The bigger picture

Tonight I will try to list the hardware components I plan to include in the system, and try to figure out how to connect them all.

• Input/output devices outside the door, controlled by an Arduino-like microcontroller:
• Card reader for ISO 14443A cards (like MIFARE and NFC).
  MFRC522 chip, SPI interface.
• Card reader for EM4100 125kHz RFID cards.
  Serial 9600/8/N/1 interface, 3 pins: Serial data, Data ready pulse, RESET.
• Some sort of keypad.
  Until recently I planned to use a cheap ($2) 4x3 key membrane switch matrix keypad. It is very easy to use with the Arduino Keypad library - but this eats 7 I/O-pins. However the library now supports the use of I²C port expanders (like PCF8574 or MCP23017) which lowers the number of required pins to 2: SDA and SCL - and they can be reused for other I²C devices.
  Two weeks ago I discovered the Freescale MPR121 Proximity Capacitive Touch Sensor Controller. It is a small (3mm x 3mm) chip that allows up to 12 electrodes to be used as touch inputs. The interface is I²C. I requested a free sample (3 pcs) from their website and 5 days later FedEx arrived with an envelope from Malaysia. Thank you, Freescale! So now I have to try and see if capacitive touch can provide a better interface than the membrane switch keypad.
• Nokia 5110 84x48 pixel display.
  Philips PCD8544 controller, SPI interface.
• Sensors and actuators in the door and door frame, controlled by a Raspberry Pi:
• Ruko/Abloy EL582 solenoid lock.
  1 output to control the solenoid (through a MOSFET).
  2 inputs for position of bolt and handle.
• A reed contact and a recessed magnet mounted in door and frame to detect if the door is really closed.
  1 input.
• A microswitch in back of the strike plate to detect if the dead bolt is in place.
  1 input.
• Power supply - supplied by 230VAC mains.
• A 12V 7.2Ah lead acid battery so the access control system can keep running during power failures.
• A battery charger (230VAC => 12VDC).
• 12V => 5V and/or 3.3V regulators for the electronics.
• Perhaps use an analog input to monitor the voltage of the 12V battery.
• Wild ideas:
• A PIR-sensor to detect unathorized entry.
• Monitor my Kibbi-sensors for open windows.
• A 12V sirene - if unauthorized entry is detected.
  1 output (through a MOSFET).
• A laser-grid in front of the door - to signal "armed" status ;-)

And now it is late, and the connections must wait for another time. SPI, I²C, serial, RS485 - who knows.