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I want to build a small box with two audio inputs and two audio outputs, with two switches to control the current input/output. Something like this: http://www.youtube.com/watch?v=mQzLnt0_-zU

I've wired up the following diagram in CircuitLab for how I think this should work:

https://www.circuitlab.com/circuit/4nq8kz/audio-toggle-box/

Do I have it right? Am I missing something? How can I add some LEDs to show which audio source/output is current? I was thinking I could probably power some small LEDs directly off the audio sources with a diode and/or capacitor to smooth out the AC audio, does that make sense?

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  • \$\begingroup\$ Just for completeness, here's a link to a new diagram using the two 'parallel' switches. Note that in the diagram I'm using DPDT but in actuality I'm going to be using DP3T with a center "off" toggles. circuitlab.com/circuit/hg64u6/audio-toggle-2 \$\endgroup\$ – davertron Mar 8 '13 at 13:22
  • \$\begingroup\$ I also just realized that there's really no reason not to wire up both sides the same way, so that it doesn't matter which way you plug in your inputs/outputs...so if you use the diagram in the comment above I would go with both sides looking like the right side of the diagram. \$\endgroup\$ – davertron Mar 12 '13 at 18:15
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First, I'm assuming that the speakers you show have their own power and amplification, so that throughout your schematic we're dealing with low-level audio signals, right? Ie: you're not hoping that the PC output will drive raw speakers?

Assuming this is correct, your schematic looks like it does what you describe, but here's a thought. As you have it wired, you are committed to using only the speakers or the headphones, but not both at once. You couldn't listen to the ipod on the headphones while the PC sends its sound to the speakers, for example.

Instead, consider wiring up two parallel "copies" of SW1, where the output of one of the "SW1"s is wired direct to the speakers (possibly through a jack) and the other "SW1" has its output direct to the headphones. Essentially these SW1s function as input selectors for the speakers and headphones respectively. It won't be a problem if both select to listen to the PC, or both the ipod.

As for driving LEDs off the audio signal, this will not be satisfactory. An LED needs at least 1.4 to 2V to illuminate, which your audio will be below much of the time. In addition, when lit the LED will draw more current than the low level audio signal can provide (without distorting the signal a lot).

You might consider replacing the DPDT switches with DP-3-throw switches, and use the extra throw to control LEDs. You could conveniently get power from the computer USB jack. Use 150 to 300 ohm resistor in series with the LED to operate off 5V -- adjust to suit your taste for brightness.

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  • \$\begingroup\$ You are correct in that I'm assuming properly amplified audio sources. I'll think about doing a switch per input, as I like the idea of being able to play from both inputs to the separate outputs at the same time, but I'll have to tinker with my diagram to fully understand what you're saying. The LED piece of this whole thing is a nice-to-have, and since I don't want to run additional power to it I'll probably just leave the LEDS off for the first one I build. \$\endgroup\$ – davertron Mar 7 '13 at 13:19
  • \$\begingroup\$ @davertron: You presently have one switch that connects the "input" of SW2 to either the computer or the dock. I think gwideman is suggesting that you should use one switch to connect the headphones directly to either the computer or the dock (without going through SW2) and the other switch to connect the speakers to either the computer or the dock. If you go that route, it might be good to use three-position "center off" switches. Note also that depending upon the designs of the headphones, computer, and dock, turning headphones to the same input source as the speakers may reduce volume. \$\endgroup\$ – supercat Mar 7 '13 at 15:56
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    \$\begingroup\$ @davertron: Such reduction in volume may be harmless except that if it is significant, and you turn up the speaker volume to compensate, the volume may be unpleasantly loud if the headphones are then switched off. \$\endgroup\$ – supercat Mar 7 '13 at 15:57
  • \$\begingroup\$ Agreed with supercat. My suggestion was to implement an input selector switch for the headphones, and an input selector switch for the speakers. Yes, the headphones (which create sound direct from the signal) would likely load down the signal a bit, which would indeed reduce the volume to the speakers if both are connected at once, leading to the effect supercat mentions. The speakers (which have their own power source) probably would not load the signal much, so switching the speakers probably would not cause surprise blasting in the headphones. These effects are probably not much concern. \$\endgroup\$ – gwideman Mar 7 '13 at 23:00
  • \$\begingroup\$ OK I get the idea now :) I don't think I'll be playing sound to both the headphones and the speakers very often, but it's not any harder to wire it up that way and it's nice to have the option. Thanks again. \$\endgroup\$ – davertron Mar 8 '13 at 3:26

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