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iPad / iPhone / iPod Touch Microphone Adapter Wiring Diagram

Previously, I wasn't aware of any commercially available cables to let you connect a line level audio source to the microphone jack of an iPhone/iPad/iPod Touch, so I came up with one of my own.

Since this was first written, I came across the KM-IPHONE-MIC-A22 cable, which from the description appears to be exactly what is needed. Note however that I have not tried this commercial cable, so I do not know for sure that it works.

If you would like to make your own cable, please read on...

Several of our apps make use of sound input:
Spectrum Pad
Morse Pad
NAVTEX Pad
Packet Pad
PSK31
ACARS
SSTV
HF Weather Fax
iGeiger

While it is possible to set your iPhone or iPad next to the radio's speaker or headphones, this is often less than ideal, due to the problem of picking up external sounds.

Here's what you need:

3.5 mm 4 conductor phone plug, such as the 171-7435-EX from Mouser
4.7k resistor
10K potentiometer
600 ohm transformer, such as the 42TL016-RC from Mouser
Audio cable to plug into your radio, usually with a 3.5 mm mono plug

The 172-7447-E cable with a 4 conductor 3.5"" plug may also be useful.

Here is the wiring diagram:

The iDevice needs to see a resistance in the neighborhood of 5k between the microphone conductor and ground. That tells it that a microphone has been plugged in. If it is a direct short, it thinks a headphone was plugged in. Open circuit means nothing was plugged in. The exact value is not important, from experimenting I found that 4.7k seems to work. This resistor is put in series with the isolation transformer.

The other side of the isolation transformer goes to the cable that plugs into the radio, through a 10K potentiometer, which serves to adjust the input level as needed. Again, the 10K value was chosen from experimentation.

While there are 4 conductor audio plugs with an attached cable that could be used, to neaten things up, they all seem to put the shield on conductor 4, which makes sense for most applications where that is ground. For the iDevices, that is the microphone line, and as a result, rather than shielding the signal, I've found that you get extra noise pickup.

As always, be careful with your wiring, double check everything, and check that the schematic and drawing above actually make sense. Any damage due to wiring errors, including in our drawings, is your responsibility.


Email your comments and questions to support@blackcatsystems.com

Last modified March 29, 2012