Theory of Operation
Chapter 3: Theory of Operation
Page 3 - 20 Isolette® Infant Incubator (Model C2000) Service Manual
as a function of humidity; the net range of capacitances is from
approximately 160 pF to 200 pF. The sensor connects to the amplifier
U7A/U8A, which is set up as a multi-vibrator. The sensor capacitance
charges up through R20 and R21 to a threshold voltage established by
R30 and R26.
When the capacitor voltage reaches the threshold, the capacitor
U7A/U8A goes high to 2.5 V as controlled by R19 and R28, and turns
on Q2. This discharges the humidity sensor through R20 until it reaches
the lower threshold established by R24, R26, and R30. The capacitor
voltage goes from approximately 0.2 V to 0.7 V. At this point, the
comparators output goes low, releasing the drive to Q2 and allowing the
humidity to start charging again. This produces a frequency output as a
function of capacitance, such as humidity.
The output signal, which is only 2.5 V peak, is then input into U7B/U8B
to condition the 5 V signal. Hysteresis is provided through the use of
R23, R29, and R32 to ensure stable frequency switching. The output
signal, JUMPUL, is then sent to the microcontroller for processing. A
typical frequency would be around 37 KHz.
Microcontroller
The microcontroller is a Priority-Interrupt Controller (PIC) 16C73, used
for signal-processing and for control of all signals on the sensor
module. The device has three external ports, configurable as inputs and
outputs. The microcontroller operates from a precise time-base of
crystal Y1, operating at 4 MHz. The instruction cycle time of the PIC is
¼ of that, namely at 1 MHz or 1 microsecond.
To ensure a clean power-up, U10 provides a fixed power-up reset to the
microcontroller. This integrated circuit also generates a rest in the event
of a brownout condition when the D+5 falls below a predetermined
threshold.
If the main controller determines that the sensor module requires reset
intervention, the reset line of the microcontroller, SMRES, is available to
the main controller.
The PIC device operates with an internal watchdog timer device that
asserts SMRES if the program execution operates outside normal
conditions.
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