There's a lot packed into the FONA 3G lets go thru all the pins, buttons and indicators and what they do

Connectors

There's three external connectors along the left side, from the top, a mini JST 2-pin, a microUSB and a headphone jack.

• JST 2-pin - this is the battery input connector. It works with any of our Lipoly batteries but since the charge rate is 500mA (and the cellular module can spike high current draw!) we suggest our 500mAh or 1200mAh batteries. You can also connect a JST cable here if you have other plans.
• MicroUSB connector - this is the LiPoly/LiIon battery charging port. The SIM808 has a USB interface but its ONLY for reprogramming the module with an expensive and unavailable IDE. So charge only! The charge rate is 500mA max.
• Headset jack - this is a 'standard' TRRS 3.5mm phone headset jack with stereo earphone and mono microphone. Any 'iPhone' or 'Android' compatible (but not iPhone original) should work. We tried about 10 different ones, and basically the more expensive once are more comfortable and louder but our official iPhone headset mic did not work for unknown reasons. Sleeve is Mic+, first ring is ground, then the second ring and tip are for stereo audio. The module does not have stereo out so we just tied both together.

Antenna ports

Up top is the place where you can plug in your GSM antenna. An antenna is required to use the module for any kind of voice or data communications as well as some SIM commands!

You can either use a uFL GSM antenna like this, or use a uFL to SMA adapter and then an SMA antenna

On the left is a GPS antenna port. While you can connect a passive GPS antenna directly, we've found the performance is lacking. We recommend shorting the solder jumper labeled 'bias' and connecting an Active GPS antenna instead, such as the one we stock in the adafruit shop

SIM Connector (on Back)

A 2G or 3G Mini SIM card is required to use the module. Nearly any cell phone shop can sell you a SIM card. For USA customers, we have a known-working TING SIM card which has a great billing system and works very well!

MicroSIMs won't fit - so make sure its a "Mini" SIM. Mini SIMs are 1" x 0.6" / 25mm x 15mm. These are by far the most common size.

Most cards come with a voice and/or data plan. If you want to make phone calls and SMS's you'll need a voice plan. If you want to transmit data like fetching a webpage, you'll need a data plan.

Bottom Breakouts

The most important pins are broken out at the bottom of the board. Not all of these are required, but they are all hella useful

These are in rough order of most important (not in linear order like we usually do)

These pins are all 3-5V input safe and if they are an output, the logic level is whatever Vio is set to.

• Vio - THIS IS THE MOST IMPORTANT PIN! This is the pin that you MUST drive with an external voltage from 3V-5V to set the logic level converter. The converter also buffers the indicator LEDs so NOTHING will appear to work unless this pin is powered! You should set the voltage to whatever voltage your microcontroller uses for logic. A 5V micro (like Arduino) should have it be 5V, a 3V logic micro should set it to 3V.
• Key - This is also a super important pin (but not as important as Vio). This is the power on/off indicator. Its also tied to the button in the top left. Tie this pin to ground for 3 to 5 seconds to turn the module on or off. It's not a level signal so it isn't like "low is off, high is on" - instead you must pulse it for ~5 seconds to turn off/on. The module comes by default off. Tie this permanently to ground if you never want your micro to turn off the FONA for power saving
• 5V - this is the USB 5V from the microUSB connector when its in and powered. Good if you need to know when the microUSB is plugged in and/or want to recharge the battery from an external plug.
• PS - this is the Power Status pin. It is low when the module is off and high when the module has power. If you're using the Key button or pin, you can monitor this pad to see when the module's booted up. This is tied to the Pwr LED too.
• NS - this is the Network Status pin. It lights up/blinks to signal the current status of the module. This is also tied to the Net LED so for more detail see the LEDs section below.
• Reset - this is module hard reset pin. By default it has a high pull-up (module not in reset). If you absolutely got the module in a bad space, toggle this pin low for 100ms to perform a hard reset.
• RX & TX - OK now that I made you read all that you can actually use the UART pins. The module uses UART to send and receive commands and data. These pins are auto-baud so whatever baud rate you send "AT" after reset or boot is the baud rate is used. RX is into the module, TX is out of the module.
• RTSin - this is the hardware flow control pin. If you turn on flow control on the FONA you can use this pin to stop and start data transfer from the FONA 3G to your microcontroller
• CTSout - this is the hardware flow control pin. If you turn on flow control on the FONA you can use this pin to determine when the FONA 3G's serial buffer is full and you should stop and start data transfer to the FONA 3G from your microcontroller
• DTR - this is a hardware flow control pin, it can be used with the FONA 3G to hang up calls, control data/command mode for TCP/IP, ec.
• RI - this is the Ring Indicator. It is basically the 'interrupt' out pin from the module. It is by default high and can be configured to go low when a call is received. It can also be configured to go low when an SMS is received.
• SPK+ and - : This is for connecting an external 8 ohm speaker. The two pins are differential so they don't have output DC blocking capacitors. You cannot connect this to a stereo, powered speakers or other non-differential amplifier without adding a 100uF+ blocking cap in series to the + pin and then not using the - pin. Instead, your amp should use GND for the - reference
• MIC + and -: this is for connecting an external electret microphone, it will bias the mic with 2V. Most electrets will work just fine. No extra circuitry is required for the mic such as a biaser or amplifier, just wire it up directly!

LEDs

• PWR - Green! Lit when the module is booted and running
• NET - Red! You can use this for checking the current state without sending an AT command:
  Always on - the module is running but hasn't made connection to the cellular network yet
  800ms on, 800ms off - the module has made contact with the cellular network and can send/receive voice and SMS
  200ms on, 200ms off - the GPRS data connection you requested is active
  By watching the blinks you can get a visual feedback on whats going on.
• Charging - Orange! This is next to the microUSB jack. Indicates the onboard lipo charger is charging
• Done - Green! This is next to the JST jack. Indicates that the battery charging is done and the battery is full