In the agricultural sector, climate change with water bombs and drought periods leads us to search solutions in terms of efficient use of resources, especially water, and to create weather independent systems. Monitoring and controlling the growth of plants and environmental parameters are essential in Smart Agriculture for making farms and greenhouses more intelligent and efficient. This paper proposes a new demonstrator with a modular architecture, which allows the creation of new services without starting over and redesigning the entire system from scratch whenever it is necessary to add new devices or functionalities such as closed control loops, considering options to potentially increase services for precision agriculture rapidly. In this paper, we design and implement a smart hydroponic greenhouse, which keeps environmental parameters stable and allows to grow tomato plants in an ideal environment. In addition, we design and optimize a PID algorithm to control and stabilize the intensity of visible light in cultivation using lamps and a spectrometer, even in the absence of sufficient natural illumination. We also compare the responsiveness of our PID controller and the threshold comparator to understand which algorithm is better. All monitoring data and system parameters are made available to the end-users through a smartphone app. Our proposed system may represent a step forward towards sustainable and precise agriculture, serving as a practical demonstrator that could provide farmers with a key tool during this crucial digital transition. The demonstrator may enable farmers to optimize crop growth and resource management, thereby showcasing the benefits of smart and precise agriculture.