June
2006, Issue 191
Nontraditional
Cursor Control
ATmega32-Based
Motion Sensing
by
Andrew Sawchuk & Joseph Tanen
HARDWARE
DESIGN
The
hardware for our Airmouse project is based on information
gathering and processing (see Figures 3 and 4). The
accelerometer output goes through three stages. The
first stage is a low-pass filter that removes the high-frequency
noise from the accelerometer.
|
(Click
here to enlarge)
|
Figure
3—The path of the data and signals through the system’s
major components is straightforward. |
|
(Click here to enlarge)
|
Figure
4—This circuit filters high-frequency noise out
of the accelerometer signal, aligns the accelerometer
output with the microcontroller voltage sampling
range, and amplifies the output to fill a larger
part of the sampling range, allowing for more resolution
on sampled signals. The circuits for the accelerometer’s
x- and y-axes are the same. Only one copy of the
filter/amp circuit is shown. |
The
second stage is the differential amplifier stage, during
which the signal is amplified by a factor of two. This
expands the accelerometer output from 1.5 to 3.5 V (representing
–1 to 1 g) to 0.5 to 4.5 V (still representing –1 to
1 g). Introducing gain into the accelerometer output
enables us to use the ADC range more fully. The signal
almost fills the ADC’s 0- to 5-V range instead of filling
only two-fifths of it, as was the case before the signal
was amplified. The negative end of the differential
input is the accelerometer output. The positive end
of the differential input is the output of a potentiometer
whose inputs are attached to 5 and 0 V. This potentiometer
enables us to tune the accelerometer 0-g level to a
voltage of our choice.
The
third stage is a protection stage for the ADC. Because
the amplifier’s output level isn’t rail-to-rail with
its power supply, we had to use a power supply that
could produce potentially damaging output to the ADC.
The output of the amplifier goes through a resistor
and then through a diode to 5 V. The output is taken
off of the junction between the resistor and the diode.
This prevents the amplifier output from getting higher
than 5 V plus the internal voltage drop of the diode
(approximately 0.67 V).
In
addition to the two accelerometer inputs to our ATmega32
microcontroller, we poll four push buttons, which provide
enable/disable functionality, left click, right click,
and mouse scrolling. These push buttons are tied to
the ATmega32 microcontroller’s pins and ground. The
pins are internally pulled up, so the buttons are active
low. Pushing the button ties the pin to ground.
