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The general and simplified view of my circuit is something like that:

General Scheme

Using a microcontroller I want to select between two sound signal sources (S1 or S2). S1 is a signal that comes from a microphone (low voltage), S2 comes from an Arduino microcontroller (0-5v). When connected, the signals are aplified and transduced to sound waves by means of a speaker.

I've tried to use a conventional Relay but when switching it creates a spike that is amplified and can be heard. I want a silent switch.

The signals S1 and S2 are never simultaneoulsy "on", so another possibility would be something like that:

Second possibility

Where a device (triac) blocks (opens or closes the circuit), the triac could also be controlled by 5v from the microcontroller. A diode could be placed to block the signal and protect S2.

Looking to a device that could achieve that I found the Solid-State relays and the TRIACs. After trying to understand the differences between them and which could be the most suitable for this application I still have some doubts.

Which scheme and device would you use?

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up vote 4 down vote accepted

For this sort of application, you can use a simple SPDT analog switch such as the MAX4544:

enter image description here

You can get them in an 8-pin DIP from Digi-Key for $1.32.

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Will it create any spike/noise when switching or is unnoticeable? – Maties Pons Apr 29 '14 at 9:14
@MatiesPons To mitigate the switching noise, you could try AC-coupling the two sources to the inputs of the switch. This would eliminate the DC offsets which I think is creating the spike. If you have the relay still hooked up, you could try it with that. Try connecting a 1 µF capacitor between each source and the relay (or analog switch) inputs. The 1 µF is just a suggestion, you may want to experiment with different values. – tcrosley Apr 29 '14 at 12:59
I can't believe that! I was thinking a lot about which could be the source of the problem and I ended up believing that was the switch itself who was creating the spike. But the problem really was that the S1 comes from a microphone that it's being biased, so the signal has a DC offset of 2.2V. After connecting it to a Capacitator there are no spikes anymore. The first thing before ask a question is to find the correct source of your problems... At least using the IC that you have proposed the switch will be much more quick. Thank you very much. – Maties Pons Apr 29 '14 at 14:04

Why don't you use an analog multiplexer like AD8170 or 74HC4051?

enter image description here

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Because I am dump enough for not having thought about it before... Thanks. – Maties Pons Apr 29 '14 at 9:16
Another reason for not using it is that it is not terribly well suited for the purpose. The AD8170 is a video multiplexer, as should be apparent from the 75 ohm termination resistors R2 and R3. With a gain-bandwidth product of 250 MHz, It is gross overkill for the intended application (microphone output). Making a PC board for the unit probably requires excellent high frequency techniques to keep the internal amplifier from oscillating. – WhatRoughBeast Apr 29 '14 at 16:43

If the two signals are at a different potential then there will be switching noise when you switch, whether you use a relay or a solid state switching element.

If you already know that the two will not be active at the same time, you can probably just use a resister from each to the amplifier input. The resisters will mix the two signals at the amplifier and you will have no switching noise since both sources are always connected.

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