We developed a series of inexpensive mobile radiation detectors, which we have named Pocket Geiger (POKEGA), to address the desire of ordinary people to own a radiation detector following the March 2011 Daiichi Nuclear Power Plant accidents in Fukushima, Japan. In order to reduce costs while maintaining accuracy and flexibility, we used a combination of a PIN photodiode detector connected to a smartphone via a microphone cable. The detector circuit design was optimized for simplicity and low cost, while the smartphone software application was tasked with handling the complex processing required. Furthermore, the device also utilized the GPS and networking capabilities of the smartphone for logging and data sharing. The 137Cs measuring range for a POKEGA equipped smartphone is approximately from 0.05 μSv/h to 10 mSv/h, which covers most radiation levels measured in Japan. Approximately 12,000 POKEGA units were shipped in the six months following its release, and more than 1,800 users have joined a Facebook community where they report measurement results and discuss hardware and software improvements.