  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 sensor's 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 (Pearson's r equal to 0.977) was obtained between our prototype and
a Geiger-Muller detector.
