MOSFET needed to short two pins of analog signal?

Hi folks,

I'm about to build a circuit (on breadboard), which should do the following: A PC and a smartphone should be connected by audio cable (audio out from pc goes to smartphone's mic in , audio out from smartphone goes to PC's mic in). Moreover, when receiving a message via usb, an arduino board ("between" PC and smartphone) should simulate pressing the play/pause-button of a wired headset's audio controls (used to trigger a specific event in a smartphone app). According to http://dl3jin.de/galaxy-data/headset_scm.jpg , basically two lines of the audio cable need to be short-circuited. I'm a electronics noob (only basic, theoretical knowledge), so I don't know how to realize the circuit. A colleague told me that this could be achived by using a MOSFET which is controlled by one of arduino's I/O-ports.

Is there an easier way to do this? Which components are needed/would you recommend? (Which arduino board, which additional components?) Is it necessary to protect the PC against the short-circuit? (how?)

Thanks in Advance, Cheers, Martin

I imagine a simple part like this would do. http://www.digikey.com/product-detail/en/ADG419BNZ/ADG419BNZ-ND/820067

Just to make sure I understand: The switch switches between "nothing" (empty pin) and a short-circuit between the two lines, right? Are any additional components needed? (resistors, diodes (to protect PC from short-circuit)?) Which arduino board would you recommend for this purpose?

Mosfet are designed for power switching. Have you kW audio signal to switch ? I think no, it would be rather mW.

As "R2" is the ground a simple NPN low power bipolar transistor seems appropriate. The NPN emiter must be connect to the ground --> connect the NPN emitter to the ground, the NPN collector to one end of the 330 ohms. Connect the other end of the resistor to "S". Do not forget put a resistance in series between the Arduino Output and the NPN base. A value between 2.2k ohms and 4.7kohms is appropriate.

A transistor passkey is the 2N2222 but there are many other models. Among the transistors you will find, choose which has a gain (HFE or Beta) of at least 100 (no upper limit) and the lowest VCEsat. To get easily datasheets: http://www.datasheetcatalog.com/

You may find that a simple transistor to short/mute an audio line will not be sufficient depending on the impedance of the audio.
Even when the transistor is conducting, there will be some resistance between the collector and the emitter which depending on the impedance of the audio line, the input impedance and gain of the next stage, you will still be able to hear some audio as the transistor will form part of a voltage divider and depending on the ratio, you may not get enough attenuation.

Rather use a small JFET.

AudioMute.jpg

68tjs: Mosfet are designed for power switching. Have you kW audio signal to switch ? I think no, it would be rather mW.

As "R2" is the ground a simple NPN low power bipolar transistor seems appropriate.

Hm... I don't think this is an appropriate justification for one type of transistor over another. MOSFETs are voltage-controlled, and BJTs are current-controlled. The difference is not so much based on quantity of signal, but the mechanism by which the switch is controlled.

Yes MOsfet X_field effect transistor are voltage-controlled I never said the opposite, And in the x_fet family Mosfet are generally for high power. And when on this forum one speak about X_fet it is at 99.999 % for high power switching , so MOSFET.

But for the application of this thread a bipolar transistor is much easy to use than a x_fet. that's all that I want to say, sorry if I'm not as clever as you to handle the English language. If I wrote "don't forget to put a resistor between ........" it was to control the base current.

I write a tuto in French for beginners "How to use à bipolar transistor in switching mode": http://forum.arduino.cc/index.php?topic=100727.0 If you don't understand French you can use google translator. Anyways drawings are always readable.