Radio Shack 26-3801 Service Manual page 74

7o
10-
Buzzer Control Circuit
There are two ways in which
the buzzer can be sounded by
the
buzzer control circuit. One
is
by emitting
a
signal from PB5
of 81C55 at
a
frequency which sounds
the buzzer, and the
other
is
by
using the timer output
of 81C55.
(1)
Signal from PB5 of 81C55
With PB2 of 81C55 at
"H"
level in the circuit diagram
(Fig. 33)
,
the buzzer
is sounded by the repeated
-OFF
and
ON switchings
(of
the transistor for buzzer
drive)'
caused by the output from
PB5 of "H", "L", "H", "L"
.
.
signals synchronized with
the
frequency for sounding the
buzzer.
(2)
Using 81C55 timer output
With
PB5 at "H" level in the circuit diagram
(Fig.
33)
the
buzzer
is
sounded when, with
the 81C55 timer in the
square-wave
output mode and
set to the value
corresponding
to the
frequency which will sound the
buzzer, PB2
is
switched
to
"L"
,
causing the square-wave
pulse emitted from the Timer Out
(To)
terminal of 81C55
to be
applied to the base of the transistor for buzzer
drive.
PB2 uses this
as the buzzer ON/OFF control signal.
(1)
Signal from
PB5 of 81C55
(2)
Using 81C55 timer output
Fig.
33