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And I didn't leave the mortar & pestle on the floor."
The sensor which immediately held my interest was the geomagnetic sensor, found in most smartphones. App Inventor converts the output from the HTC's internal AK8973 3-axis geomagnetism sensor to an azimuth reading (0 - 360 degrees), for utility as a compass. I was disappointed with this, and had hoped that the device's raw data would be available. Anyway, the graph of azimuth vs time proved most interesting.
The trace shows intriguing hunting behaviour when close to active electronics (laptop etc.) and also I was surprised by the sensitivity. A two pence coin (post 1992, bronze to steel) moved near the phone distorts the Earth's magnetic field and the trace moves considerably. At this point I realised I could interface with the phone; I swiftly cut a spare charger lead up, to provide a couple of feet of wire, stripped both ends and, with a 18650 Li-Ion battery ripped from a scrap Vaio power pack, I recreated Hans Christian Ørsted's momentous experiment of 1820. He would have recognised only the wire.
Joe uses 30 turns of enamelled copper wire (about a meter) to form a coil with an internal diameter of 1cm. He's energising the coil using an arduino (5V) through a 120 ohm resistor to limit the current to 40 mA. He uses a free magnetic sensor app to determine where the sensor is within the phone, then determines the maximum speed he can send data to the phone from the arduino.
communications between an Arduino and an Android cellphone. See this blog post for details: