I'm sorry I know I already asked for the similar question but now I learned how to properly ask my question and I will detail it more on this thread.

I have a DIY mixer on breadboard right now.

It's only 1 channel for the moment and I used a SPDT switch to pass the signal on or off. The center pin of the SPDT is connected to the output. Left pin is at the input where the signal comes from, and the third pin is on ground to minimise the noise when the switch is on the muted status.

I successfully added an LED with a 3904 transistor on this channel so when the SPDT is on, passing the signal to the output, this audio signal (AC I guess) is turning the 3904 transistor on and lets the green light monitors the signal, this LED reacts to the audio signal, like a level meter LED bar graph but only 1 LED, it flickers as the sound frequencies are being played.

Now my question is I want to add another LED, a red one, that would turn on ONLY when the SPDT switch is turned off (this is the muted state for the chanel mixer.)

I tried many things already, with 3906 or other simple circuits I found online for toggling 2 leds but since all I find online is using a steady voltage instead of an audio signal like I do right now, it makes my circuit light up both leds when the signal is on.

So I guess either I would have to block some portion of the audio signal triggering both LEDs (which should trigger just the green ON LED) or convert the audio signal to a DC voltage like something 3V for on and 0V for off. If I can know how to convert this audio signal to 0V or 3V it would solve my problem because right now K can turn on the muted red LED when the audio signal is muted but I can't make it so that when the spdt switch is on it only light up the green led, not the red led.

I hope it is more clear now, I'm sure this is basic electronics but 'm new to all this so it's complicated for me.

transistor led on mixer circuit

  • 3
    \$\begingroup\$ The easiest way is to use a DPDT switch and devote one contact to the LED. \$\endgroup\$ Commented Jul 4, 2020 at 20:48
  • \$\begingroup\$ Your added circuit (the LED) may distort the audio. Perhaps not a lot, but some. I'd want a high impedance detection of the presence of audio. What comes to mind is a bandpass (to improve S/N) followed by threshold detection with some hysteresis followed by a retriggerable one-shot. There are professional products that achieve this kind of function somehow (not necessarily as I described it), and more. But I'm not sure if there is an IC for it. \$\endgroup\$
    – jonk
    Commented Jul 4, 2020 at 20:57
  • \$\begingroup\$ Hmm. Now that I think about it, much could be combined into a single opamp and a handful of parts. You'd set up a three-resistor voltage divider, null out one node with a capacitor, supply the signal via a capacitor to the other node of the divider, then pass along these two nodes to the opamp input. One would be rock solid (capacitor), the other would vary with signal. When the signal reaches enough level, the opamp will saturate to the rails. Beforehand, not so much. A diode at the output would detect the rail and drive an RC for detection. I almost want to write it up. (I will test it now.) \$\endgroup\$
    – jonk
    Commented Jul 4, 2020 at 21:47
  • \$\begingroup\$ Thanks spehro but dpdt are too big, i thought of it but i cant use them and i already have the spdt slide switch format i want to use. \$\endgroup\$ Commented Jul 4, 2020 at 23:14
  • \$\begingroup\$ Ok thanks jonk, i have really no idea at the moment what you suggest how to do it, i will wait if you can mockup a circuit test. \$\endgroup\$ Commented Jul 4, 2020 at 23:15

1 Answer 1


Turn the switch around!

make the common terminal go to ground, the mute position connect the end of R55 to ground, and the remaining terminal short out the red led (or use an NPN transistor if sinking the current is undesirable)....


simulate this circuit – Schematic created using CircuitLab

You will want to refine it a bit, as any DC on the output of the opamp will produce switching clicks.

Your level detector also has a problem in that the 1k base resistor will only draw current on a positive going signal, where it looks like 1k in series with a diode, but when not conducting that chain looks like an open circuit. Your source impedance is 1k, so that produces pretty horrible distortion.

  • \$\begingroup\$ It will be interesting to see if the OP wants a grounded output when off. They may! \$\endgroup\$
    – jonk
    Commented Jul 6, 2020 at 18:05
  • \$\begingroup\$ I'm not sure to understand... where is the in and out on your schematic? and where is the second led? \$\endgroup\$ Commented Jul 7, 2020 at 18:06
  • \$\begingroup\$ your r2 connects to what? and the right line connects to what? \$\endgroup\$ Commented Jul 7, 2020 at 18:07
  • \$\begingroup\$ R2 heads off to the +12V rail to power the mute LED. Your signal presence detector is off to the right connected to the right hand wire, I ONLY drew the mute switching. Incidentally a dual opamp and buffer the drive to that signal indicator will make things much lower distortion, and you may wish to read the chapters on signal switching and level indication in "Small signal audio design" by D. Self, it is good stuff. \$\endgroup\$
    – Dan Mills
    Commented Jul 8, 2020 at 11:30

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