If you live in Bat country, this is good project- A bat detector is an instrument that will detect the presence of bats by tuning into the echolocation ultrasounds they produce. There are a variety of commercial bat detectors available but what excited me were the numerous internet sites with home-built detector circuits. I stumbled across these sites while trying to decide what to do with a 1968 Braun Cylindric T2 cigarette lighter. What resulted was the Cylindric Ultrasonic Bat Detector. Link.
Cylindric Ultrasonic Bat Detector