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We developed a series of inexpensive but accurate and mobile radiation detectors, which we 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. To reduce costs while maintaining accuracy and flexibility, we used a combination of a p-i-n photodiode detector connected to a smartphone via a microphone cable. The detector circuit design is optimized for simplicity and low cost, whereas the smartphone software application is tasked with handling the complex processing required. Furthermore, the device also used the GPS and networking capabilities of the smartphone for logging and data sharing. The 137Cs measurement range for a POKEGA-equipped smartphone is approximately from 0.05 to 10 mSv/h, which covers most radiation levels measured in Japan. Approximately 12000 POKEGA units were shipped in the six months following its release, and 2000 users have joined a Facebook community where they report measurement results and discuss hardware and software improvements. In parallel, we have addressed practical problems for POKEGA, such as vibration noise, energy consumption, and operating temperature, by conducting field tests in the Fukushima evacuation zone. The POKEGA series has been improved by solving such issues. This article reports on a new style of pragmatic sensor networking methodology, from the aspects of emergency response engineering, open-sourced development, and consumer-generated measurements.