•
SPK+ and SPK-
– Differential speaker interface. If using this
interface, the two pins of a speaker can be tied directly to these two
pins.
•
EAR_MIC
– “Earpiece” microphone. This is a single-ended audio
input. If you use this interface, connect the other microphone pin to
ground.
•
MIC+ and MIC-
– Differential microphone input. If you’re using this
interface the two microphone pins can be connected directly to these
pins.
There are a variety of speakers and microphones that can be connected to
these pins. If you just want to try a simple proof-of-concept, you can use our
Electret Microphone and Thin Speaker to test the interface out.
Section 4.4 of the MG2639 Hardware Development Guide includes some
excellent information about connecting an audio interface to the MG2639.
Consult that document for more information about the impedances, offset
voltages, and sensitivities.
Supplying Power
Beyond finding a suitable SIM card, the most important part of getting the
MG2639 Shield working is supplying it with enough power.
Power Supply Requirements
Depending on which state it’s in, the MG2639 module can be a relatively
power-hungry device. The maximum current draw of the shield is around
350mA. It usually won’t pull that much, but may require around 260mA
during phone calls or 80mA during network transmissions. This chart from
the datasheet summarizes what you may expect:
In selecting your power supply, it’s important to note that the MG2639
Shield will
not work
if your Arduino is only powered off USB – an
external
power supply is required
to power both the Arduino and Shield. The
shield includes a 3.8V regulator to supply the MG2639 within its 3.4-4.2V
range. That regulator is sourced by the
Arduino’s VIN pin
. We recommend
a barrel jack power supply with a voltage output in the acceptable range of
your Arduino (or at least 4.5V for the shield).
Our 9V power supply is a good choice, and the one we used in our
prototyping and testing.
(Optional) Using a Battery Supply
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