A Modified FM Radio for Touch Sensing


Looking into the possibilities of touch or proximity sensing–as the mechanism by which the user interacts with our model of the ecosystem, and brings that system into “balance,” or “harmony”–I looked into repurposing an FM radio receiver from an undergraduate RF Design course.

I remembered, from when I was building it, that the local oscillator would change frequency if I put my finger near the reactive components. Since the demodulator emphasized any change in frequency, this effect was detrimental to the operation of the radio. Now, however, this effect can be used to our advantage.

I “tapped” the oscillator circuit with a piece of copper, and monitored the change in frequency on a spectrum analyzer as I moved my hand near it. As the peak shifts left and right, the oscillator frequency changes by a few hundred kHz, which can be easily demodulated into a DC signal, which can then be used to control other parts of the overall system.

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