An active device for radon detection in the air was developed. The monitor operates in pulse counting mode for real-time continuous measurements. The presented prototype has a relatively simple design made of low-price and easy to acquire components which made it possible to develop an inexpensive device. The device used as a sensor, the SLCD-61N5 Si-PIN planar photodiode, which has an area of 9.67x9.67 mm2, is sensitive to alpha particles. An Arduino Uno microcontroller was used as a data acquisition system. Signals were observed when placing an 241Am or 226Ra source near the sensor. The sensors sensitivity has small bias dependency and the device can operate even at modest voltage. As a result of a one-month test in a radon-rich atmosphere, a positive high correlation (Pearsons r equal to 0.977) was obtained between our prototype and a Geiger-Muller detector.
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