I have an idea for an infrared remote interface for handhand Android devices which have an LED camera light and does not tie up the audio interface.
A high speed photodiode or phototransistor is connected to one input of an AND gate with the other input of the AND gate being connected to an infrared carrier generator, with the output of the AND gate being connected to an output transistor for the infrared emitter/s - if the LED camera light is ON, an infrared signal is transmitted.
The camera light can "gate" an infrared carrier generator at a low frequency, either on a USB PWM generator dongle connected to the USB On-The-Go port or a 38kHz generator connected to a battery via a power switch.
Using the "gating" of the infrared carrier is easier than generating the carrier frequency via the LED camera light, especially at the B&O remote carrier frequency of 455kHz.
To extend battery life on the handheld device, the brightness of the LED camera light should be limited by software PWM, and a high frequency filter is connected after the photodiode or phototransistor to prevent problems with infrared transmission.
This circuit can either be entirely powered by the USB On-The-Go interface via a power switch in some cases or a rechargable battery with a power switch.
Idea for Android infrared transmitter
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Idea for Android infrared transmitter
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Idea for Android infrared transmitter
I forgot to say that this could be a LIRC project.
"The best thing possible to do with knowledge is to share it".
I always like it when people have some new ideas. 
The new idea here is to use the camera LED, grabbing it without electrical contact but optically. For this: do you have access to an API for controlling that LED? I would expect that it is specified to be turned on and off approximately in second rate, so it is not clear that it can be controlled with the (as such moderate) requirements of IR signals, even if you delegate the modulation to dedicated hardware. It is not a question of rise- and fall-times in the electronics, rather it is the question of the realtime properties of the operating system. It could be that the LED is served in a slow, low-priority task. Putting it differently, why should the designer of a real-time system treat something as a camera LED as a high-priority thing, there are always sufficiently many things that has to be serviced with high priority.
To LIRC: You should take a look at the "serial" driver, it probably does pretty much what you want, as soon as you have found out how to access the LED: There a command line parameter called "hardcarrier" or some such that inhibits the carrier generation in software. You may also like to use this patch to make the LIRC server useful.
For alternatives: some Android thingies come with build in IR transmitter (Sony tablets, Samsung?, possibly other). Real C-IR, not IRDA. That might be a better idea? Then there is of course USB dongles, like Iguana.
The new idea here is to use the camera LED, grabbing it without electrical contact but optically. For this: do you have access to an API for controlling that LED? I would expect that it is specified to be turned on and off approximately in second rate, so it is not clear that it can be controlled with the (as such moderate) requirements of IR signals, even if you delegate the modulation to dedicated hardware. It is not a question of rise- and fall-times in the electronics, rather it is the question of the realtime properties of the operating system. It could be that the LED is served in a slow, low-priority task. Putting it differently, why should the designer of a real-time system treat something as a camera LED as a high-priority thing, there are always sufficiently many things that has to be serviced with high priority.
To LIRC: You should take a look at the "serial" driver, it probably does pretty much what you want, as soon as you have found out how to access the LED: There a command line parameter called "hardcarrier" or some such that inhibits the carrier generation in software. You may also like to use this patch to make the LIRC server useful.
For alternatives: some Android thingies come with build in IR transmitter (Sony tablets, Samsung?, possibly other). Real C-IR, not IRDA. That might be a better idea? Then there is of course USB dongles, like Iguana.
^^^
The Griffin Beacon is another example of that. (But don't get one. It sucks beyond belief).
And iRule uses an IP to IR gateway device.
But it would be nice to be able to use just the phone itself without any additional hardware though, as originally proposed.
The Griffin Beacon is another example of that. (But don't get one. It sucks beyond belief).
And iRule uses an IP to IR gateway device.
But it would be nice to be able to use just the phone itself without any additional hardware though, as originally proposed.