I wanted to conduct a recorded interview earlier this month, but I really didn’t want to be dragging around a bagful of kit. My first thought was to rush out and buy a handheld recorder (which appealed to my love of acquiring equipment), but then it occurred to me that I had a perfectly good iPhone which could probably do exactly the same job.

Now, for a scratch recording to use as an aide memoire, the internal microphone is easily good enough. But I wanted a recording that would be good enough to use in a podcast, which meant using a decent external microphone. There are many options for the iphone, from plug-in lavalier’s to adapters for hand-held microphones, and none of them are desperately expensive, but I wondered if I could build a little adapter that would allow me to use the kit I already have.

A quick note on the 3.5mm connector for the iPhone, then. A standard headphone connection uses a 3.5mm TRS jack. If you’re using a microphone or adapter, you’ll find it uses a 3.5mm TRRS jack. With the iPhone, the tip and first ring of the jack are the headphones left and right connections. The second ring is the earth connection and the sleeve is the microphone connection. When you plug in a TRS headphone jack, the mic and earth connections are shorted and the iphone assumes there is no mic plugged in, and defaults to the built-in microphone. With a TRRS jack, the iphone puts out a small voltage and looks to see if this is being dropped across a resistance. If it detects the appropriate resistance, it assumes an external microphone is plugged in, and defaults to that.

So here’s what I did. I soldered up a TRRS jack to connect the headphones pins to a 3.5mm headphone socket, and the mic pin to a 3.5mm mono TS socket. In the TS socket, I connected a 1kΩ series in series with the mic input pin. The iPhone specs are a little vague about what resistance is required; it seems that it varies between models and between iOS version. Mine is an iPhone 4 running iOS 6, and with a little experimentation, 1kΩ seemed to work fine. I tested it with an Audio Technica AT804 interview mic and it worked perfectly, as it did too with a Shure SM57.

Both of these mics have transformer outputs with an impedance of around 300Ω. That presents in series with the 1kΩ resistor, putting 1k3 across the ring and sleeve pins of the TRRS jack, and iPhone seemed happy to accept this as indicative of an external microphone being connected. The transformer also isolates the microphone capsule from the small DC voltage the iPhone puts out across the ring and sleeve pins.

So it all worked out fine, with a total installed cost £3.90. I’m running the Rode REC audio recording app, but I would imagine any recording app will work fine. One tip; plug in the microphone before you launch the app to ensure that it detects the external microphone over the built-in mic.