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Motorola Sensor Device Data
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The bulk of the circuit hardware is contained in a display box
mounted on the top front side of the cage. Since the accel-
erometers are physically located far away from the mother
board (about 10 feet of wiring), op–amps were used to buffer
the accelerometers’ output and drive the transmission line.
The four accelerometer signals are then simultaneously fed
into a comparator network and four of the ADC inputs on an
MC68HC11 microcontroller. The MC68HC11 was selected
because it has the capability of converting four A/D channels
in one conversion sequence and operates at a higher clock
speed. These two features reduce the overall time interval
between digitizations of the analog signal (that result from the
minimum required time for proper A/D conversion and from
software latency) thus allowing a more accurate representa-
tion of the acceleration waveform to be captured. The
comparator network serves a similar purpose by eliminating
the additional software algorithm and execution time that
would be required to continually monitor the outputs of all four
accelerometers and determine whether impact has occurred
or not. By minimizing this delay (some is still present since the
output signal must exceed a threshold, and a finite amount of
time is required for this) more of the initial and more significant
part of the signal is captured.
The comparator network employs four LM311’s configured
to provide an OR function, and a single output is fed into an
input capture pin on the MCU. A potentiometer and filter
capacitor are used to provide a stable reference threshold
voltage to the comparator network. The threshold voltage is
set as close as possible to the accelerometers’ offset voltage
to minimize the delay between ball impact and the triggering
of the conversion sequence, but enough clearance must be
provided to prevent false triggering due to noise. Because the
comparator network is wired such that any one of the accel-
erometer outputs can trigger it, the threshold voltage must be
higher than the highest accelerometer offset voltage. Hystere-
sis is not necessary for the comparator network, because
once the MCU goes into the conversion sequence it ignores
the input capture pin.
The system is powered using a commercially available 9 V
supply. A Motorola MC7805 voltage regulator is used to pro-
vide a steady 5 Volt supply for the operation of the MCU, the
accelerometers, the comparator network, and the op–amp
buffers. The 9 V supply is directly connected to the common
anode 8–segment LED displays. Each segment can draw as
much as 30 mA of current. Therefore, to ensure proper opera-
tion, the power supply selected to build this circuit should be
capable of supplying at least 600 mA. Ports B and C on the
MCU are used to drive the LED displays. Each port output pin
is connected via a resistor to the base of a BJT, which has the
emitter tied to ground. A current limiting resistor is connected
between the collector of each BJT and the cathode of the
corresponding segment on the display. To minimize the
amount of board space consumed by the output driving cir-
cuitry, MPQ3904s (quad packaged 2N3904s) were selected
instead of the standard discrete 2N3904s. The zero bit on Port
C is connected to a combination BJT and MOSFET circuit that
drives the “Your Speed” and “Best Speed” LED’s. The circuit
is wired so that the LED’s toggle, and only one can be ON at
a time.
Figure 4 shows a schematic of the circuit used. Part (a)
shows the accelerometers, the op–amps used to buffer the
outputs and drive the transmission lines, the comparator net-
work and the potentiometer used to set the detection thresh-
old. Part (b) shows the MCU, with its minimal required
supporting circuitry. Part (c) shows the voltage regulator, a
mapping of the cathodes to the corresponding segments on
the LED displays, the BJT switch circuitry used to drive the
seven segment display LEDs (although not shown on the
schematic, this circuit block is actually repeated 15 times), and
finally, the circuitry used to drive the “Your Speed”/“Best
Speed” LEDs.
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Freescale Semiconductor, Inc.
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