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Chapter 21
Semiconductor Input Devices
Section 21-1
Section 21-2
Section 21-3
Section 21-4
Section 21-5
Section 21-6
Section 21-7
Section 21-8
Thermistors
Photoconductive Cells
Photoconductive Diodes
Pressure Sensors
Flow Detection Sensors
Hall Effect Sensors
Proximity Sensors
Ultrasonic Sensors
Chapter 21 — Semiconductor Input Devices
Objectives
Section 21-1 Thermistors
•
•
Define thermistor and describe the classes of
thermistors.
Explain how to test thermistors.
Chapter 21 — Semiconductor Input Devices
A thermistor is a
temperature-sensitive
resistor whose
resistance changes
with a change in
temperature.
Chapter 21 — Semiconductor Input Devices
In the presence of fire,
the increase in
temperature lowers the
resistance of an NTC
thermistor, which
increases current and
activates an alarm.
Chapter 21 — Semiconductor Input Devices
The hot and cold
resistance of a
thermistor can be
checked with a DMM.
Chapter 21 — Semiconductor Input Devices
21-1 Checkpoint
1. What happens to the resistance of a PTC
thermistor when it is heated?
2. What happens to the resistance of an NTC
thermistor when it is heated?
3. What is the most commonly used thermistor type?
Chapter 21 — Semiconductor Input Devices
Objectives
Section 21-2
Photoconductive Cells
•
•
Describe photoconductive cells (photocells) and
give examples of how they are used.
Explain how to test photocells.
Chapter 21 — Semiconductor Input Devices
A photocell can be
used to determine if the
pilot light on a gas
furnace is ON or OFF.
Chapter 21 — Semiconductor Input Devices
A photocell can be
used to determine
when a streetlight
should turn on or off.
Chapter 21 — Semiconductor Input Devices
Humidity and contamination are the primary causes of
photocell failure.
Chapter 21 — Semiconductor Input Devices
21-2 Checkpoint
1. What happens to the resistance of a
photoconductive cell as light on it increases?
2. What happens to the current flowing through a
photoconductive cell as light on it increases?
Chapter 21 — Semiconductor Input Devices
Objectives
Section 21-3
Photoconductive Diodes
•
Define photoconductive diode (photodiodes) and
explain how it operates.
Chapter 21 — Semiconductor Input Devices
A photodiode is a diode
that is switched on and
off by a light.
Chapter 21 — Semiconductor Input Devices
Photodiodes are used
to position objects and
turn machine functions
on and off.
Chapter 21 — Semiconductor Input Devices
21-3 Checkpoint
1. What happens to the resistance of a photodiode
when light is shining on it?
2. What happens to the current flowing through a
photodiode when light is shining on it?
Chapter 21 — Semiconductor Input Devices
Objectives
Section 21-4
Pressure Sensors
•
•
Define and describe pressure sensors.
Explain how to test a pressure sensor.
Chapter 21 — Semiconductor Input Devices
A pressure sensor is a transducer that changes
resistance with a corresponding change in pressure.
Chapter 21 — Semiconductor Input Devices
Pressure sensors are
tested by checking the
voltage or current
output and then
comparing the value to
the manufacturer
specification sheets.
Chapter 21 — Semiconductor Input Devices
21-4 Checkpoint
1. If a pressure sensor that is rated to output 0 VDC
to 10 VDC with a specified pressure operating
range of 0 psi to 500 psi outputs 2 VDC, is it
working properly according to manufacturer
specifications?
2. If a pressure sensor that is rated to output 4 mA to
20 mA DC with a specified pressure operating
range of 0 psi to 500 psi outputs 2.5 mA, is the
pressure sensor working properly according to
manufacturer specifications?
Chapter 21 — Semiconductor Input Devices
Objectives
Section 21-5
Flow Detection Sensors
•
Define and describe flow detection sensors.
Chapter 21 — Semiconductor Input Devices
A solid-state flow
detection sensor
operates on the
principle of thermal
conductivity.
Chapter 21 — Semiconductor Input Devices
A flow detection sensor
can be used to monitor
product flow in a pipe.
Chapter 21 — Semiconductor Input Devices
A flow detection sensor
can be used to monitor
airflow in painting or
welding exhaust
system applications.
Chapter 21 — Semiconductor Input Devices
21-5 Checkpoint
1. In a thermal-type flow detection sensor, does the
sensor’s thermistor produce higher or lower
electrical signal when there is no flow?
2. Is a thermal-type flow detection sensor a fastacting or slow-acting change detection type?
Chapter 21 — Semiconductor Input Devices
Objectives
Section 21-6
Hall Effect Sensors
•
•
•
•
Define Hall effect sensor and explain the Hall
effect.
Explain how Hall effect sensors operate.
Explain how Hall effect sensors may be actuated.
Describe Hall effect sensor applications.
Chapter 21 — Semiconductor Input Devices
A Hall generator is a thin strip of semiconductor material
through which a constant control current is passed.
Chapter 21 — Semiconductor Input Devices
Hall effect sensors are
available in a variety of
packages for different
applications.
Chapter 21 — Semiconductor Input Devices
In head-on actuation, a
magnet is oriented
perpendicular to the
surface of the sensor
and is usually centered
over the point of
maximum sensitivity.
Chapter 21 — Semiconductor Input Devices
In slide-by actuation, a
magnet is moved
across the face of a
Hall effect sensor at a
constant distance
(gap).
Chapter 21 — Semiconductor Input Devices
Pendulum actuation is
a combination of the
head-on and slide-by
actuation methods.
Chapter 21 — Semiconductor Input Devices
In vane actuation, an
iron vane shunts or
redirects the magnetic
field in the air gap
away from the Hall
effect sensor.
Chapter 21 — Semiconductor Input Devices
A Hall effect sensor
may be used for
monitoring a remote
conveyor operation.
Chapter 21 — Semiconductor Input Devices
Each change in polarity
results in an output
from a Hall effect
sensor used in a shaft
speed sensor
application.
Chapter 21 — Semiconductor Input Devices
A Hall effect sensor
can be used to monitor
the level of liquid in a
tank.
Chapter 21 — Semiconductor Input Devices
A door-interlock system
can be designed using
a Hall effect sensor, a
magnetic card, and
associated electronic
circuitry.
Chapter 21 — Semiconductor Input Devices
Hall effect sensors are
used in beverage gun
applications because of
their small size, sealed
construction, and
reliability.
Chapter 21 — Semiconductor Input Devices
Length measurement
can be accomplished
by mounting a disk with
two notches on the
extension of a motor
drive shaft.
Chapter 21 — Semiconductor Input Devices
Hall effect sensors may
be installed in the base
of a machine to
indicate the level or
degree of tilt.
Chapter 21 — Semiconductor Input Devices
Hall effect sensors may
be used in a joystick
application.
Chapter 21 — Semiconductor Input Devices
21-6 Checkpoint
1. Is the output of a Hall effect sensor of the digital
(ON/OFF) type or analog (varying) type as a
magnet moves closer to the sensor?
2. Can a magnet actuate a Hall effect sensor by
moving in a sideways or straight manner?
Chapter 21 — Semiconductor Input Devices
Objectives
Section 21-7
Proximity Sensors
•
•
Define proximity sensor and explain how it
operates.
Explain the difference between an inductive
proximity sensor and a capacitive proximity
sensor.
Chapter 21 — Semiconductor Input Devices
Proximity sensors are available in an assortment of
sizes and shapes to meet as many application
requirements as possible.
Chapter 21 — Semiconductor Input Devices
Inductive proximity
sensors use a
magnetic field to detect
the presence of a
metallic target.
Chapter 21 — Semiconductor Input Devices
Capacitive proximity
sensors use a
capacitive field to
detect the presence of
a target.
Chapter 21 — Semiconductor Input Devices
Capacitive sensors
work based on the
dielectric constant of
the material to be
sensed.
Chapter 21 — Semiconductor Input Devices
21-7 Checkpoint
1. What type of proximity switch detects metallic
objects?
2. What type of proximity switch detects any object
that has a high dielectric constant?
Chapter 21 — Semiconductor Input Devices
Objectives
Section 21-8
Ultrasonic Sensors
•
•
Define ultrasonic sensor and explain how it
operates.
Explain the difference between a direct mode
ultrasonic sensor and a diffused mode ultrasonic
sensor.
Chapter 21 — Semiconductor Input Devices
Ultrasonic sensors detect objects by bouncing highfrequency sound waves off the objects.
Chapter 21 — Semiconductor Input Devices
An ultrasonic sensor
used in the diffused
mode can provide an
analog output that
varies linearly with the
target’s distance from
the sensor.
Chapter 21 — Semiconductor Input Devices
21-8 Checkpoint
1. What is the operating mode of an ultrasonic
sensor called when the emitter sound waves
travel in only one direction to the receiver?
2. What is the operating mode of an ultrasonic
sensor called when the emitter sound waves
travel in one direction to the detected object and
bounces back in the opposite direction to the
receiver?