Footswitch
Board
4-16
the
associated photo-transistor to conduct. The collectors
of
the transistors
are
cormected to
input
pins
of
an
I/O
port
on
the
microcontroller
where
they
activate
the
desired
mode
of
operation.
Audio
Circuit
The
audio
system
consists
of
an
audio
oscillator,
tone
control signals, a
volume
control potentiometer,
an
audio
amplifier,
and
a speaker. A
reference voltage (Vref) is
used
throughout
the
audio
circuit
and
is
generated
by
dividing
the
+12 V
power
supply
down
to
about
2 V
by
R9
andR8.
The
audio
circuit armunciates
the
presence
of
RF
output
and
provides
an
auditory
indication
of
alarm
conditions. A
potentiometer
adjusts the
volume
of
RF
output
activation tones.
The
speaker
volume
carmot
be
turned
off entirely. The
volume
of
the
tone
issued
during
alarm
conditions
is
not
adjustable.
The
audio
oscillator is
enabled
when
UP
_TONE\
or
RF
_TONE\
is
pulled
low. Diodes
Dl
and
D2
provide
a
wired
OR
function for
the
two
signals.
Since
UP
_TONE\
and
RF
_TONE\
are
+5 V (logic level) signals, resistors
R4
and
R6
divide
the
+12 V
audio
power
supply
down
to
about
4.85 V to
prevent
Dl
and
D2
from sourcing
current
into
the
output
pin
of
U3
on
the
Control board.
When
either
UP
_TONE\
or
RF
_TONE\
is
enabled
low,
the
voltage
at
the
noninverting
input
of
UlB
is
pulled
below
the
Vref
threshold
present
at
UlB's
inverting
input,
the
open
collector
output
of
UIB
is
turned
on,
grounding
R31
and
allowing U6A to oscillate.
U6Ais
a relaxation oscillator
whose
frequencies
are
determined
by
the RC
time constants
of
R30, C35,
and
CIS. This
design
allows
the
oscillator
to
produce
two
distinct frequencies
that
can
be
selected
by
the
state
of
the
LO _TONE signaL
•
When
LO_TONE is
not
asserted, R30
and
C35
determine
the
frequency
of
operation
of
the oscillator (approximately 900 Hz).
•
When
LO_TONE
is
asserted
(+5 V),
the
voltage
at
the
noninverting
input
of
UIA
exceeds the 2 V Vref
signal
at
the
inverting
input,
turning
on
its
output
transistor. This effectively cormects C18
in
parallel
with
C35 to
produce
a
higher
RC
time
constant
for
the
oscillator,
which
results
in
a
lower
audio
frequency
of
approximately
700
Hz.
The ALARM signal selects
the
user-controlled
audio
volume
or
the
fixed
alarm
level
volume.
UIC
and
UID
are
configured
in
an
exclusive
OR
arrangement
in
which
the
state
of
the
output
transistors
of
UIC
or
UID
is
complementary.
In
other
words,
the
output
transistor
of
one
of
these
two
devices is
always
on,
but
both
carmot
be
on
simultaneously.
Under
normal
operating
conditions, the ALARM signal
is
low, allowing
the
UIC
output
to float
while
the
UID
output
transistor is
turned
on.
The
output
of
UID
creates a voltage
divider
through
RU,
R12 (the
volume
control
potentiometer),
and
R32
to
attenuate
the
audio
signal
to
levels acceptable
for
input
to the
audio
amplifier. R32
determines
the
maximum
audio
volume
and
Rll
determines
the
minimum
audio
volume.
RIO
determines
the
audio
alert
volume
level. R34
provides
an
alternate
audio
signal
path
in
the
event
of
an
open
volume
control potentiometer.
Force
F)(TM-C
Service
Manual
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