This is a MODBUS-node with a capacitive water level sensor, a DS12S80 temperature sensor and a RS485 interface to the main controller.
This is my first project based on CADSOFT Eagle free-version. I still have to learn a lot for creating 2-layer PCBs :-/
2013-05-01
The board features are:
- Onboard power supply
- ATMEGA328P processor running on 16MHz
- RS485 serial line driver
- 1-WIRE bus for connection to an remote DS12B80 sensor
- A capacitive water-level-meter.
The boards is based on an ATMEGA328p processor - same is used on the current ARDUINO UNO SMD version. The idea is, that I once program the ARDUINO bootloader into the ATMEGA328p using the 6-pin header. Later I can update the code using the regular FTDI-6-pin header via the ARDUINO development environment.
The capacitive sensor is based on the work Nathan Hurst.
2013-04-30
And this is my first PCB: 
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The board was produced by PCB-DEVBOARDS in Germany for a reasonable price.
I'm still waiting for some components, so I started testing the power-supply and the capacitive level-meter 
and it works!
2013-05-04
Today Reichelt delivered the missing components - first of all the ATMEGA328P. Most component on the board are now populated.
After full testing I will remove the solder flux and clean the board.
Next action was flashing the bootloader using my ATMEL AVRISP MKII. (The ISP connector is the left 6-pin header). Inside the ARDUINO IDE I've selected the ARDUINO DUEMILANOVE w/ ATMEGA328 board.
Flashing was an action of about 2 minutes using the ARDUINO IDE.
And the first version of SKETCH is also now loaded using the FTDI cable. (This cable is plugged onto the right 6-pin header)
And - yes it's working as expected :-)
Here some more pictures of the populated PCB 
MODBus registers
The boards features the following MODBus registers:
R0 + R1 double waterTemp; // water temperature at the electrode
R2 unsigned int waterLevel; // compensated water level
R3 unsigned int waterLevelRaw; // the uncompensated raw water level just for debug purposes
R4 unsigned int chipVcc; // onboard voltage measured internally in the ATMEGA328p. Raw ADC value.
R5 unsigned int chipTemp; // onboard temperature measured internally in the ATMEGA328p. Raw ADC value.
Read-on
Read on about my experiments regarding the capacitive level sensor.