An active device for radon detection in air has been developed in the frame of a project carried out by the Physics Department of the University of Torino with the aim to create a monitoring network of radon concentration in the University workplaces. The device uses a commercial planar photodiode, sensitive to alpha particles, and a dedicated electronic chain integrated in an approx. 30 mm by 50 mm printed circuit board (PCB), designed in order to minimize the power consumption. The device can be used not only as alpha particles counter (using the digital output) but also for alpha spectroscopy, connecting the analog signal output from the shaper amplifier to a Multichannel Analyzer (MCA). In this paper, the first prototype of the detector will be presented with the preliminary experimental results. The spectroscopic performances of the device were tested, integrating the sensor in a commercial aluminum box and acquiring the alpha particle spectrum from a calibration source and from rock samples used as radon sources. The radon sensitivity of the detector, in terms of (Counts per hour)/(Bq m−3), was assessed by the comparison with a calibrated commercial radon detector (RAD7 Durridge Inc.). In conclusion, the final prototype will be presented. It is characterized by a new 3d printed plastic case in black PA12 for light shielding, covered by a conductive paint in order to minimize the electromagnetic noise.