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CONTROL
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•Control Systems
• Open and Closed Loop (with Feedback)
•Analog and Digital Systems
•Digital Ics
• Logic Gates
• 555 Timer
• Op-Amp
CONTROL SYSTEMS - OPEN SYSTEMS
A system that does not have feedback is an open system. An open system
normally works once and then stops. A good example is seen below. A digital
camera is used to take a photograph, it is transferred to the computer where
processing of the picture takes place and finally a printout is produced. This is a
closed system because there is no feedback and no attempt is made to improve
the picture.
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CONTROL SYSTEMS – WITH A FEEDBACK ( AN EXAMPLE )
An Automatic Sprinkler System
An automatic water sprinkler system has been ordered by a farmer. The system
must have sensors that detect dry weather and turn on water sprinklers to water
valuable crops.
The company manufacturing the system have decided that a starting point is to
think in terms of INPUT - PROCESS - OUTPUT and also include FEEDBACK. The
basic plan is set out below.
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This is the layout to the systems diagram for the automatic sprinkler system
The moisture sensor detects when the soil is dry. The darlington pair is a simple
electronic device that amplifies the signal sent by the sensors so that the
computer can read it.
When the sensor determines that the soil is moist/damp the signal to the
computer ends and the computer turns off the sprinkler. This is called FEED BACK.
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ANALOGUE AND DIGITAL SYSTEMS
Electrical signals are in two forms;
Analogue signals
Digital signals
Analogue signals: These are usually older electronic
gadgets (introduced before the mid 1990’s). A good
example of an analogue signal is the loud-speaker
of a stereo system. When the volume is turned up
the sound increases slowly and constantly.
Digital signals: Modern electronic products such
as computers and mobile phones depend on
digital signals. However, a good example of a
digital signal is Morse Code. The signal is sent as
a series of ‘on’ and ‘off’ pulses. Morse code was
introduced in 1837 by Samuel Morse, as a
method of communication.
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Both analogue and digital systems can be used as sensors. A thermistor is analogue as
resistance slowly changes, a micro-switch is digital, as it is ‘on’ or ‘off’.
Computers are digital devices and the various electronic parts communicate using 1’s
and 0’s.
1 = ON
0 = OFF
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Digital ICs
TTL (Transistor-Transistor Logic) uses the BJT technology
• Logic zero (0) or low (L): less than 0.8V
• Logic one (1) or high (H): greater than 2.0V
• The input voltage range 0.8V to 2.0V is a dead zone
where the input state is undefined.
• The digital output of a TTL device ranges between 0
and 0.5V for low and between 2.7V and 5V for high.
CMOS (Complementary Metal Oxide Semiconductor) uses the FET technology:
• The logic levels depend on the supply voltage.
• Very little power consumption, but susceptibility to
damage from static electricity.
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DIGITAL ELECTRONICS - LOGIC CIRCUITS
LOGIC circuits are normally composed of ‘gates’. A combination of gates make up a
circuit and some digital circuits can be extremely complex. It is the logic gates that
produce pulses of electrical current (1s and 0s).
The simplified AND gate shown
above has two inputs, switch A and
switch B. The bulb Q will only light if
both switches are closed. This will
allow current to flow through the
bulb, illuminating the filament.
The simplified OR gate shown above
has two inputs, switch A and switch B.
The bulb Q will light if either switch A
or B are closed. This will allow current
to flow through the bulb, illuminating
the filament.
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BASIC LOGIC TABLES
A range of logic gates exist
and they are represented as
symbols, each with its own
truth table (sometimes called
a logic table). Gates have
inputs and produce outputs
and these are in the form of
1s and 0s.
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ALTERNATIVE REPRESENTATIONS OF LOGIC GATES
A ‘1’ means that current is present. For instance, if current is present at an
output of a gate then this is represented as a ‘1’. Instead of placing a ‘1’ at
the output other terms can be applied - high, true, on or up - all mean that
current is present.
A ‘0’ means that current is not present. For instance, if current is not present
at an output of a gate then this is represented as a ‘0’. Instead of placing a'0’
at the output other terms can be applied - low, false, off or low - all mean
that current is not present.
Alternative ways of representing the AND gate are written below.
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EXAMPLE LOGIC CIRCUITS
Below is the logic circuit for a simple house alarm. The alarm protects the front
and back doors and six windows. Once the alarm is set if any of the doors or
windows are opened the alarm will sound. OR gates have been used. The
TIMER allows the house owner to enter the house by either the front or back
door and turn of the alarm system before the alarm sounds.
The inputs for each of the gates representing the doors and windows can be
connected to a vast range of sensors (eg. movement and magnetic sensors).
On the circuit below the input states of each of the sensors are ‘0’ (false, low,
off). This means that they have not detected an intruder. As a result the alarm
does not sound.
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EXAMPLE LOGIC CIRCUITS
The animation below shows what happens when the micro-switch has been
switched 'ON' as the guard is in the correct position. This means that the logic
states of both inputs are 1 (true, on, high, up), consequently the output logic state
is 1 (true, on, high, up) and the machine works.
Remember, for the AND gate to output 1 both inputs must be 1.
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THE 4081B LOGIC CIRCUIT
Logic gates are usually electronic circuits (based on an integrated circuit) and they
are used to make simple decisions. A good example of this type of circuit is based on
the 4081B integrated circuit (IC)
For example, a dog owner wants to build an
automatic animal feeder to work at night and
when his dog presses a switch (pressure pad).
This type of device would automatically feed
the dog when the owner is asleep. A diagram of
a simple prototype design is shown opposite.
The 4081B integrated circuit will detect when
the two switches are activated, one by the dog
and the other as darkness falls - the motor
allows food to be released from a tube. If only
one switch is activated, food will not be
released.
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The logic diagram is shown below. A micro-switch (pressure pad) is used as one input
device and a dark sensing circuit as the other. The AND gate has two inputs. If both
are activated - the dark sensor and the micro-switch - the logic state of the output
changes to high and the motor releases food to the hungry dog.
The diagram below shows that the micro-switch has not been pressed and that it
is daylight, consequently the motor is off.
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THE 4081B CIRCUIT DESIGN
A circuit based around the 4081B integrated circuit can be seen below. The
integrated circuit (IC) contains a number of AND gates, although for this sample
circuit only one of the AND gates has been used.
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DIGITAL LOGIC EXAMPLE
Sensor A is a temperature sensor which outputs false(0, low, off) when the
room temperature falls below a set level.
Sensor B is a light sensor and is attached to a lamp. The sensor outputs true (1,
high, on) when the lamp is switched on.
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DIGITAL LOGIC EXAMPLE
Below is a simple AND gate logic circuit designed for a dog. The dog’s owner is very
concerned that when he is at work he can gain entrance to the kennel he has made. The
kennel is situated outside. However, recently a cat has been entering the kennel and
eating the dogs food. The owner has fitted an electronic device that is activated when
the dog passes close to a light / dark sensor and presses a hidden pressure pad. Once
this has been completed successfully, a motor opens the kennel door.
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The prototype circuit made from modules such as a light/dark sensor and AND gate
module . It has be converted into a circuit diagram.
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THE BINARY NUMBER SYSTEM
Binary numbers are closely related to digital electronics. With digital electronics a
‘1’ means that current / electricity is present and a ‘0’ means it is not present.
The different parts of a computer communicate through pulses of current (1s and
0s).
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Decimal Number Systems
• dn  d3d2d1d0 = dn 10n+…+ d2 102+ d1 101+ d0 100
• Base: 0,1,2,3,4,5,6,7,8,9
• Example: 123
• Binary Number Systems
• (dn  d3d2d1d0)2 = dn 2n+…+ d2 22+ d1 21+ d0 20
• Base: 0 (OFF), 1 (ON)
• Example: 11012
• Bits are the digits of a binary number
• MSB (Most Significant Bit): the first bit
• LSB (Least Significant Bit): the last bit.
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INTEGRATED CIRCUITS
Integrated circuits (ICs) are very important components found in many circuits.
They are also called silicon chips or microchips. Basic 555 timer circuits ranging to
complex PIC Microcontroller circuits and computer processors (CPUs) are based on
the use of integrated circuits.
555 TIMER
PIC MICROCONTROLLER
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The packages of popular integrated circuits used in schools and colleges are sometimes
called a Dual In-Line Packages. They are also called either DIP packages or DIL packages
and two examples are shown opposite. The PICAXE 18 is a DIL package with 18 pins and
its smaller relation is the PICAXE 08 with 8 pins.
The pins of integrated circuits can be delicate and
trying to solder each pin to a PCB can be very
difficult. It is often a good idea to solder a cheap
chip holder to a PCB and then press the integrated
circuit package into it.
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THE 555 INTEGRATED CIRCUIT
This integrated circuit is used for timing. Many circuits are composed of timers
and the most common of them all is the 555 Integrated Circuit. The 555
integrated circuit (IC) is a chip that is used in many school projects and
commercially made items such as video recorders and timers. You must
understand the basic workings of this important IC.
The 555 has eight pins (legs) but the
function of two are very important.
These are pin two and three.
PIN 2. This pin is where the current /
voltage enters the chip and starts the
timing sequence or starts to count.
PIN 3. This is where the current comes
out after the timer has completed
counting.
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The inside
Standard 555
package on a silicon
chip installed in an
8-pin mini dual-inline package (DIP8), includes:
1. over 20
transistors,
2. 2 diodes and
3. 15 resistors
Gest where does the name of the IC come from…
WHAT THE 'PINS' OF THE 555 ACTUALLY DO
The pin (leg) that triggers the 555 IC is leg two. In other words leg two starts the
timing sequence once a voltage is applied to it and after the 555 timer has ended it’s
timing sequence a signal (output) is sent down leg three. In the circuit at the top of
this page, the signal down leg three starts the buzzer. The variable resistor VR1 can
be used to increase or decrease the timing cycle.
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3 operating modes
 Monostable : in this mode, the 555 functions as a "one-
shot". Applications include timers, missing pulse
detection, bouncefree switches, touch switches,
frequency divider, capacitance measurement, pulsewidth modulation (PWM) etc
 Astable: free running mode: the 555 can operate as an
oscillator. Uses include LED and lamp flashers, pulse
generation, logic clocks, tone generation, security alarms,
pulse position modulation, etc.
 Bistable: (Schmitt trigger) the 555 can operate as a flipflop, if the DIS pin is not connected and no capacitor is
used. Uses include bouncefree latched switches, etc.
THE 555 TIMER - ASTABLE CIRCUIT
When the 555 IC is used to produce an ASTABLE circuit - it will continually pulse
until power is removed. Astable circuits can be used to flash lights/LEDs on and off
or to turn a buzzer on and off repeatedly. They are also used in many more school
based circuits.
Look at the circuit drawn below. Pins 6 and 2 are connected and go to the negative
(0 volts). This is the easiest way of recognising that a 555 IC has been set up as
astable.
Astable means that the 555
can operate repeatedly, it
will switch on, then off, then
on, then off, continually. The
555 is sometimes called an
oscillator.
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555 ASTABLE EXAMPLES
The circuit continually resets itself
and the LED flashes on and off until
all power is removed. The circuit
diagram shows that pins 6 and 2 are
connected, this means that the circuit
resets and triggers itself.
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The bulb is turned on and off repeatedly. This cycle stops when the toggle switch
is turned off.
Increasing the value of the resistors and capacitor extends the time the bulb is
illuminated and the time it is off.
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The motor is turned on and off repeatedly. This cycle stops when the toggle switch
is turned off.
Increasing the value of the resistors and capacitor extends the time the motor is on
and the time it is off.
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THE 555 TIMER - MONOSTABLE CIRCUIT
Monostable means that once the circuit is switched on it will time once and then
stop. In order to start it again it must be switched on manually a second time.
When the 555 IC is used to produce an MONOSTABLE circuit - it will only pulse once.
Monostable circuits can be used to turn lights/LEDs on or off just once.
Look at the circuit drawn below. Pins 6 and 7 are connected and go to the positive
(+9 volts). This is the easiest way of recognizing that a 555 IC has been set up as
monostable.
When the switch is pressed current flows
into pin 2. Current then flows out of pin 3
switching the transistor. Current can now
flow from +9 volts to -0 volts and the LED
lights. In this monostable circuit when the
switch is pressed the LED only lights once.
The switch has to be pressed each time for
the LED to light. In this example the LED
stays on for approximately 8 seconds.
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555 MONOSTABLE EXAMPLES
A number of 555
monostable example
circuits are seen
below. The switch
must be pressed each
time to light the LED
as the timing cycle
only works once.
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The bulb only illuminates when the push
switch is pressed. It works once and
then stops. The switch must be pressed
again for the bulb to light.
Increasing the value of the resistors and
capacitor extends the time the bulb is
illuminated.
The motor spindle only turns when the
push switch is pressed. It works once and
then stops. The switch must be pressed
again for the spindle to rotate.
Increasing the value of the resistors and
capacitor extends the time the spindle
turns.
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THE 741 OPERATIONAL AMPLIFIER
41 Operational Amplifiers (also known as Op Amps) are used in a range of
circuits. They are generally used to amplify weak electrical current in a circuit.
Radios, stereo systems, headphones, TVs and many other electrical products
include an operational amplifier as a component in many of their circuits.
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THE OPERATIONAL AMPLIFIER USED AS
AN AMPLIFIER WITH SENSORS
Sometimes it is necessary to increase the current in a circuit. This is especially
important if a sensor is being used as an input. For example, a temperature
sensor may to used to detect fire and then to turn on a water sprinkler system
to put the fire out. Look at the example below.
When the rise in temperature (caused by the fire) is detected the sensor circuit
(including an operational amplifier) allows a small amount of current to flow
through it. However, the current is too small to activate the sprinkler system.
The current must be increased for this to happen. An operational amplifier is
used to increase the current (called GAIN). Then the sprinkler system is turned
on putting out the fire.
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The diagram below clearly shows how a small current (sometimes called a signal) is
amplified by the Operational Amplifier to produce a larger current (signal)
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THE 741 OPERATIONAL AMPLIFIER
The Operational Amplifier is probably the most versatile Integrated Circuit available. It is
very cheap especially keeping in mind the fact that it contains several hundred
components. The most common Op-Amp is the 741 and it is used in many circuits.
The OP AMP is a ‘Linear Amplifier’ with an amazing variety of uses. Its main purpose is
to amplify (increase) a weak signal - a little like a Darlington Pair.
The OP-AMP has two inputs, INVERTING ( - ) and NON-INVERTING (+), and one output at
pin 6.
The important pins are 2, 3
and 6 because these
represent inverting, noninverting and voltage out.
Notice the triangular
diagram that represents an
Op-Amp integrated circuit.
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THE 741 IS USED IN TWO WAYS
1. An inverting amplifier. Leg two is the input and the output is always reversed.
In an inverting amplifier the voltage enters the 741 chip through leg two and
comes out of the 741 chip at leg six. If the polarity is positive going into the chip,
it negative by the time it comes out through leg six. The polarity has been
‘inverted’.
2. A non-inverting amplifier. Leg three is the input and the output is not
reversed.
In a non-inverting amplifier the voltage enters the 741 chip through leg three
and leaves the 741 chip through leg six. This time if it is positive going into the
741 then it is still positive coming out. Polarity remains the same.
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1. An inverting amplifier - Leg two is the input and the output is always reversed or
inverted.
2. A Non-inverting amplifier - Leg three is the input and the output is not reversed.
INVERTING AMPLIFIER
GAIN (AV) = -R2 / R1
NON-INVERTING AMPLIFIER
GAIN (AV) = 1+(R2 / R1)
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EXAMPLE CIRCUIT INVERTING AMPLIFIER
GAIN (AV) = -R2 / R1
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OP-AMPS AS COMPARATORS
741 is also used as a comparator. The difference between the two is small but
significant. Even if used as a comparator the 741 still detects weak signals so that
they can be recognized more easily.
A ‘comparator’ is an circuit that compares two input voltages. One voltage is called
the reference voltage (Vref) and the other is called the input voltage (Vin).
When Vin becomes below or above Vref the output changes polarity (‘+’ becomes ‘–’
or vice versa).
Positive is sometimes called HIGH.
Negative is sometimes called LOW.
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OP-AMPS AS COMPARATORS-TEMPERATURE SENSOR
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EXAMPLE CIRCUIT - LIGHT ACTIVATED ALERTER
The buzzer emits a tone when light falls on the light dependent resistor. Resistor 2
controls the sensitivity of the circuit.
The 741 is working as a comparator and the piezo buzzer sounds when the output
from the 741 goes ‘high’ .
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