I am trying to make a mute button for a phantom powered microphone using a latching DPDT switch and an LED indicator for when the mic is muted. The LED will be powered by the mixer's phantom power. I need a few more eyes on this project. Does my attached schematic make sense?

 

There's usually about 6.8kΩ driving pins 2 and 3 at the mixer end, so the voltage across R7 and R8 will be about 6V, That would only give 185uA through the LED, which will light up but rather dimly.
Normally muting is simply accomplished by shorting pins 2 and 3.

 

Shorting pins 2 and 3 makes a loud bang, a better solution is shown in this schematic.

C1,C2 = 6.3V. R2,R3 1% tolerance.
LED must be low power type (2mA) to keep within the phantom power standard.
This is basically a very low lowpass filter, R1 resistor ensures that the voltage across the caps stabilizes,
so it doesn't cause a loud noise when the switch is operated.
C1,C2 can be up to, 2200µF or higher, then reduce R1 to 20, 24k.

 

There is a better circuit, which I have seen, called a "cough switch". But the website that had it seems to have vanished.
Rather thatn a switch with all of those noisy contacts, a simple variable resistor to shunt the two signal lines.

 

There's usually about 6.8kΩ driving pins 2 and 3 at the mixer end

So I guess it goes without saying then, if there is only a maximum of 7mA available, there simply isn't any way to power an LED without an external circuit (short of tapping into the 48V behind the 6.8K resistor within the mixer itself). Bummer!

 

Back in the mid 1980's era I found that a few of the "high brightness LEDs will light up with a very low current. BUT it would take some research and experimenting to find the right ones, and that might not be bright enough for the particular application.

 

Most modern LEDs are clearly visible at 1mA. The red ones might be the old type, but any 525nm greens would be GaN, and very efficient.

 

If the microphone is a low impedance type, 50 to 200 ohms, use a 0.47 MFD capacitor in series with a 4700 ohm resistor across pins 2 and 3,the microphone's balanced output. Then use a SPST switch across the resistor to mute the audio. The resistor will keep the capacitor charged to any residual voltage and the capacitor will absorb all of the audio output when the resistor is bypassed.