Signal Presence Detectors?

[Haible Juergen]

A few hints:

(1) Don't switch LED current on and off. Steer it thru the LED when
     in should be "on" and bypass it with a transistor when it should be
     "off". Place the transistor next to the LED to keep the magnetic 
     radiation low.

(2) If you don't want to do (1), at least don't pollute the signal GND.
     If you don't want an extra GND, derive it from the +/- supply rails.
     A slow Opamp unity gain follower with input at signal GND, and
     its output serving as Dirty_GND will often do. Do not bypass the
     opamp output to signal GND with a cap, though. If you need
     a cap, it goes to +Vb or -Vb.

(3) If you're doing (2), treat +/- rails as polluted. Don't
    use them for initial VCA gain pots or such.

(4) If you're using capacitors (or RC filters) to "buffer" the
     local supply voltage for the LED(s), be aware of the
     path the current will take in the first moment of switching.
     Capacitor is you voltage source then. That path is a closed
     loop, and the cap and LED are both part of it. Make sure
     that this path does not overlap with any signal GND path.
     The signal GND *can* be connected to that LED loop,
     but that point of contact must be a point, not a line.
     Better to have no contact at all, or have it in the PSU.

JH.