Tuesday, January 22, 2013

Soft switch sound switcher using a few 2N3904 and resistors

Having two sound sources connected to the same output may be good for a while but sometimes you don't want to have both sources outputting sound at the same time and may want to switch between them. I wanted to be able to control this with a micro controller and not use a servo, so a mechanical switch is out of the question.

Transistor as switch
I know that you can use transistor as soft switches so I started researching how to build this.

It seems that this is not something original as it has been done before. I actually found this instructable that does just that (and a lot more).
Using that schematic and the same components I set out to build my own.
My result is quite good but it is nothing you want to look at.

Soft Switch audio switcher
Soft Switch audio Switcher back side

 For the control I used standard header pins. I also clearly marked the output so there would be no confusion. The lonely pin near the output is the ground (haven't market it too well) and the other two correspond to the respective inputs.
As I write this I am trying to figure out why the output has to be the output. The way I see it you can also have a continuous input and a selectable input while also having a mute function for the output if you would use as output one of the inputs.

To test this, again I am using my battery and usb to header cable although I think putting 5V for control is a bit much, using something like a 3V power supply for the control should be better as to not damage the transistors.
Testing the Soft Switch audio switcher
 This setup works well but you should know that powering both inputs at the same time will give you a lower volume to the output compared to only selecting one input. I think there are some crossing currents as both input devices can operate at different voltages and that might damage the one using lower voltage so I would recommend not to have both inputs selected for output. This is not a problem for me as I wanted just to switch from one to the other.

Good luck and enjoy!


I realized that I needed to add some pins for monitoring the inputs. I need this because i want to be able to select an input only if there is something playing on it. To do this I soldered on pin on each input plug to one of the legs. Make sure not to solder it to the middle pin as that is usually the ground.

Monitoring pins to the Soft Switcher
Monitoring pins on the back side

Check out my testing of the board here.

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