Mallinckrodt Nellcor N-20 Service Manual page 60

Technical Supplement
TO: N-20
MAIN PCB
JP18
1
2
MEASURE
BUTTON
The standard user controls consist of two momentary push-button switches (measure and
check-battery). The Measure button is an elastomeric contact switch, and the Check-Battery button is
a mechanical momentary switch.
The CPU input lines BAT_BTN and GO_BTN are normally pulled to the high state by R71 and R78.
Whenever a button is depressed, the CPU input line is pulled low through R74 and R80. The switch
contacts are debounced with C64 and C66. L11, L12, C126, and C123 provide a current path for ESD
protection.
In addition to being read by the CPU, the Measure button also activates the power supply via the
power control circuit. Note that the Measure button has circuitry on both the main PCB as well as the
auxiliary PCB.
9.10.15 Power Supply/Power Control Circuitry
Power supply circuitry is located on the auxiliary PCB and consists of the following subsections:
9-22
GO_SW
CHECK BATTERY
TP88 L12
R74
1K
TEST
SW1
R80
221
S2
Figure 9-23: User Controls Circuit
VCC
R102
15K
GO_BTN
R79
3.32K
L11
C123
100PF
VCC
R71
150K
BAT_BTN
C126
C64
100PF
.01UF
GO_SW
TP72
GO BTN
TEST
VCON
To U21
pin 4
R78
150K
C66
.01UF
TP71
TEST
TP81
TEST
TO: N-20
AUX PCB
JP8
1
2