the PSU?

[Neil Johnson]

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