Turning your Android into a metal detector is very simple, you will only need an application and your cell phone must have a magnetic sensor, practically all current smartphones have such a sensor.
Para saber se seu dispositivo tem esse sensor basta baixar o aplicativo CPU-Z and check in the sensors tab if it has the magnetic sensor
CPU-Z
Having seen that your device has the sensor, simply download the Metal Sniffer, and then just bring it close to any metal and the graphics on the screen will indicate if there is metal near your cell phone.
Metal Sniffer
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