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CONTROL

•Control Systems• Open and Closed Loop (with Feedback)

•Analog and Digital Systems•Digital Ics• Logic Gates• 555 Timer• Op-Amp

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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 SystemAn 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 signalsDigital 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 = ON0 = OFF

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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.

Digital ICs

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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).

• 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.

The inside

Gest where does the name of the IC come from…

Standard 555 package on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8), includes:

1.over 20 transistors,2.2 diodes and3.15 resistors

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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.

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, pulse-width 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 flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.

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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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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.

THE 555 TIMER - MONOSTABLE CIRCUIT

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 ASAN 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, non-inverting 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’ .