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I recently purchased the Arduino GSM shield so that I could build my own low-tech cellphone. Now that I've successfully been able to send and receive text messages, I would like to add the ability to make voice calls.

I was able to find a schematic in the modem documentation that describes how to build the earphone circuit. It seems fairly straightforward but I have a few questions:

  1. What does "differential layout" mean?
  2. What is the purpose of the transient-voltage-suppression diode in this circuit?
  3. What type of device is the "amphenol 9001-8905-050"? Based on the diagram it looks like 3.5mm 4 pole phone jack.
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  1. "Differential layout" means that the PCB tracks should be a close, a constant distance apart and close to equal length. This has the effect that most electronmagnetic interference applies to both lines equally, and will vanish when one is subtracted from the other.

  2. TVS is to protect against electrostatic discharge; you may get multiple kilovolt spikes when a human touches the device. The TVS diode shorts this to ground to protect the GSM module.

  3. Yes, it's a jack for a headphone-and-microphone accessory.

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