Human Humalyzer 2000 - Photometer; Temperature Contro; Flowcell and Vacuum Pump; Display, Keypad, and Thermal Printer

4
Rev.
B
Principles
of
Operation
Photometer
The light from a tungsten-halogen lamp is passed horizontally though the tube or flowcell and
the sample it contains. The sealed box on the opposite side of the read well contains a rotating
filter wheel and photodiode. The filter wheel contains eight interference filters of various
wavelengths, and is speed controlled to approximately 3 rps under software control. As each
filter passes in front of the photodetector, an infrared optical switch triggers a sampling of the
peak voltage produced by the photodetector. The voltage is then fed to a comparator which
compares the sampled voltage to the output of an exponential capacitor decay circuit. The pulse
at the output of the comparator enables a 16 bit counter in an 8254 programmable timer. The
photodetector output is proportional to the intensity of the light, whereas the width of the positive
phase of the comparator output is proportional to the absorbance. The resistance across the log
cap (RC decay) determines the base of the log (10 for absorbance) and is used to adjust the low-
end absorbance calibration (gain). Another potentiometer is used to adjust the high-end
absorbance (offset). =
Temperature control
The incubation block and read well are maintained at 37°C by means of thermistors (10k
(025*C): These thermistors are multiplexed at 0.5 second rate into the feedback path of .
oscillator U14A. The waveform at U14A is a sawtooth of approximately 1.2 Vpp amplitude
centered-at +2.5 V. Schmitt triggers U11 convert this to a square wave for current reversal and to
drive the, CLK2 input of U4. The microprocessor heats the system to maintain the frequency of
this signal at 2.08 kHz, the 37°C set point.
Power resistors located on the bottom of the incubation block are switched to 115VAC via an
opto-triac. A red LED located on the block PCB illuminates when heat is on. A similar system is
employed for the read well except that driver U10 applies 12VDC (raw) across heater resistors
located on the cell PCB.
“Flowcell and Vacuum Pump
An integral DC vacuum pump maintains a constant vacuum on an external waste bottle, by .. ]
means of a solid-state vacuum sensor circuit on the pump control PCB . A solenoid-operated
pinch valve is opened for a precise interval under microprocessor control, to apply this vacuum
to the flowcell. Sample (or air) is drawn through the flowcell and into the waste bottle. Sensor
wires built into the bottle cap are used in conjunction with a detector circuit on the pump control
PCB to indicate a full condition on the waste bottle.
“Display, Keypad, and Thermal Printer
A 24x 2:line character Liquid Crystal Display (LCD) is mounted inside the cover. Two 4 x 4
membrane switch keypads are sealed behind the front panel overlay. The LCD connects to the
interface PCB via a 14-pin DIP cable. The tails of the Keypad switch layers are fed through the
cover
and
are
connected
to
the
interface
PCB.
The
printer
assembly,
consisting
of
a
20-column
Humalyzer 2000 Service Manual