January
2006, Issue 186
Electronic
Scarecrow
REMOTE
UNIT
Photo
2 shows the remote unit’s hardware, which includes a
modified SARD board and various sensor boards. The modular
system can include up to four SARD boards. Each board
uses one of its on-board accelerometers and can connect
to as many as three external sensors. Also included
are a PWM audio output for an external amplifier and
speaker and a logic-level output for future expansion.
You may download a schematic of the complete system
from the Circuit Cellar FTP site.
|
(Click
here to enlarge)
|
Photo
2—The SARD board is in the middle. The set of wide
traces on the left edge acts as an antenna. Also
included are the MC33794 e-field sensor, accelerometers,
pressure sensor, and switch expander boards.
|
The
system is designed around an MC9S08GT16 microcontroller
(see Figure 2). The SARD board actually contains an
MC9S08GT60 microcontroller, but all of the hardware
and software is based on an MC9S08GT16 microcontroller.
Changes aren’t required when switching to this part.
All of the MC9S08GT16 microcontroller’s I/O pins and
most of its internal peripherals are used.
|
(Click here to enlarge)
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Figure
2—The MC9S08GT16 microcontroller is at the center
of all the action. It coordinates all the sensors
and transmits their statuses to the base unit over
the radio link. |
The
SARD board includes an MC13192 2.4-GHz radio transceiver
with separate transmit and receive antennas. It also
has an RS-232 port, which is used only for debugging,
and a pair of accelerometer sensors. Only the XY-axis
accelerometer is used. The board has several connectors
for the various external sensors. It also has a system
Enable/Disable switch. Two address select jumpers enable
up to four boards to be individually addressed.
A
NiCd battery charged by a photovoltaic array powers
the remote unit, so minimizing power consumption is
a primary design goal. Sensors are turned on only when
necessary. Sensor circuitry is designed to minimize
settling time. Between readings, the microcontroller
goes into Stop 3 mode and the radio goes into Doze mode.
I spent a lot of time evaluating which clock modes would
yield the lowest overall power consumption.
