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3700 Portable Samplers
Section 6 Servicing
6-10
and other components provide the LCD with a temperature com-
pensated driver voltage. By providing temperature compen-
sation, the display quality is optimized over a wide temperature
range. IC U9 and IC U18 are I/O devices that allow the micropro-
cessor to read the keypad, sound the beeper, advance the dis-
tributor, measure pump counts, and so on.
A 24-position keypad, connected at P5, directs the micropro-
cessor through the program. Many discrete components along
with IC U11 are used to decode pressed keys. All user-originated
instructions enter the system through the keypad.
The 3700 Sampler is supplied with a liquid detection system that
gives it the ability to accurately deliver specified sample
volumes. The liquid detection system consists of a piezoelectric
sensor and filtering circuitry. The sensor, mounted on the
exterior of the control box, produces a signal proportional to an
induced strain on the pump tubing. This signal is routed through
P8 to IC U15 and its associated circuitry where it is massaged
into a YES or NO indication of liquid presence. IC U15, the main
c i r c u i t c o m p o n e n t o f t h e d e t e c t i o n s y s t e m , c o n t a i n s a
switched-capacitor, low-pass filter and two Op Amps. A collection
of transistors, resistors, and diodes are used to control the dis-
tributor motor. The motor-control circuitry connects to the dis-
tributor through P6 and can drive the distributor in either
direction. As the distributor moves, its position is monitored by
an optical device mounted on the distributor assembly. IC U20
supplies the necessary current for the optical device.
An RS-232 serial communications port is connected to the system
at P9. This port is used to output sampler status, program set-
tings, and sampling results to a printer or an interrogator. IC
U21, IC U19, and a handful of discrete components are used to
transmit serial information, receive serial information, and sense
the presence of the externally connected device.
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