Table of Contents
Advertisement
Transistor Q1 is a series switch which applies battery
voltage to the remainder of the circuitry.
controlled by transistor Z2, which in turn, is controlled
by "watchdog timer" circuit U11.
To initiate power-on Q1 is turned on through
momentary closure of membrane pushbutton S1
("ON / (OFF) / ZERO") and diode D2 (located on
display board 48901/500043); diode D1 (located on
display board) is used to signal input pin "P2" of I/O
expander U4 (also on display board) that pushbutton
switch S1 is pressed.
A secondary turn-on circuit occurs through diode D4
(located on the display board) and pushbutton S4
("RECALL PRIOR WEIGHT") to allow display of the
previously stored weight if the scale is presently
turned "off". The pushbutton closure is also coupled
through diode D3 (located on the display board) to
signal input pin "P1" of I/O expander U4 (located on
the display board) that the "RECALL PRIOR
WEIGHT" pushbutton is pressed.
Once Q1 is on and voltage is applied to the circuit,
watchdog timer U11 will keep transistor Q2 on
through
output
line
subsequently keeping transistor Q1 energized. If no
further action occurs, an internal timer contained
within watchdog timer U11 will time-out after
approximately 1.6 seconds and switch off Q2,
causing Q1 to turn off and remove power from the
scale's circuitry.
Once energized and properly running, microcomputer
U10 will keep resetting watchdog timer U11 by
periodically pulsing U11's input line, labeled "WDI".
Should the scale's operating program call for shut-off,
or a hardware/software failure of microcomputer U10
occurs, the reset pulses to U11 will no longer occur
and 1.6 seconds later U11 will time-out and cause the
circuit power to switch off.
Resistors R1, R2, R3, R5 and R14 are included for
proper circuit biasing. Capacitor C6 is used as an
output filter.
4.25.2 VOLTAGE REGULATION
Voltage regulators VR1 and VR2 render regulated
sources of +5 Volts D.C. for operation of the analog
(VAA) and digital (VCC) circuits, respectively. Use of
two separate +5V regulators helps to prevent noisy
digital signals from entering the sensitive analog
circuits. Capacitors C3 and C8 are used to insure
regulator stability.
/WDO
and
resistor
4.25.3 +9.5V/-7.5V SUPPLY
Q1 is
Integrated circuit U2 is used to convert +5 Volts D.C.
to +9.5V and -7.5 Volts D.C. for use in the analog
circuits. It contains an internal oscillator (operating at
approximately 8 KHz) and a series of switches.
During one half of the cycle, capacitor C25 is
connected between VAA and ground, charging C25
to VAA's potential of +5 Volts. During the other half
cycle, capacitor C25 is reconnected between VAA
and pin 8 (negative lead of C25 to VAA) so that its
voltage adds to VAA and charges filter capacitor C26
to approximately twice VAA or 9.5 to 10 volts.
The remainder of U2 is used to generate a negative
supply voltage. Capacitor C25 is connected between
ground and the +9.5 Volt source on pin 8 during one
half cycle of the internal oscillator. During the other
half cycle, it is reconnected between ground and pin 4
such that its negative lead is connected to pin 4. This
transfers C28's charge into filter capacitor C27 and
produces a negative voltage. Diodes CR8 and CR9
reduce the voltage slightly to obtain the desired -7.5
Volts.
R5,
4.25.4 BATTERY MONITOR
Integrated circuit U3 is included to monitor the voltage
of the battery and provide an indication to the scale's
operator when battery replacement is required. Two
states of weak battery operation are detected;
"low-battery" (battery is usable but will soon need
replacing) and "low-low battery" (battery is too weak
to properly operate the scale).
Pins 1, 2 and 3 of U3 are connected to a voltage
divider network consisting of R8, R9 and R10, to form
the "low-battery" detector. The output of this circuit
(pin 1) is normally low when the battery is good and
switches high when the battery is low. It is coupled to
I/O pin "P2.6" of microcomputer U10.
software in U10 will process this signal and after a
slight delay cause the "LOW BATTERY" annunciator
on the display board to flash.
The remaining half of U3, pins 5, 6 and 7, are
connected to resistors R11, R13 and R13 to form the
"low-low" battery detector. The output on pin 7, which
is normally "high" with good batteries, goes "low"
when the batteries are too weak to reliably operate
the scale. This output is connected to I/O pin "P2.4"
on microcomputer that "low-low" battery has been
recognized.
Microcomputer U10 will process the "low-low" battery
signal and cause "bAttrY" to appear on the scale's
14
Operating
Hide quick links:
Advertisement
Table of Contents
