thanks for feedback. you are right, the battery life is poor.

the problem with the battery life is not the arduino itself - a bare atmega328 (the hearth of arduino) will live for several months from a 3AA battery.

the real power consumption comes from the ultrasonic sensor - I tried to completely cut it off from power between measurements, however the ultrasonic circuit is not recovering properly from these on&offs (I was looking for help on arduino forums but without response: http://forum.arduino.cc/index.php?topic=216079.msg1589123#ms...)