  Augmenting everyday surfaces with interaction sensing capability that is
maintenance-free, low-cost (about $1), and in an appropriate form factor is a
challenge with current technologies. MARS (Multi-channel Ambiently-powered
Realtime Sensing) enables battery-free sensing and wireless communication of
touch, swipe, and speech interactions by combining a nanowatt programmable
oscillator with frequency-shifted analog backscatter communication. A
zero-threshold voltage field-effect transistor (FET) is used to create an
oscillator with a low startup voltage (about 500 mV) and current (< 2uA), whose
frequency can be affected through changes in inductance or capacitance from the
user interactions. Multiple MARS systems can operate in the same environment by
tuning each oscillator circuit to a different frequency range. The nanowatt
power budget allows the system to be powered directly through ambient energy
sources like photodiodes or thermoelectric generators. We differentiate MARS
from previous systems based on power requirements, cost, and part count and
explore different interaction and activity sensing scenarios suitable for
indoor environments.
