Potrace is a tool that converts bitmaps to vector images. It is especially good for converting hand made drawings.

This setup worked for me:

pdftoppm -singlefile input.pdf | potrace --output output.svg --blacklevel 0.72 --color '#404040' --svg; xdg-open output.svg

Potrace expects some some specific formats on input. My scanned drawing is in pdf and pdftoppm happaned to be installed on my Linux machine.