From the survival of biological systems to the surface electrochemistry of materials, ambient and surface water plays a key role, and understanding its contribution is essential to describe the observed behaviours fully. Thus, precise measurement and control of relative humidity are essential in laboratory applications. In this paper, we present an open source hardware and software design of a low-noise and high precision standalone relative humidity controller, whose versatility and potential applications we demonstrate through basic calibration and performance tests. The hardware and software implementation are discussed in detail, with all plans and codes provided.