Battery Voltage
The analog input voltage range of the CPU is 0-5 VDC. Because the battery voltage may be as high as 6.2 V,
R69 and R70 form a voltage divider to decrease the measured battery voltage to a usable level. The gain is 0.75;
thus, if the battery voltage was 6 V, then the voltage of BAT VOLT would be 6 x 0.75 which equals 4.5 V.
The software has the ability to determine when battery power is too low. If the software determines that the
battery voltage is too low to provide accurate information, the software generates an audible signal and
automatically switches the unit off. If an optional printer is installed, the battery voltage data are used to
compensate for battery voltage changes that can affect printout quality.
Battery Type
The unit can operate with either disposable or rechargeable batteries. Battery type input is digital; a high input
informs the CPU that rechargeable batteries are in use. If rechargeable batteries are used, the battery and the
VRECHARGE terminals are mechanically connected. This applies the battery voltage to VRECHARGE,
pulling BAT TYPE high. R100 and CR27 are a current-limiting resistor and a voltage-clamping diode that are
used to protect the input port from excessive battery voltage. If disposable batteries are used, VRECHARGE is
electrically isolated, which allows R101 to pull BAT TYPE input low.
The nominal voltages and voltage discharged curves are significantly different between rechargeable and
disposable batteries. In order for the CPU to predict how much "battery life" remains, the nominal voltage and
discharge curves must be known; the BAT TYPE signal provides that information.
Printer Control
Printer circuitry is divided into two subsections: the printer interface and the printer flex circuit. Printer interface
circuitry is present on both models, but is disabled by software in the N-20.
•
Printer interface circuit (auxiliary PCB) – This circuit detects the presence of the flex circuit, and supplies
power to the printer heads and paper advance motor. Electrical noise generated by the printer motor is
filtered. The circuitry is protected from excessive battery currents by a fuse.
•
Printer flex circuit (N-20P only) – The printer flex circuit is added when the printer is present. The printer
generates a timing signal that is read by the CPU and sent to the flex circuit. This circuit signals the CPU
that a printer is present by connecting one CPU input to ground. Power and power control signals from the
auxiliary PCB generate an output load for a resistor array; heat from this process produces a dot matrix
pattern on thermal paper.
User control is provided by momentary push buttons: ON (on/off), ADV (advance), and D/D (day/date). ON
enables or disables the printer, ADV controls the advance of printer paper, and D/D sets date, time, and other
clock parameters.
When a low battery voltage condition is present, the N-20P adjusts power to the printer's head; however, a weak
battery voltage condition causes the printer to shut off, thereby allowing the N-20P to continue to display oxygen
saturation and pulse rate readings until the batteries are exhausted. An ambient temperature sensor adjusts
printout quality to compensate for environmental conditions.
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