This turbidity monitoring and control system (often called turbidostat) is one of three units I worked in an attempt to build an advance photobioreactor. This work was carried out while I worked at Biodesign Swette Center for Environmental Biotechnology, Arizona State University. The scientific contributions of this system to advance algal cultivation was published in Algal Research Journal. The contribution of this system is open hardware design and open software, espcially the lowest cost system to minitor and control biomass density for microalga. Some commerical systems could cost a $10 thousands to a Tesla ($60k).