The emission in the Ca II K line is modulated by the magnetic activity. The magnetic field produced by the action of a dynamo in the solar interior emerges on the surface leading to the formation of magnetic features. It extends further in the chromosphere where it causes the formation of plages, which are bright extended structures. Our goal is to study the long-term brightness variation of the chromosphere in this line taking into account the different chromospheric structures. Those structures are the main contributors to the variability of the solar irradiance, which plays a important role in the chemistry of the Earth atmosphere. For this we exploit the archives of synoptic full-disc images taken on-site at the Royal Observatory of Belgium with the Uccle Solar Equatorial Table (USET). The station carries 3 solar-patrol telescopes producing full-disc images in 3 different wavelengths (white-light, Ca II K and H alpha). Ca II K images are recorded since 2012 so we can make our study for the second half of the cycle 24 and the beginning of the cycle 25. I will show the methods to estimate the brightness variation of the plages and the plage fraction area from the USET images. In the approach of the Sun seen as a star, I will compare my results with the S-index data from the TIGRE telescope, located in Guanajuato, Mexico, observing the Sun as reflected by the Moon. As Ca II K observations are used since many decades for solar irradiance modelling, our results can be useful for climate model construction.