Scale-Tronix 6702 - Page 22

The battery power remains on until microcomputer
U10 decides to shut it off in response to lack of scale
activity by setting pin 4 of Ut and setting the latch
“off”.
Resistors R1, R2, R3, and R5 are included for proper
circuit biasing. Capacitor C6 is used as an output
filter.
23005 R02 board: Transistor Qi is a series switch
which applies battery voltage to the rernainder of the
circuitry. Q1 is controlled by transistor Q2 which, in
turn, is controlled by "watchdog timer" circuit U14.
To initiate power-on Q1 is turned on through
momentary closure of membrane switch S9 ("ON")
and diode CR10; diode CR13 is used to signal input
pin “PB.7" of port expander U7 that the "ON" switch
is pressed.
A secondary turn-on circuit occurs through diode
€R12 and switch $3 ("RE CALL”) to allow display of
the previously stored weight if the scale is presently
turned "off, The switch closure is also coupled
through diode CR14 to signal input pin "PB.1” of port
expander U7 that the "RE CALL" switch is pressed,
Once GI is on and voltage is applied to the circuit,
watchdog timer U11 will keep transistor Q2 on
through output line ANDO and resistor R5,
subsequently keeping transistor Q1 energized. If no
further action occurs an intemal timer contained
within watchdog timer U11 will time-out after
approximately 1,6 seconds and switch off Q2,
causing Q1 fo turn off and remove power from the
scale's circuitry. (Because the modei 5102 remains
on constantly, transistor Q1 is permanently wired
"on" by use of a jumper across J2 or JTEST)
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. The model 5102 has
a jumper added at location J7; this causes U11 to
reset U10 in the event of a hardware/software failure,
Resistors R1, R2, R3, R5, and R14 are included for
proper circuit biasing. Capacitor C6 is used as an
output filter.
4.24.2 VOLTAGE REGULATION
Voltage regulators VR1 and VR2 render
regulated sources of +5 Volts D.C. far 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 Cé are used 10 insure
regulator stability.
4.24.3 +9.5V/-7.5V SUPPLY
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 intemal 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 O28 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 à negative voltage. Diodes CR8 and CR9
reduce the voltage slightly to obtain the desired -7.5
Volts.
4.24.4 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 R&, 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
20 5002, 5102, 5602, 6002, 6102, 6202, 6502, 6702