RESUMO
Monitoring systems are ubiquitous in many environmental science fields due to the technological advances in the last decades, which contributed to the migration from conventional to automated observing networks. However, the cost of acquisition and operation of sensor networks is still a limiting factor for their wide use. Under this scenario, low-cost and affordable open-source sensors and systems emerge as an alternative to research-grade instruments. To fulfill the quality requirements defined by international organizations, sensor calibration has to be performed and shall be considered as a basic requirement. Best monitoring practices including established quality control procedures should be implemented for gathering environmental data. With a focus on data gathering, this paper reviews basic concepts on open source technologies, calibration methods, quality control procedures, applications and trends, and possibilities for improving the hardware of low-cost and affordable systems.