I'm having way too much fun torturing my dear wife in the name of science. I'm working on the KType iPad app and need to figure out a reliable way to detect whenever an external button is pressed. Instead of requiring my users to touch the iPad screen, I want them to be able to press a physical button with their hand/foot/chin and register it as a click on the iPad app.

One of my initial ideas was to pair a low-cost bluetooth num-pad with the iPad and detect whenever a key is pressed. While it sort of works, there are some technical issues that cannot be resolved trivially. So I'm still looking for a better/cheaper way to detect from my iPad app that an external button has been pressed.

One idea I've considered exploring is high-frequency sound. It's easy to make a small battery-powered device that emits a specific high-frequency tone when pressed. If the iPad's microphone can detect this tone easily, then it can register a click. Of course, the sound must be inaudible to humans and pets but still be detectable by the iPad microphone. To test it out, I started playing these clips at a low volume.

I could hear 8kHz-16kHz sounds fine but not the 17kHz and above clips so I cranked up the volume all the way. Juliet started freaking out in the other room as she could clearly hear everything up to 20kHz. While it was hilarious to watch her get so flustered, it is also bad news for my research. It appears I might not be able to rely on high-frequency sound for input and need to keep looking for a better way.

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