This is a plug-and-play Geiger counter that fits any smartphone headphone jack (iOS or Android).
The Smart Geiger is actually made by a Korean firm called FTLab.
The Smart Geiger dongle is every bit as effective as stand-alone handheld Geiger counters that run well into three figures – whereas this one is ¥4,980 (~$42). They do of course include the caveat that this device provides general reference data and not necessarily highly accurate, definitive measures.
Dimensions: 10 x 10 x 47mm (32mm when plugged-in)
Smartphone Compatibility: iOS 7+; Android 2.2+
Radiological Sensing: Gamma Rays; X-Rays
Sensor Range: 0.1 ~ 200 uSv / h
Price: ¥4,980 (~$42)