wheeee! i don't believe it, but i've just managed to get the whole thing working perfectly. turns out, i was reading the bits returned from QT300 backwards, shifting left instead of right. silly me. but yes. i shall post this as a tutorial in a week or so when i get back from travelling.
next up is improving the stability of the system - currently, using a 100nF sampling cap, the data returned ranges from about 2200 (far) to 800 (near), but has a noise level of about +/- 15 which is totally unuseable for precision control.
some improvements i'm considering:
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isolating the QT300 from arduino, powering it on it's own battery supply to get rid of power rippling (i'm sure arduino's USB power output isn't crash hot in terms of ripple - can anyone verify this?)
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adding some shielding around the QT300 (could anyone suggest some good literature on how shielding works? i know absolutely nothing, except that i should keep my ground traces away from the sensor electrode...)
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perhaps try using optocouplers or some other kind of circuit isolation (again, i know nothing. do optocouplers work fast enough to allow microsecond-accurate transfer of data, or does anyone know of some other similar gadget that does?)
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trying to increase the sensitive distance of the antenna. starting with researching antenna design - how shape, length, material, etc. affect the sensitivity characteristics. i don't even know if the QT300 is capable of projecting the sensitivity field, or whatever you call it, through more than about 10cm of air. i'm hoping i can overcome this somehow, because 10cm is pretty useless to a theremin player...
hmmmm. so i've sill got quite a fair way to go on this one. but at least the first big hurdle is pretty much sorted out. any suggestions welcome!
jon.