part 2
Analog board
The analog board is used both for the sensors and for the XBee socket, see figure 3.
both gas detectors Methane CNG gas sensor – MQ-4 (http://www.sparkfun.com/commerce/product_info.php?products_id=9404 )
and Carbon Monoxide sensor – MQ7 (http://www.sparkfun.com/commerce/product_info.php?products_id=9403 ) need continuously 5V to operate in their working temperature (heating),so the board is always connected to 5V power source. This is also why the board is used in router configuration and can not be used as end device.

There is also a temperature sensor Thermistor 10k connected and used to measure temperature changes. (http://www.sparkfun.com/commerce/product_info.php?products_id=250 )

Figure 3: analog board description

The two gas detectors are using resistors to reduce reading output from 5V to 3.3V (MCU ADC working volts) see figure 4 schematics.
For the gas sensors there are two leds (one for each) when the gas level passes user defined up thresholds, a time window will start, if in this time window the gas will not go below the down threshold, then critical time led will light and a buzzer will start.
There is one MCU “alive led” that flash every second to indicate that the MCU is still running.
In addition there are also three leds connected to XBee module (associated, rssi, sleep).
The analog board size is the same as the digital board size, this enables to pack them back to back
Figure 4 shows the schematics of the analog board.

Figure 4: analog board schematics

Digital board
The digital board is made by ALVIDI company ( http://alvidi.de/avr32_module.html ), more details can be found in their web page.
This board holds an ATMEL avr32 MCU with other peripherals that let it work without any other needed hardware. The MCU T32UC3A1512 (http://www.atmel.com/products/avr/default.asp ) was chosen as it had more than enough flash and ram memory for this project and future projects to be used with the same programed code.
The board is fed from 5V usb cable and uses regulators to reduce it to 3.3V. The same 3.3V is used as power to the MCU and to the XBee module so there is no need for another regulator.