Smartphone Microphone for Acoustic Instruments

In this Instructable, I will show you how to use an old smartphone as a pickup for your instrument!

Smartphone - I used an iPhone 4s
Microphone/Megaphone App that outputs through the headphone jack or over Bluetooth
Acoustic Instrument of your choice - in my case an acoustic guitar.
3.5mm headphone to guitar cable adapter
Guitar cable
Something to mount the phone with I suggest either tape, velcro or sugru - I used blue-tack and a belt
An amplifier or speaker - Beware of feedback!
Audio recorder - I used a Zoom H4n

Firstly, you'll have to find an app that allows your phone to output whatever sound it picks up through the headphone jack or over Bluetooth. - This will take some experimentation to find one that offers low latency and audio input and output level control.

Some apps can output directly through the phone's internal speakers. - I do not recommend using that since it will cause instant audio feedback that can wreck your phone (and your hearing).

I found that these apps are most often called microphone or megaphone. The one I used was available on iPhone 4s and was free. It had level control, Reverb, and some audio samples on a soundboard.

Simplified:
You need to be able to choose from where the phone will play sound from - Speakers, Headphones, or Bluetooth.
You need to be able to choose from where the phone will hear sound from - Phone internal mic or Headset.
The app needs to have a low latency - As little delay as possible from the phone hearing sound to playing sound.
A control for how sensitive the phone is for picking up sound.
A control for how loud the phone will play the sound it picks up.

I prefer using a cable but if you're going wireless you'd also require a Bluetooth speaker or receiver for the signal.

To get the best sound you will have to move the phone around on your instrument to find the best spot for it.

I recommend using an audio recorder with a guitar cable input and headphone monitoring.
This way you'll capture a clean signal from your smartphone pickup.

Have fun!