You can find the first part on how to built a water level sensor, here:
-Raspberry Pi with
Sendmail or postfix if you want a email warning.
What the software will do:
The command part
This part is invisible to the user and is (and should be ) independent of the webpage.
Using cron, it will read the water level every minute and save the information to a file. If the water level reaches a certain level, an email will be sent. This is the most important part.
The Web page
In this part, the web page reads the information from the file written by the cron job and displays an image of your pit.
Step 1: Download and copy the software
Download the zip file and uncompress it to the webfolder of your Raspberry. I recommend creating a subfolder at the root of your www folder. Typically, the folder path is /var/www/sumppump.