To show that iPhone is not just a fashion accessory, as many like to say, read below how to make it a useful device. It is a frequency generator with solid characteristics. Quality will satisfy the needs of amateur electronics and it will be sufficient for the basic measurements and adjustments in audio technology.
Only two components are required, assuming that you have the iPhone:
- Application "Freqgen" which is free and available in the AppStore.
- Adapter that you have to make. Construction is very simple, and the whole process is shown further in instructable.

Step 1: Freqgen

Is a frequency or test tone generator application over the range of 20Hz - 20 kHz, and it is possible to choose between waveform: sine, square, triangle and saw. Thanks to quality of audio output for headphones, which is built into iPhone, frequency accuracy is very high. The output is near CD quality (44,100 samples per second, 16 bits/sample)
Frequency is selected by typing the numeric value in the box with the keypad, rotary selector, using a small keyboard at the bottom of the screen, or by shifting octaves with octave up/down buttons.

Step 2: Cable

Construction of cable is very simple, and the whole process is shown on my site:

Step 3: Video

A short video of everything in use:

This is cool. But I don't understand - why wouldn't I just plug the iphone's audio out into some speakers or something?<br />
&nbsp;Depends for what you need&nbsp;freq gen. You can&nbsp;connect&nbsp;it directly if you want test audio equipment or&nbsp;something&nbsp;else with audio input.

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More by damirvk:Magnetizer / demagnetizer iPhone as a frequency generator Desoldering tool 
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