LED Mystery

[Mike McLane]

I built a tube preamp that has a "standard" foot switch bypass and a second foot switch that toggles an additional component in the tone stack in/out. Both have LED indicators that are wired in parallel as shown and drawing current directly from the 9v DC jack. The LED cathodes are foot switched to GRD as is typically done in pedals. Both work as expected when actuated independently, BUT when I have the pedal in "default" operation (only LED A on) everything is good. . . when I actuate B its LED B comes on and LED A goes off. If I switch B off A comes back on. What kinda Voodoo is this???? Is there a threshold current required such that the 4K7 passes enough juice to power one LED, but not two? In that case is this like a "soaker hose" analogy. . . the hole at the end of the hose gets more "pressure" than the one at the beginning, so the starved one doesn't reach the current threshold?

 

[Robert]

The LED with the lowest forward voltage drop will be the one to light.

The other won't have high enough forward voltage because the lit LED will be dropping it.


You need two current limiting resistors, one for each LED.

 

[Mike McLane]

Even if they're both RED LED's? So even if the variance in forward voltage drop is minuscule the "squeaky wheel" will get the "grease"?

 

[Feral Feline]

I don't see the problem. You know when A is active, you know when B is active.

 

[Mike McLane]

Don’t lie awake at night over it. Just curious about what’s going on. Next thing on my agenda is learning how to play the guitar