The camera I use to shoot video doesn't have an audio input. In order to have audio on my video I have to use a separate microphone and sync the video and the audio I recorded in post production.
This is usually done with clapper board : the shutting of the clapstick identified on the video and the clap identified on the audio are used to sync both tracks precisely.
I decided to make another type of "clapper board" for which the syncing process could be automated and made easier.
I made a simple electronics circuit that fires a flash (taken from a disposable canon camera) and makes a beep (with a piezzo buzzer) : both the flash and the beep produce a short and sudden change in the audio and the video track that can be identified by my script latter.
The script uses ffmpeg to identify the beep and the flash and to produce the final track with video and audio!