It’s also a very handy thing to have when paragliding, which is why [mircemk] decided to build a hand-held version using the Arduino Nano ... microcontroller, sensor, buzzer, and the 9 V battery ...

In the microcontroller era today, Arduino is more than just a tool ... is used to measure the light level. The alarm system is triggered when a “Logic High (H)” level signal is detected at its sensor ...