The core part of the design is TP-Link TL-WR1043ND wifi router running OpenWrt with small 16GB USB stick [/dev/sda1] containing ext3 partition with OpenWrt configuration + swap.
There are also 16GB USB stick and 2 thermometers connected using USB <-> Serial connector (bought on eBay):
I'm going to use the last stable version of the OpenWrt firmware: openwrt-ar71xx-generic-tl-wr1043nd-v1-squashfs-sysupgrade.bin
Upgrade the firmware and remove the old configuration:
rm -r /tmp/opkg-lists/
sysctl -w vm.drop_caches=1
sysupgrade -v -n http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/openwrt-ar71xx-generic-tl-wr1043nd-v1-squashfs-sysupgrade.bin
Here are the notes how way how I configured it.
If you don't like the commands feel free to check the configs here: https://github.com/ruzickap/linux.xvx.cz/tree/gh-pages/files/openwrt