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1-wire temperature sensors. With USB to 1-wire adapter.  Works well. 

opkg install usbutils
lsusb now works
 ID 04fa:2490 Dallas Semiconductor DS1490F 2-in-1 Fob, 1-Wire adapte                            r

I have 1-wire USB module DS9490R and not the more wide spread serial adapter 
Took me some time to figure out that need to run owfs with option -uall.

http://owfs.sourceforge.net/WRT54G.html
need DS2480B
DS2438


Driver for 1-wire Dallas network protocol.
usb 1-1.1: new full speed USB device using ar71xx-ehci and address 5
usb 1-1.1: configuration #1 chosen from 1 choice

opkg install owshell owfs

opkg install kmod-w1
opkg install owhttpd
opkg install bc   # arithmetics for floating point


cd /home 

mkdir 1wire
owfs -uall  /home/1wire

owfs -uall  /home/1wire --debug

owhttpd -uall -p 3002 
http://10.0.0.1:3002/
http://10.0.0.1:3002/28.ACE1FA020000
cat /home/1wire/28.ACE1FA020000/temperature12
cat /home/1wire/28.3C9AC5020000/temperature12

 
24.937  WORKS!!!

cd /www
rrdtool create t.rrd --step 300  \
DS:temp:GAUGE:600:-20:100  \
RRA:AVERAGE:0.5:1:2016  \
RRA:AVERAGE:0.5:6:1344  \
RRA:AVERAGE:0.5:24:2190  \
RRA:AVERAGE:0.5:144:3650  \


A=$(cat /home/1wire/28.3C9AC5020000/temperature12 | awk '{printf "%3.2f\n",($0)}')
printf $A
rrdtool update /www/t.rrd N:$A
rrdtool fetch /www/t.rrd AVERAGE

rrdtool graph temperature.png   \
--start -2h --end now --vertical-label "sensor, C"    \
DEF:average=t.rrd:temp:AVERAGE              \
LINE1:average#FF0000     \