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I have an idea for a project but need some help on achieving what I want to do. I want to create a device which will insert into the audio jack of my iPhone which will emit IR signals to control my TV. Most IR devices use signals in the 35-45K range and the audio out has a max frequency of around 20K. So, since I cant directly tell the IR LED what frequency to emit, I'm thinking this device will need to have a microcontroller in the middle.

So what I'm thinking of doing is, having my iPhone app send a signal via the headphone jack to the device, have the device interpret it and then generate the appropriate IR signal to control the TV.

Any suggestions on how I might achieve this at a high level?
Any suggestions for kits which might allow me to easily prototype this kind of thing?
Any alternatives to my suggested solution which might work better?

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This problem is really not that bad at all. You do not need to send the carrier frequency over the headphone jack. Instead, you should build the oscillator into your device a-la http://jap.hu/electronic/infrared.html (or just use a micro) and then you just need to switch it on and off based on a much slower signal from the headphone jack.

According to the internets, almost all remote control receivers expect a 38kHz carrier wave. You will need to turn it on and off at 2-4kHz. See also http://en.wikipedia.org/wiki/RC-5

If you are as clever as off-the-shelf iPhone headphone jack infrared transmitters then you can get your power from the audio signal, but if I were you I'd just use batteries.

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  • \$\begingroup\$ Thanks joeforker. So are you saying I could for example, build an oscillator into the device which is capable of producing the IR signal of 32-56K and then have the iPhone app; (a) tell the oscillator what frequency in the IR band to use and also the oscillating frequency? (b) to tell the device the frequency in the IR band and then just control the oscillation by turning the beam on and off via a signal from the headphone jack? \$\endgroup\$ – conorgriffin Oct 3 '10 at 20:27
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This might be possible with a small FPGA, and a digital rate multiplier circuit (measures the period of the input pulses from the audio jack, and (re)output the pulses at, say, 4X the frequency). For IR input, a 4X frequency divider working the other way might also be interesting to try. Might be possible to implement this in software as well on a reasonably fast microcontroller.

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