We assumed an input DC voltage as our sensor and built a voltage-controlled oscillator (VCO) to turn different DC voltages into correspondingly different frequencies, which the microphone input into the smart phone’s audio port reads as different tones.
Step 1: What You Need
Oscilloscope & lead wires
Device that plays audio and takes microphone over a 3.5mm audio jack (we are using a Samsung Galaxy Vibrant).
3.5mm TRRS Audio cable
Resistors (two 100kΩ and one 100Ω)
OPA 551 [http://www.ti.com/lit/ds/symlink/opa552.pdf] (1)
You can use a breadboard. If you’ve never used one before, see http://www.instructables.com/id/Breadboard-How-To/.
You can use an oscilloscope. If you’ve never used one before, see http://www.instructables.com/id/Oscilloscope-How-To/.