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Almost
Ultrasonic
Motion
Sensor
This
sensor
uses the
sound
generator
in the
RCX to
make a
15kHz
audio
tone,
which is
almost
ultrasonic.
The tone
is
received
with
circuitry
similar
to my
Sound
sensor.
The
output
of a
crystal
microphone
MIC is
amplified
and then
only the
very
high
frequencies
are
further
amplified
(see
plot).
This
signal
is
enveloped
detected
with a
diode D1
and
capacitor
C1. The
voltage
on the
capacitor
will
equal
the
average
volume
of high
frequency
sound
the
microphone
is
picking
up at
any
moment.
Motion
of
objects
near to
the RCX
and
microphone
cause
the
volume
level to
vary as
the
signal
path and
distance
between
them
changes.
The
variation
in level
is
further
amplified
and fed
to the
RCX. The
RCX sees
a level
of about
31 when
there is
no
motion,
and
peaks to
over 50
when
there is
rapid
significant
motion.
The
circuit
diagram
is
below.
An LM324
quad
Opamp is
used for
all
stages,
and the
diodes
are all
1N4148s.
This
is a
plot of
the
frequency
response
of the
High
Frequency
Amplifier
stage.
The gain
has been
set to
peak at
about
15kHz so
that
interference
from
other
sounds
is
minimized.
Here
is a
photo of
my
finished
sensor.
I built
it on a
piece of
PC board
and
mounted
it
inside
of a
LEGO
#5391 9V
Battery
Box. The
microphone
is
attached
to the
end with
double
face
foam
tape.
Here
is how I
tested
the
circuit.
The RCX
has a
Visual
Basic
program
that
produces
a 15kHz
tone and
watches
the
input
for
levels
over 33.
If is
sees
one, it
turns on
an
output
which
just has
a light
hooked
up to
it. The
operating
range
depends
on many
mechanical
factors.
It can
pickup
motion
of large
objects
8" away
while
small
objects
may need
to be
only a
few
inches
between
the RCX
and the
microphone.
Here
is the
very
simple
VB code:
.BeginOfTask 0
.SetSensorType 1, 3
.SetPower "0", 2, 7
.Loop 2, 0
.PlayTone 15000, 25
.If 9, 1, 0, 2, 33
.On "0"
.Else
.Off "0"
.EndIf
.EndLoop
.EndOfTask
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