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could the iPhone's headphone jack be used as an input for a device? Answered

I was thinking that the headphone jack could be used as an input for a 3-lead ECG (EKG)  and used to cheaply monitor arrhythmias in remote areas (or in the field) and record and/or send these to health professionals. It needs to be able to detect milivolt impulses from a pos/neg/ground input.


Actually you could use the speaker output wires as an input voltage for the amplifier and use an encoder to encode the information into a serial signal down the the input wire for the mic. But I like the idea.

This was posted over the weekend on the MAKE Web site.

Not directly. The iPhone jack is a combination earpiece (output only) and microphone (input only) connector. The microphone input is a single wire, with common ground to the earpieces.

Essentially you need to isolate the three-lead system from the mic, and do a voltage offset. Then you can put a unipolar signal into the iPhone so that the "trace" looks right visually and can be analyzed with an App. If you're going to do data-logging to ship to medical professionals, you'd want the App to subtract the offset.

I wonder if you could do this with surface-mount components and make it small enough to fit inside a simple-looking phono plug with a dongle that just plugged right into the iPhone.

there is a build i saw where a mp3/video device that was able to record audio had been converted into an ocilloscope through the use of the headphone jack. similar to the dso nano. http://www.sparkfun.com/products/9625

I'm not sure I'm following the question. Do you want to route the phone's audio output into another device (probably possible, though you might need to attenuate or amplify the output), or do you want to feed the phone's headset-microphone input from another device (probably possible but more complicated -- especially at those signal levels -- undoubtedly needs external circuitry to filter out noise and match the signal to the mic input).

There's also the question of whether the phone, and/or the phone connection, could handle those frequencies, or if you'd have to do what past remote EKGs have done and put them through a modem. Especially since cell phone bandwidth is even narrower than land-line bandwidth.

In other words, the answer is "yes it could probably be done, but probably not as cheap as you hope, at least not until you get into mass-manufacturing quantities."

And if you want cheap, why are you starting with such an expensive phone?

Interesting question. I'd search for technical manuals on the iPod/headphone jacks.

(But how would the arrhythmias' signals get to the iPod...?)