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Technical Manual
BACKLIGHT CIRCUIT
The backlight switch, S3, when pressed, provides a low input to the microprocessor U4-
38. The microprocessor output U4-52 is then set high turning on the backlight inverter
circuit contained in H1 and which drives transformer T1. The output of T1 is an AC
waveform of about 150 volts P-P and 700 Hertz frequency which drives the
electroluminescent backlight panel DS2.
Wall Mount Security Switch (Model 9000 Only)
The reed switch, S7, is normally open when the thermometer is away from the security
wall mount. When the thermometer is placed into the security wall mount, a magnet
located inside the wall mount closes S7 which is sensed by the microprocessor (U4-35)
which then shuts down all thermometer circuits.
THEORY OF OPERATION (MECHANICAL)
The probe cover dispenser initially contains 100 covers and is inserted into the bottom
of the thermometer handle. As the slide handle is pushed forward, the rack gear on the
slide rotates the idler and ring gear assembly causing the pawl to drop below the top
most probe cover in the dispenser and push the cover upward until the cover is fully
seated in the window. The dispensing of the cover occurs within about the first half of
the slide handle travel and the pawl is then stopped by a ledge on the window.
As the slide is pushed forward, the O ring/clutch assembly located between the ring
gear and the bell crank slips to allow the slide and gears to continue moving without the
pawl. As the probe tip is pushed through the probe cover and window, the plastic film
of the probe cover stretches over the probe tip. When the slide is almost fully
extended, the perforation connecting the stretched cover with the next cover in the
dispenser is broken and the fully stretched cover and tip extend out of the thermometer
case for temperature measurement.
The stretched probe cover is latched onto the probe tip at this point by the paper and
plastic base of the cover which has been forced over the larger retainer of the probe tip.
As the slide is retracted, the used probe cover is peeled off as it passes through the
window and then drops off the end of the retracting probe.
During the retracting of the slide, the gear and pawl motion is reversed causing the pawl
to retract until it hits the pawl stop. This causes the pawl to pull away from the
dispenser. The O ring/clutch then slips to allow the slide and gears to finish traveling
without moving the pawl. The lifting of the pawl against its stop allows removal of the
dispenser when the slide is in its retracted position. If the pawl does not lift away from
the dispenser, probe covers will remain inside the instrument as the dispenser is
removed.
The ratchet and ratchet spring ensure that complete slide strokes are performed when
loading and unloading probe covers. The ratchet mechanism prevents the direction of
slide travel from being changed until the slide is at one end of its travel.
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