We'll start by making a version that fits on a breadboard. In the end we did fit the whole thing into the Lockitron casing but its a bit of work and for most people, they might want to keep the parts outside so they can be poked-at and modified

Wiring for the FONA & Metro mini

• BAT from FONA to VIN on Metro Mini - this will let the metro mini run off of the same lipoly battery the FONA runs off of - red wire. This is technically undervolting it but the metro mini is happy to run at a lower voltage of ~3.7V!
• GND from FONA to GND on Metro Mini - common ground - black wire
• RST (reset) from FONA to #5 on the Metro Mini - this is so the Metro can reset the FONA when it first starts up - blue wire
• KEY on FONA to GND - this will keep the FONA on all the time - black wire
• RI (Ring Interrupt) on FONA to #2 on Metro Mini - will let FONA alert the Metro when a text message comes in - yellow wire
• TX on FONA to #3 on Metro Mini - data sent from FONA to Metro - white wire
• RX on FONA to #4 on Metro Mini - data sent to FONA from Metro - green wire
• Vio on FONA to 5V on Metro Mini - will tell the FONA to use the same logic level as the Metro when sending/receiving data. - red wire
• 5V on FONA (near the micro USB connector) to USB on Metro Mini - will let you also use the Metro's USB connection to recharge the FONA battery - white wire

Wiring for the TB6612 Motor Driver & Metro Mini

• V+ (not Vm!) on TB6612 connects to 5V on Metro Mini - will tell the TB6612 to use the same logic level as the Metro - red wire. Technically this will be 3.7V or there abouts but that's fine
• GND on TB6612 to GND on Metro Mini - common ground - black wire
• PWMB on TB6612 to A1 on Metro Mini - motor direction control 2 - white wire
• BIN2 on TB6612 to A2 on Metro Mini - motor direction control 2 - yellow wire
• BIN1 on TB6612 to A3 on Metro Mini - motor direction control 1 - blue wire

Wiring from TB6612 Motor Driver to Lockitron

• Red wire from battery pack connects to V+ on TB6612 (we recommend the '3.5mm terminal block' spot)
• Black wire from battery pack connects to GND on TB6612 (we recommend the '3.5mm terminal block' spot)
• Red wire from the motor connects to MOTORB on TB6612 (last pin)
• Black wire from the motor connects to MOTORB on TB6612 (2nd-to-last pin)