  Low-power a.c. generators of square-wave or sinusoidal signals can be used in
combination with impedimetric sensors to detect stimuli on the basis of the
voltage drop taking place at the sensor electrodes. When a.c. generators with a
power of only a few micro-Watts are used, this approach becomes extremely
sensitive. A very low-power generator is the LCD back panel driving signal,
which has a flipping polarity with a voltage of 3-5Vpp, depending on the
generator model. This type of square-wave generator is contained in many
low-cost handheld digital multimeters, and it is used as signal tracer to test,
for example, low-frequency amplifiers. As an example, this method has been used
to acquire a human breath rate pattern, by using a zeolite-based water sensor.
If the generator I-V characteristics has been measured, the achieved breath
pattern can be converted from a voltage drop vs. time graph to an impedance or
current intensity vs. time graph.
