If you have one ultrasonic sensor and a microcontroller, you can detect plenty of useful things, like the water level in a rain barrel or the distance traveled by a tablet along a rail.

The treat in question is detected by an SR-04 ultrasonic distance sensor connected to an Arduino Nano. All in all this was a highly successful build as far as neighborhood entertainment value goes.

A low-cost (~$100), do-it-yourself (DIY) ultrasonic water level sensor/datalogger has been designed, constructed, and tested for use in education, citizen/community science, and research settings. The ...