The objective of this Instructable is to show you one method of turning DC values, such as those from a thermometer, pH sensor, or pressure sensor into a frequency which can be used to send information over the microphone band of an audio jack to a smartphone.

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
Frequency generator
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)
1μF Capacitor
OPA 551 [http://www.ti.com/lit/ds/symlink/opa552.pdf] (1)
10Ω-10kΩ Potentiometer

Assumed Knowledge
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/.


<p>Would it be possible to use the output from a Theramin instead of the wave generator successfully?</p>
<p>In the old days, before digital they were used to make signals that could be recorded on tape. Industry used 9 channel tape for recording data using different freq. VCOs so that several could be recorded on each channel, if I remember right you could record 24 different data streams on each channel. They were also used to tune radio transmitters and receivers.</p>
Interesting project. This turns anything that can control a voltage into a signal that can be used as sound right? I'm guessing it's not 1 octave per volt or anything particularly musical, but the idea of making analogue events generate audible tones or digital signals has all kinds of interesting applications.

About This Instructable




Bio: A maker, addicted to sewing, cooking, and crafting. Sometimes an engineer. Spent a summer at Instructables; got a degree in E: Neural Engineering at Olin ... More »
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