Greenhouse Shield for Arduino

This is a board my son Robert is building to monitor and control conditions 
in our back yard greenhouse.  The objective is a "shield" for an Arduino
Duemilanove board, with the following features:

	- temperature sensor				MCP9700A
	- light sensor					PDV-P9007
	- humidity sensor				HCH-1000 + TLC555
	- i2c real-time clock				DS1307
	- i2c non-volatile memory for data logging	24AA1025
	- 4 relay-switched mains (110 volt AC)
		- electric heater (high current!)
		- electric fan
		- hydroponics pump
		- spare
	- spi TeleDongle interface for RF link
		http://altusmetrum.org/TeleDongle

The idea is that we log temperature, light, and humidity every so often to
the non-volatile memory with timestamps.  On each sample, we check current
conditions and turn the heater on if it's too cold, and the fan on if it's
too hot.  The hydroponics pump gets turned on and off on a regular schedule.
And the RF link provides a mechanism for remotely downloading the logged data,
changing setpoints and schedules, manual override, etc.

References:

    various relevant circuit fragments:
	http://www.ladyada.net/learn/sensors/cds.html
	http://klk64.com/arduino-spi/
	http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1264143673

    pcb layout in gEDA of an Arduino shield:
	http://lowvoltagelabs.com/products/videooverlayshield/

    5 to 3.3 volt level shifting with a MOSFET ...
	http://www.rocketnumbernine.com/2009/04/10/5v-33v-bidirectional-level-converter/

    DS1307 i2c example code
	http://www.glacialwanderer.com/hobbyrobotics/?p=12