How To Transistor Switch LEDs In This Scenario?

[Paul Marossy]

I have a case where there is a pair of 3PDT switches that select between a bypass mode, an effects send/return and an LED (two circuits in one box). I am having problems with one of  the LEDs causing a popping noise. Every single thing I have tried to get rid of it has resulted in failure. Nothing has worked at all.

So what I would like to do is switch the LEDs with a transistor using the middle pole of the 3PDT switch. Problem is that I can't do it like the Mill 2 Bypass or Joe Davisson's LED Trick because I can only use one half of the middle pole and I can't use the output signal as a "trigger" because of how everything is wired. Is there a way to do it otherwise?

This is how the switch is wired:

[Paul Marossy]

I am of the opinion that I am just screwed in this situation. I don't see how I could make it work. 

[R.G.]

So the only problem with the switch as shown is that the LED pops?

If so, you don't have a problem. Use the middle pole to switch the transistor on and off. Set up an NPN with its emitter grounded, base connected to a resistor to +V, and the base also connected where you have the LED connected now. The resistor lets a current into the base or into ground, depending on which way the switch is. The collector is then open circuit, or shorted to ground by the transistor turning on. You can put whatever you want on the collector, including a soft on/off RC network for an LED, or use the collector to trigger almost anything else that switches between ground and some voltage.

[armdnrdy]

You can wire the led and transistor as in this image.

http://www.mayothi.com/transistors.html

Use the two open poles on your switch (one pole voltage in, the other out to R-2) to send voltage to the base of the transistor through R-2. (R-2 should be a 10K)

When the base gets voltage, the LED turns on.

R-1 should be whatever value you usually use to adjust the brightness of the LED.

Edit:
Taking a second look at your switching, I noticed that the ground has to stay on the center lug. So....follow the PNP switching diagram that's in the link that I provided.

[Paul Marossy]

So the only problem with the switch as shown is that the LED pops?

If so, you don't have a problem. Use the middle pole to switch the transistor on and off. Set up an NPN with its emitter grounded, base connected to a resistor to +V, and the base also connected where you have the LED connected now. The resistor lets a current into the base or into ground, depending on which way the switch is. The collector is then open circuit, or shorted to ground by the transistor turning on. You can put whatever you want on the collector, including a soft on/off RC network for an LED, or use the collector to trigger almost anything else that switches between ground and some voltage.

Yeah, it's pretty much confined to the LED making the popping noise. Thanks for the suggestion, I will try this.

[Paul Marossy]

I tried both versions. They both work. Looks like I have to go with the PNP version though as the NPN version makes the battery get hot, so that's not good.

Anyway, thanks for the help!