basic electronics handout

J u n i o r C e r t i f i c a t e

Technology

Energy and Electricity

Contents

Work, Energy and Power...............................................................................1

Electric Circuits .............................................................................................4

Resistors.........................................................................................................15

Components ...................................................................................................23

Switches .........................................................................................................27

Transistor and Voltage Divider Circuit..........................................................38

Relay ..............................................................................................................45

555 Integrated Circuit ....................................................................................52

Logic Gates ....................................................................................................57

Project Circuits...............................................................................................64

Energy & Electricity JC Technology

Work, Energy and Power

In this lesson you will learn about:

• Work, Energy and Power

• Different types of energy

• Sources of energy

When anything happens movement is involved. To cause movement a force must be applied. So when a force is applied and movement occurs it is said that work is done.

Work = Force x distance moved

Where force, is measured in newtons and the distance in meters. The unit of measurement for work is the Joule (Nm).

However no work can be done without energy. Energy is the ability to do work .

Every time we do anything we use energy. When we walk, swim, mow the lawn energy is being used. Appliances and devices also need energy to work. The lawnmower needs energy to cut the grass. A car needs energy to allow us drive to our destinations. A cooker needs energy to cook all sorts of delicious foods. An IPod needs energy so we can hear our favourite music.

Where does all this energy come from? The lawn mower and car get their energy from the fuel they use. People get their energy from the food they eat. A cooker gets it energy from the gas or electricity it uses. An IPod gets its energy from rechargeable batteries.

There are different types of energy:

• Electrical

• Mechanical

• Chemical

• Light

• Heat

• Sound

Electrical energy is the energy given up to become heat, light etc. as current flows around a conductor.

Figure 1

There are two types of mechanical energy: Potential and Kinetic. Potential energy is energy that is stored in a body and waiting to do something. Kinetic energy is the energy a body has due to its movement.

Light energy is energy that travels in the form of waves at the speed of light (approx.

300,000 km/sec). Light allows us to see colours and shapes when light rays are reflected into our eyes. The biggest source of light energy is the Sun. Candles and bulbs also give off light energy.

Heat (thermal) energy is the energy of a body that increases with temperature. When a kettle is filled there is very little movement of the water molecules. As the water heats

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JC Technology Energy & Electricity up the molecules become much more active and their movement can be seen in the boiling water.

Chemical energy is the energy a body has because of what it is made from. Chemical energy is released when a chemical reaction occurs.

Fuels, food and batteries are all sources of chemical energy. When a battery is connected to a circuit a chemical reactions takes place that forces current to flow through a circuit.

Sound energy is energy that moves through the air by means of vibrating air molecules. When a mobile phone rings it causes air molecules beside it to vibrate. These vibrating molecules vibrate the

Figure 2 molecules beside them and so on until they hit the ear drum.

Principal of Conservation of Energy

Energy cannot be created or destroyed but can only be transferred from one form to another.

When energy is used it is converted into other forms of energy. Some examples are given below.

Device From To

Car

Table 1

Chemical Kinetic, heat, sound

Phones Sound electrical

Power

Is the rate at which work is done. The unit of measurement for power is the Watt.

Power =

Work done

Time taken

Where work done is measured in joules and time is measured in seconds.

Sources of Energy

Energy sources are divided into renewable and non-renewable. Non-renewable energy sources are source that will run out. Examples include oil, peat, coal, and gas.

Collectively they are referred to as fossil fuels.

Renewable energy sources will not run out. Examples include: solar, wind, wave, tidal, hydro and biomass.

2 © T4 Galway Education Centre


Hydro Energy

Flowing water is used to rotate a turbine which causes a generator to produce hydroelectricity.

Figure 3

Biomass Energy

Plant and animal waste is used to produce fuels such as methanol, natural gas, and oil.

Rubbish, animal manure, woodchips, seaweed, corn stalks and other waste can also be used.

Wind Energy

A coastal area where the wind is strongest is the best place for wind generators. To be effective an average wind speed of around 25 km/h is required.

Figure 4

Solar Energy

Is energy from the sun. Solar panels can be used to provide heat and photovoltaic cells can be used to generate electricity directly from the sun.

Figure 5

Tidal Energy

As the tide goes in and out the changes in water levels is used to turn a turbine which is connected to a generator.

Exercise 1: Answer the following questions.

1.

Explain the terms Work, Energy and Power in your own words.

2.

Calculate the work done when a 10N force moves a distance of 30m.

3.

List three types of energy and give two examples for each type.

4.

Why is renewable energy important?

5.

Write a short report on one source of renewable energy.


JC Technology Energy & Electricity

Electric Circuits


• Electric Current, Voltage and Resistance

• Series and Parallel Circuits

• Ohm’s Law

Since its discovery electricity has become the most widely used form of energy. It is used by millions of people every day to operate a wide range of appliances and devices: televisions, washing machines, computers, MP3 players, digital cameras and iPods to name a few. It is also used to provide essential services such as light and heating in our homes, schools and work places.

Before any appliance or device can be used it must be connected to a power source and switched on making it part of an electric circuit. Electricity flows around the circuit and operates the appliance or device. Electricity is the flow of electric current around a circuit. The two main sources of electric current are the mains supply and batteries.

Figure 6: Sources of Electric Current

Electric current is a very useful invention that would be difficult to live without today.

But what is electric current? To answer this question we need a little knowledge of atoms and electrons.

Everything in the universe is made up of atoms. The centre of the atom is called the nucleus and it contains protons and neutrons. Electrons orbit the nucleus on shells.

Nucleus

Neutron

Proton (positive)

Shells

Figure 7: Structure of Aluminium Atom

Electrons (negative)



Protons and electrons carry equal but opposite charges. Protons have a positive (+ve) charge and electrons have a negative (-ve) charge. Neutrons have no charge. Normally there are an equal number of electrons and protons so the atom is electrically neutral.

A 3D representation of the Aluminium atom is shown above in figure 2.

Electrons are held to the atom by the strong force of attraction to the protons. This force of attraction is greatest on the shells closest to the nucleus but decreases as the distance between the shells and the nucleus increases.

In some materials the electrons on the outer shell do not remain attached to the atom. The copper atom on the left is an example of this. Copper has

29 electrons orbiting the nucleus arranged on the shells as shown.

Figure 8: Copper Atom

Materials that allow electrons to move freely between atoms are called conductors. The electrons that move are referred to as free electrons. Metals are good conductors of electric current due to their atomic structure. The weak attraction between electrons on the outer shell and their positive nucleus allows these electrons to break away and move randomly between atoms figure 4.

Figure 9: Electron Movement

When a power supply such as a battery is connected to a conductor all the ‘free’ electrons move in the same direction as shown in figure 5. They move from the negative terminal of the battery to the positive terminal of the battery.

This movement of electrons through a conductor is called

Electric Current .

Figure 10: Current Flow



Electric current is represented by the letter I . The unit of measurement is the Ampere often abbreviated to Amp and its symbol is A . Small amounts of electric current are measured in milliamps mA . A milliamp is one thousandth of an amp.

1mA = 0.001A

Figure 10 shows the current flowing from the negative terminal of the battery to the positive terminal of the battery. This is what really happens but before electrons were discovered it was thought that electric current was the movement of positive particles through a conductor. That is why electric current is always shown flowing from the positive to the negative terminals of a battery in circuit diagrams. This is referred to as conventional current

.

As copper is a very good conductor of electric current it is used to connect electrical components such as bulbs, motors, switches etc, together to form an electric circuit.

An electric circuit forms a complete route or path from the positive to the negative terminal of the battery for the current to flow around. To save time when drawing circuits symbols are used for electrical components as shown below in figure 11.

When the switch is off

Figure 11 the circuit is broken.

No electric current can flow around the circuit and the bulb is off.

This is called an open circuit.

When the switch is on the circuit is complete and electric current flows around the circuit

Figure 12 lighting the bulb. This is called a closed circuit.

An Ammeter is used to measure the amount of electric current in a circuit. The ammeter is placed into the circuit (in series) and must be connected the correct way.

Figure 13

The current in this circuit measures 89.92mA.



Voltage

The force that makes electrons flow around a circuit is called the Electromotive Force

(EMF). It is provided by the battery. Its unit of measurement is the Volt. For this reason EMF is more commonly referred to as voltage. Increasing the voltage in a circuit will increase the current.

Figure 14

The symbol for voltage is V . The unit of measurement for voltage is the volt, symbol

V. Large voltages are measured in kilovolts, kV and small voltages are measured in millivolts, mV

.

The difference in voltage between any two points in a circuit is call the Potential Difference (p.d.) and is also measured in volts.

A voltmeter is used to measure voltage. To measure the voltage drop across a component the voltmeter is connected in parallel as shown below.

The voltmeter in the simulation software is polarised therefore it must be connected the correct way.

Figure 15

When using a multimeter set to read volts it can be connected any way round.

Resistance

All conductors and electrical components oppose the flow of current to some extent.

The moving electrons bump against each other and atoms causing them to slow down sometimes. This opposition to current flow is called resistance. Resistance reduces the amount of current in an electric circuit.

Circuits A and B have the same applied voltage, 9V. Bulbs 1-3 are identical.

Adding a second bulb to circuit B has reduced the current in the circuit.

A B

Adding more components to a circuit increases the resistance and decreases the current.

Figure 16



Resistance is represented by the letter R . The unit of measurement for resistance is the ohm symbol Ω (omega). Large units of resistance are measured in kilohms k Ω or megohm M Ω .

1 k Ω = 1000 Ω

1 M Ω = 1,000,000 Ω

Ohm’s Law

The circuits above show that increasing the voltage increases the current flowing in a circuit but increasing the resistance reduces the current in a circuit. This is Ohm’s

Law which states that:

‘The electric current in a conductor is directly proportional to the applied voltage and inversely proportional to the resistance at constant temperature’

Ohm’s Law triangle is used to calculate the voltage, current and resistance in a circuit.

V

=

IxR

V

I R

Ohm’s Law Triangle

I

=

V

R

R

=

V

I

Figure 17

Example:

1. Calculate the current in a circuit with an applied voltage of 9V and a resistance of 100 Ω .

2. A circuit has a current of 10mA flowing through it and a resistance of 300 Ω . Calculate the voltage.

I

I

=

V

=

R

9

100

= 0 .

09

A

V

V

V

=

IxR

= 0 .

01 x

300

= 3 V



Exercise 2 : Use Ohm’s Law to fill in the blanks in the table below.

Voltage (V) Current Resistance ( Ω )

12 2

2 3

9 0.3

12 4

5 8

6 100

Series Circuit

In a series circuit components are connected one after the other. There is only one path for the current to flow around. When the switch is opened both lights go off. In a series circuit all components are on or off together. If a component is damaged and cannot conduct electric current, this breaks the circuit and it does not work.

Figure 18: Series Circuit

The current in a series circuit is the same at all points in the circuit.

The current of 5.54mA is the same on all three ammeters.

Figure 19

The voltage in a series circuit is shared by each component in the circuit. How it is shared depends on the resistance of the components. In Figure 15(a) the two bulbs are identical therefore they have the same resistance. The voltage is divided equally.

When the components have different resistances the voltage is not shared equally figure 15(b) and (c). The voltage across each component in a series circuit adds up to equal the applied (battery) voltage.



(a)

(b)

(a) 4.50V + 4.50V = 9V

(b) 3.07 + 5.93V = 9V

(c) 3.87V + 3.51V + 1.61V = 9V

(c)

Figure 20

The voltage across any components in a series circuit can be calculated if its resistance is known.

Figure 21

V1

V2

=

=

R1

R1

R1

+ R2

R2

+ R2

x Applied Voltage

x Applied Voltage

V1 =

270

270 + 330

x 6

V1 =

270

600

V1 =

270

100

V1 = 2.7V

x 6

V2

V2

V2

V2

=

=

=

=

330

600

330

100

3.3

330

270

V

+

x

330

6

x 6



Exercise 3 : Using 6V 60mA MES bulbs, bulb holders and a power pack, build the series circuit below and answer the questions.

Connect to power pack

Questions:

1.

Unscrew each bulb in turn and replace into the circuit. What happens?

2.

Explain your answer to question 1.

Measure the voltage across each bulb and record the results. Add the voltages and record the result.

3.

What do the results of question 4 tell you about this type of circuit?

4.

Use an ammeter to measure the current flowing in the circuit. Record the answer.

5.

Remove BL3 from the circuit and measure the current again. Record the answer.

6.

Why is the current measured in question 6 greater than the current measured in question 5?

Exercise 4 : Copy each circuit diagram and calculate the voltage across each component in the circuits below. For each circuit check the voltages add to the applied voltage.



Parallel Circuit

In a parallel circuit the components are connected side by side. There is more than one path for the current to flow around.

This parallel circuit has two paths for the current to flow around.

Path 1:

Path 2:

Figure 22

In a parallel circuit the components can be on and off at the same or different times.

Figure 23

The current in a parallel circuit is shared between the different paths. The amount of current in each path depends on the total resistance in the path. The current in each path adds to the total current in the circuit.

The 18mA of current is divided equally between both circuits as R1 = R 2 .

Figure 24

A2 + A3 = A1



Figure 26

Figure 25

R1 and R2 are different. The 10.80mA of current is divided in the ratio of the resistance in each path.

Ohm’s law can be used to calculate the current in each path (chapter 3).

The voltage across each path in a parallel circuit is equal to the applied voltage. The number of components in a path or the size of the resistance in a path makes no difference.



Exercise 5: Using 6V 60mA MES bulbs, bulb holders and a power pack, build the parallel circuit below and answer the questions.

Connect to power pack

1.

Unscrew BL1 from the circuit. What happens?

2.

Replace BL1 and unscrew BL2 from the circuit. What happens?

3.

Replace BL2 and unscrew BL3 from the circuit. What Happens?

4.

What do the results of questions 1, 2 and 3 tell you about the parallel circuit?

5.

Measure the voltage across each bulb.

6.

What do the results of question 5 tell you about the voltage in a parallel circuit?

7.

Use an ammeter to measure the current at the points shown below in the circuit and record the results.

8.

What do the results in question 7 tell you about current flowing in a parallel circuit?



Resistors


• Fixed resistors and their values

• Resistors in series and parallel

• Variable resistors

• Light Dependent Resistors and Thermistors

While all components have some resistance to the flow of current, resistors are electrical components designed to specifically reduce the flow of current in a circuit.

Resistors are also used to:

• protect some components from too much current (pg ??)

• create a potential divider circuit (pg??)

• create a time delay with capacitors (pg??)

The most common type of resistor is the fixed resistor.

Coloured bands printed onto the surface of the resistor are used to show its resistance. The first three colours give the resistance of the resistor (known as the nominal value) and the fourth colour shows the tolerance (accuracy) of the resistor, figure 21.

Each colour represents a number as shown in table 2 below. When calculating the resistance the coloured bands are read from left to right with the tolerance band on the right hand side .

Resistor Colour Code

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Grey 8

White 9

Table 2

Nominal Value

Figure 27

Tolerance



To calculate a resistor’s value:

Band 1 = brown = 1

Band 2 = brown = 1

Band 3 = red (2) = 00

Value of resistor = 1100 Ω

= 1.1k Ω

More Examples:

Resistor

Figure 28

Band

1

Band

2

Band

3

Value

Red, yellow, brown

1000000

Brown, black, green

56 Ω

Green, blue, black

82000

Silver, red, orange

Exercise 6: Copy the table below and calculate the values of each resistor.

Ω

Band 1 Band 2 Band 3 Resistance

Blue Grey Brown

Green Blue Orange

Red Red Red

Brown Black Yellow

Yellow Violet Black

Orange Orange Green

Brown Red Orange

Ω



To find the colour code for a resistor of known value for example 82k Ω .

First write the resistors value in Ω .

82k Ω = 82000 Ω

Digit 1 = 8 = grey

Digit 2 = 2 = red

Digit 3 = 3 zeros = orange

Colour code = grey, red, orange

Figure 29

Exercise 7: Copy the table below and list the colour code for each resistor.

Resistor Value Band 1 Band 2 Band 3

150 Ω

33k Ω

1.5k Ω

18 Ω

10k Ω

1M Ω

1.2M Ω

Tolerance

Resistors are not one hundred percent accurate. The tolerance band indicates the percentage accuracy of the resistor. The colour of the fourth band gives the percentage accuracy shown in table 3.

Tolerance Colour Code

Table 3

No band ± 20%



Example:

A 330 Ω resistor has a gold tolerance band. The operating value of the resistor will be anywhere between 330 Ω ± 5%. The maximum value will be 330 + 5% and the minimum value will be 330 – 5%.

330 x 5% = 16.5

Maximum value of resistor = 330 + 16.5 = 346.6 Ω

Minimum value of resistor = 330 – 16.5 = 313.5 Ω

Exercise 8: Copy the table below and calculate the maximum and minimum values for the resistors listed.

Resistor, Tolerance band Maximum Value Minimum Value

720 Ω , silver band

39k Ω , gold band

36 Ω , silver band

470k Ω , gold band

Table 5

Preferred Values

As resistors operate within a given tolerance they are not manufactured in numerical sequence for example 1 Ω , 2 Ω , 3 Ω , 4 Ω , 5 Ω , etc. Resistors are manufactured in

Preferred Values. The E24 Series and the E12 Series are the most common ranges manufactured. The E24 series has a tolerance of ± 5% and the E24 series has a tolerance of ±10%. The minimum values manufactured in each range are shown in the tables below. Each minimum value is also manufactured in multiples of 10. For example taking 1.1

Ω the multiples are: 1.1

Ω , 11 Ω , 110 Ω , 1.1k

Ω , 11k Ω , 110k Ω .

E12 Series

± 10%

Table 4

1.0 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2

E24 Series

± 5%

1.0 1.1 1.2 1.3 1.5 1.6 1.8 2.0 2.2 2.4 2.7 3.0

3.3 3.6 3.9 4.3 4.7 5.1 5.6 6.2 6.8 7.2 8.2 9.1


Energy & Electricity

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JC Technology

Resistors in Series

When two or more resistors are connected in series the total resistance in the circuit is greater than their individual resistances. The total resistance is calculated by:

R

Total

=

R

1 +

R

2 +

R

3 + ..........

.

R

Total

=

=

3 k

Ω

5k Ω

+ 2 k

Ω

Figure 30

When using Ohm’s Law, to calculate the current or voltage of a series circuit, the total resistance must be used.

Example: Calculate the voltage required to produce a current of 0.5A in the circuit below.

0.5A

Step 1: Calculate the total resistance

3 Ω

Figure 31

4 Ω 5 Ω

R

Total

=

=

R

3

1

+

+

4

R

+

2

5

+

R 3

= 12 Ω

Step 2: Use Ohm’s Law to calculate the voltage

V

=

IxR

=

=

0.5

x 12

6V

Exercise 9: Calculate the total resistance of the following groups of resistors connected in series.

1.

220 Ω , 300 Ω and 16 Ω

2.

1k Ω and 2.4k Ω

3.

43k Ω , 750 Ω and 68 Ω

4.

390 Ω and 1.2M Ω

19

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20


Resistors in Parallel

When resistors are connected in parallel the total resistance of the circuit is less than their individual resistances. The total resistance is calculated by:

R

1 p

=

1

R

1

+

1

R3

..........

......

R

1 p

=

1

R2

+

1

300

+

1

150

+

1

150

R

1 p

=

1 + 2 +

300

2

1

=

5

R p

300

R p

=

300

5

Figure 32 R p

= 60 Ω

When there is only two resistors in parallel the following equation can be used instead:

R p

=

R1 x R2

R1 + R2

Exercise 10: Calculate the total resistance of the following groups of resistors connected in parallel.

1.

18 Ω and 300 Ω

2.

270 Ω and 680 Ω

3.

3k Ω and 5.6k Ω

4.

10k Ω , 1k Ω and 820 Ω (Hint: work out in pairs)

Variable Resistor

A variable resistor can change the resistance between two points in a circuit. Variable resistors have a circular track with connections at both ends. A wiper moved along the track, by rotating a spindle, increases or decreases the resistance in a circuit. Only two terminals are used: terminal B with terminal A or C.

If terminal A is used rotating the wiper clockwise increases the resistance as the current must flow through a longer length of track.

If terminal C is used the wiper is rotated anticlockwise to increase the resistance.



Terminals

Figure 33 wiper

Track

Variable Resistor

Preset resistors are variable resistors designed to be mounted directly onto a PCB. They are adjusted to the required resistance when the circuit is built. Once the resistance is set it is not changed again.

Figure 34

Spindle

Light Dependent Resistor (LDR)

The resistance of an LDR varies with the levels of light falling on its surface. In darkness the resistance is vey high, 1M Ω or greater. When light shines on the LDR the resistance can be as low as 100 Ω .

Symbol

Figure 35



Thermistor

The resistance of a thermistor increases or decreases with temperature change. The most common type of thermistor is the Negative Temperature Coefficient (NTC). The resistance of NTC thermistors decrease as the temperature increases making them suitable for use in fire alarms.

Figure 36

Symbol



Components

Diode

A diode allows electric current to flow in only one direction. The arrow in the symbol indicates the direction in which the current can flow. The positive lead is called the anode and the negative lead is called the cathode. The symbol is shown below.

Anode (+ve)

Figure 37

Cathode (-ve)

When the anode is connected to the positive terminal of the battery the diode will conduct current. The diode is said to be forward biased.

Figure 39

Figure 38

When the anode is connected to the negative terminal of the battery the diode will not conduct current. The diode is said to be reverse biased.

As diodes are small components the cathode is identified by a line printed on the body of the diode.

Figure 40

Diodes are often used in circuits to protect transistors and ICs against high voltages that could damage them.

The diode on the right is connected in reverse bias across the coil of the relay. It prevents the high voltage induced in the coils when the relay is switch off damaging the transistor.

Other devices that make use of diodes are solenoids and motors.

Figure 41



Light Emitting Diode (LED)

A LED is a diode that emits light. A very popular use for LEDs is as indicators to show a device or appliance is on. They are available in different colours. The shorter of the two leads is the cathode (negative lead). A second way to identify the cathode is the flat edge on the casing. The lead nearest to the flat edge is the cathode.

Anode (+ve) Cathode (-ve)

LED Symbol

Flat edge

Figure 42

Negative lead

(short leg)

Like diodes LEDs must be connected the correct way around to conduct current. The

LED will light when it is forward bias and will be off if connected in reverse bias.

Forward bias:

LED is on

Reverse bias:

LED is off

Figure 43

In a circuit LEDs should not have more than two volts dropped across it and a maximum current of 20mA flowing through it. To achieve these conditions a resistor is always connected in series with and LED. The value of the resistor can be calculated using Ohm’s Law.

For the above circuit:

Applied V= 9V

V across LED = 9-2 = 7V

I = 20mA = 0.02A

R =

V

I

=

7

0 .

02

= 350 Ω

Using preferred values a resistor of 390 Ω or 470 Ω should be used.



Capacitors

Capacitors are devices that store electric charge. The amount of charge stored is referred to as capacitance. The unit of measurement for capacitance is the Farad (F).

As most capacitors store less than one farad the smaller unit of microfards is used.

Microfarads μ F = 10 -6 F (I millionth of a farad). Other units include: Nanofarads nF =

10 -9 F and Picofarads pF = 10 -12 F. Capacitors can be polarised or non-polarised.

Non polarised capacitors generally have small values of less than 1 μ F. It does not matter which way it is connected in a circuit.

Symbol

Cerami c Polyester

Figure 44

Electrolytic capacitors are polarised. They must be connected the correct way around in a circuit. The short lead is negative and can also be identified by a band printed on the body of the capacitor. The working voltage and capacitance of the capacitor is also printed on the body of the capacitor. When using a capacitor is used in a circuit its working voltage should always be greater than the applied voltage.

+

Symbol Indicates negative lead capacitance

Negative lead

Figure 45

Operating voltage



Capacitors are used with resistors to create time delay circuits.

The capacitor forms a voltage divider with the variable resistor.

As BCX38B is a darlington pair transistor the switch on voltage will be 1.2 -1.4V.

The LED will light after a time delay.

Figure 46

When the SPDT switch is closed the capacitor begins to charge. As the capacitor is charging, the base voltage increases. When it reaches

1.2-14.V the transistor turns on allowing the LED to light.

When the SPDT switch is

Figure 47 opened the capacitor discharges.

Using a variable resistor allows the time delay to be changed. Capacitors are not very accurate devices. They have a large tolerance. The 1000 μ F capacitor and variable resistor set to 1M Ω gives a time delay of 3-4 minutes.

For more accurate time circuits the 555 IC is generally used.



Switches


• How different switches types work

• Wiring a motor and switch for forward and reverse motion

• Using limit switches with a motor

A wide variety of mechanical switches are available to control the flow of electric current in a circuit. They are generally used to turn a circuit on or off. Mechanical switches require a physical input to switch them to their on or off position.

Toggle

Rocker

Push

Microswitch

Lever type

Slide

Figure 48: Mechanical Switches

When a force is applied to a push switch (push-to-make or push-to-break) and microswitch the switch is activated and remains activated while the force is applied.

When the force is removed the switch returns to its original state. Rocker, slide and toggle switches remain in their switched position when the force is removed. A second force must be applied to return the switch to it off position.

Switches are classified by the number of poles and throws they have. The pole refers to the number of circuits a switch can open and close at the same time. The throw refers to the number of positions the pole can be moved to.


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There are four types of switches:

Type of Switch

Single Pole Single Throw (SPST)

Single Pole Double Throw (SPDT)

Double Pole Single Throw (DPST)

Symbol


Double Pole Double Throw (DPDT)

Table 6

Single Pole Single Throw (SPST) Switch

This type of switch can only turn one circuit on or off.

Figure 49

Push-to-Make (PTM) and Push-to-Break (PTB) switches are also SPST switches. In its normal state the PTM switch is normally open (NO) and the circuit is off. When the switched is pressed the circuit is on. Releasing the pressure turns the circuit off again.

Pressure/force

Figure 50



In its normal state the PTB switch is normally closed (NC) and the circuit is on. When the switched is pressed the circuit is off. Releasing the pressure turns the circuit on again.

Pressure/force

Figure 51

Single Pole Double Throw (SPDT) Switch

This type of switch can be used to control two different circuits. When one circuit is on the other is off. The two circuits can never be on or off at the same time.

Figure 52

Two single pole double throw switches can be used to control staircase lighting. SPST

1 is positioned at the bottom of the stairs, while SPST 2 is position at the top of the stairs.

Figure 53

2



Double Pole Single Throw (DPST) Switch

This type of switch consists of two SPST switches. They have separate electric circuits but are connected together mechanically (indicated by the chain line). It can be used to turn two circuits on or off at the same time. It is not possible to turn on or

4

Figure 54

Double Pole Double Throw (DPDT)

A DPDT switch consists of two SPDT switches connected together mechanically. It could be used to control four devices, turning two circuits on and off at the same time.

30

Figure 55

A much more common use for the DPDT switch is to control the forward and reverse motion of a motor as shown in the circuit diagram below.

Figure 56



In figure 57(a) below the flow of current rotates the motor in a clockwise direction. In

57(b) the flow of current into the motor has changed. The motor now rotates in an anticlockwise direction.

(a) (b)

Figure 57

The following steps show how to convert this circuit diagram to a working drawing.

1. Identify the terminals on the symbol and switch. Mark the terminals on the circuit diagram.

Terminal view of DPDT slide switch

T1a P1 T1b

P1

DPDT Symbol

T1a

T1b

P2

T2a

T2b

T2a P2 T2b

Figure 58



2. Using the circuit diagram draw in connections on a sketch of the switch terminals and motor as shown below:

1. Join terminal T1b to T2a and T2a to battery

2. Join T1a to battery

3. Join T2b to battery

4. Show +ve battery snap lead on P.1

5. Show –ve battery snap lead on P2

Figure 59

3. A neater circuit can be made by replacing the yellow wire with one connecting T1A to T2b. Both terminals are still connected to the same side of the battery.

Figure 60

4. Final working circuit diagram.

+V

Figure 61 0V



(b)

Figure 62

DPDT Switch Operation

When the slide switch is pushed to the right the metal contacts inside the switch connect P1 to T1b and P2 to T2b. Current flows through the circuit as shown figure

60(a) and rotates the motor in one direction. When the slide switch is pushed to the left the metal contacts inside the switch connect P1 to T1a and P2 to T2a. Current flows through the circuit as shown figure 60(b) and rotates the motor in the opposite direction.

(a)

Note:

For each of the two switch positions, the rotation of the motor in a clockwise or anticlockwise direction depends on which way round the motor is connected in the circuit.

When a DPDT switch is used the motor is always rotating in a clockwise or anticlockwise direction. The motor is never off unless the battery is disconnected. To turn off the motor a DPDTCO (Double Pole Double Throw Centre Off) switch is used. The difference in the mechanical operation of the switches is shown below. The wiring of the circuit does not change.

Switch Positions

DPDT

DPDTCO

Table 7



(a)

(c)

Figure 64

Limit Switches

Switches can be added to the circuit to stop the motor rotating in a forward or reverse direction. One option for inserting the limit switches is shown below.

Figure 63

The diagrams below show how the circuit works.

(a) motor is rotating in a clockwise direction

(b) when PTB1 is pressed the motor stops

(c) motor rotates anticlockwise when the DPDT switch is changed over

(d) when PTB2 is pressed the motor stops

F

(b)

(d)

F

Figure 65

Wiring diagram for motor control with limit switches.

+V

0V



Micro-switches

These switches are called micro-switches because a very small movement of the button will turn them on or off. In addition to this the force necessary to activate the switch is also very small compared to slide, toggle or push switches. For these reasons they are usually used as limit switches in motor control circuits. They are available in different styles to suit different situations: lever (standard and long), button and roller lever as shown below.

Lever (standard)

Figure 66

Push B utton Roller Lever

Microswitches have three terminals: COM (common), NO (normally open) and NC

(normally closed). The COM terminal acts as the pole. Microswitches are SPDT switches and how they function depends on the terminals used.

When the NC terminal is used the output device is normally on and turns off when the switch is activated.

When the NO terminal is used the output device is normally off and turns on when the switch is activated.

Figure 67



Reed Switch

Consists of two metallic overlapping strips (called reeds) enclosed in a tube. The contacts at the end of the reeds can be opened (NO) or closed (NC).

When a magnet is brought near the reed switch the reeds become magnetised and will attract or repel each other depending on the type of reed switch.

Figure 68

NO Reed Switch: The switch is normally open so the circuit is off. The magnet closes the contacts and turns the circuit on.

Magnet

Figure 69

NC Reed Switch: The switch is closed so the circuit is on. The magnet causes the reeds to spring apart breaking the circuit.

Magnet

Figure 70

Standard reed switches are SPST but they are available as SPDT also.

Tilt Switches

Tilt switch consists of two terminals and a liquid (usually mercury) which acts as a conductor enclosed in a container. In its horizontal position the liquid does not bridge the gap between the terminals. The switch is open. If the switch is tilted from its horizontal position the liquid will bridge the gap between the two terminals closing the switch. For safety reasons mercury has been replaced by ball bearings or a nontoxic material in the majority of tilt switches.

Tilt switches are designed to operate when they are tilted a certain angle from the horizontal. The switch shown below will close when tilted approximately 15° from the horizontal.

Figure 71



Float Switch

A float switch can be used to detect the level of any liquid within a container and turn on an alarm, indicator light or pump when a certain level is reached. They are designed for horizontal or vertical mounting. A magnet on the float activates a reed switch enclosed in the main body when it gets to a horizontal or vertical position caused by the rising liquid.

Horizontal Mounting

Magnet is inside float

Vertical Mounting

Magnet

Figure 72: Float Switches

Exercise 11:

1. Copy the circuit diagram

(a) Mark the current flow for each change over of the switch on the circuit diagram.

(b) Will the limit switches work correctly?

(c) Draw the wiring diagram for the circuit.

2. Will the limit switches work correctly in this circuit? Explain your answer.



Transistor and Voltage Divider Circuit


• How transistors act as a switch

• Voltage divider circuits

• Designing sensor circuits

• Darlington Pair Transistors

The transistor is a small device that can be used as an electronic switch instead of a mechanical switch in a circuit. The most common type of transistor is the NPN transistor. It has three terminals called the emitter, base and collector.

Symbol

Figure 73

The collector is connected to the positive terminal and the emitter to the negative terminal of the battery. Current flows into the transistor via the collector and exits through the emitter. This only happens when the transistor is switched on.

A transistor switches on when the voltage between the base and the emitter of the transistor is 0.6V.

When the base voltage reaches 0.6V a small current enters the transistor through the base turning on the transistor. A larger current then flows from the collector through to the emitter turning on the output device. When the voltage is less than 0.6 V the transistor is off.

Figure 74

To get the right voltage at the base a voltage divider (potential divider) circuit is used.

A voltage divider circuit connects two resistors in series so that the applied voltage is divided between the two resistors to produce the required base voltage. Using two fixed resistors only gives one output voltage.



Figure 75

Using a variable resistor allows the base voltage to be changed.

< 0.6V

> 0.6V

Figure 76

The parts of a complete voltage divider circuit are shown below. R1 is used to reduce the amount of current that flows into the base of the transistor as too high a current can damage it. R1 is called a protective resistor.

Output

LED/Bulb/motor etc.

Input

Voltage divider

Figure 77

Base voltage

Process

Transistor



In this circuit the voltage divider is created by R2 and VR1. The output is a LED.

When the base voltage is greater than

0.6V the transistor turns on allowing current to flow through the collector lighting the LED.

Figure 78

Figure 79

When the base voltage is less than 0.6V

The transistor is switched off. No current can flow through the collector and the LED is off.

Automatic Light Sensing Circuit

Using an LDR as part of the voltage divider circuit will allow the output to be controlled by changing light levels. Remember the resistance of an LDR varies from 400 Ω in light to about

10M Ω in darkness.

Figure 80



Figure 81

JC Technology

During the day the resistance of the LDR is very low compared to the variable resistor. The base voltage is greater than 0.6V which turn on the transistor and lights up the LED.

Daylight

LDR: low resistance

VR1: high resistance

→

→

low voltage

high voltage

As the light begins to fade the resistance of the LDR starts to increase and becomes very high compared to the variable resistor. This reduces the base voltage below 0.6V and turns off the LED.

Getting dark

LDR: high resistance → high voltage

VR1: low resistance → low voltage

Figure 82



Using a variable resistor allows the level of light at which the LED goes off to be set or changed as required.

With the variable resistor set at 5K above the LED is off while on the right increasing the resistance of the variable resistor keeps the LED on longer as it gets dark.

Figure 83

By switching the LDR and variable resistor the LED will turn on when it gets dark.

The low resistance of the LDR in daylight means the base voltage is too low to turn on the transistor. The LED is off. In darkness the high resistance increases the base voltage to more than 0.6V and LED lights up.

Figure 84



Automatic Temperature Sensing Circuit

Replacing the LDR with a thermistor will give a temperature sensing circuit. The resistance of NTC thermistors decreases as the temperature rises. The circuit below sound an alarm at low temperatures.

At high temperatures the resistance of the thermistor is low and the base voltage is less than 0.6V. The alarm is off.

Figure 85

At low temperatures the thermistor has a very high resistance making the base-emitter voltage greater than 0.6V turning on the alarm.

The temperature at which the alarm sounds can be set by changing the variable resistor.

Changing the position of the variable resistor and thermistor will sound an alarm at high temperatures.

Figure 86

Darlington Pair Transistor

When a transistor is used in a circuit a very small base current will allow a large collector current to flow through the transistor. The amount by which the small base current is amplified is called the GAIN of the transistor. The average gain of a single transistor is 100. The average collector current is 100mA. c b

Large collector/emitter

Small base current current

Figure 87 e



A darlington pair transistor consists of two transistors connected together. The emitter of the first transistor is connected to the base of the second transistor.

This combination gives a much higher gain than a single transistor. The gain can be as big as

10,000. This makes the transistor much more sensitive to changes in the base current.

If used in the light or temperature circuits above

Figure 88 the transistor will be much more sensitive to changes in light and temperature levels.

A very small change in light or temperature will give a change in the output.

As a darlington pair transistor will have a large output current it can often be more suitable for operating power output devices such as motors and solenoids.



Relay

In this lesson you will learn:

• How relays work

• How the relay acts as an interface device

• How to latch a relay

All relays consist of a coil of wire wound around a soft iron core, an armature and switch contacts. The number of switch contacts depends on the type of rely eg. SPST,

SPDT etc. Relays are activated by an electromagnet. The diagram below shows a

SPST relay. It has two switch contacts.

Pivot

Armature

Soft Iron core

Coil Contacts Switch Contacts

Un-magnetised

Figure 89: SPST Relay

Magnetised

When electric current flows through the coil of wire it magnetises the soft iron core

(electromagnet). When magnetised, the core attracts the tip of the armature making it rotate about the pivot and forcing the switch contacts together. The relay is said to be energised. When the current to the coil is turned off the core demagnetises, releases the armature and the contacts return to their original position.

The SPDT relay has three switch contacts. The centre contact is the pole. In its normal position the pole is in contact with the NC contact.

When the relay is energised the pole switches over and is in contact with the NO contact.

NC P NO

Figure 90: SPDT Relay



The DPDT Relay has six switch contacts that can be wired and used the same way as a DPDT switch.

Figure 91: DPDT Relay

With any relay the circuit providing current to the coil is completely separate to the switching circuit. The coil circuit is called the Primary Circuit and the switching circuit is called the Secondary Circuit. This allows one circuit to turn on or off another circuit with no electrical contact between them. The advantage of this is that a low voltage circuit can be used to control a circuit with a high voltage or vice versa.

The 6Vcircuit energises (turns on) the relay turning on the 24V water pump motor.

Relay de-energised

Relay energised

Figure 93



Most sensor circuits cannot provide enough current to drive components requiring a lot of power. The temperature sensing circuit below cannot provide enough current to turn the fan motor on.

Current too small to work motor

Figure 93

Replacing the motor with a relay will provide the current needed to work the motor as shown below.

The secondary circuit (motor) is taking its power from the primary circuit but can be given its own power supply as shown

Figure 94

When a relay de-energises, it produces ‘back EMf’ which could damage the transistor.

A diode placed across the relay’s coil prevents any damage to the transistor.

A DPDT relay can be used to provide automatic forward and reverse motion of a motor in response to a sensor circuit. The circuit in figure 76 could be used to open and close curtains. SW1 and SW2 are microswitches and BCX38B is a darlington pair transistor. The switch on voltage for the primary circuit is therefore 1.2V.



Curtains open

Figure 95

In daylight the LDR has a low resistance so the base-emitter voltage is less than 1.2V.

The relay is not energised. The 3V supply to the switch contacts enables the motor rotate to open the curtains. The motor stops when the opening curtains open SW1.

In darkness the LDR has a high resistance so the base-emitter voltage is greater than

1.2V. The relay is energised. The switch contacts switch over causing the motor to rotate in the opposite direction closing the curtains. The motor stops when the closing curtains open SW2.

Figure 96

Curtains closed



Latched Relay

The circuit below could be used as a door alarm. The NC reed switch is placed on the door frame and the activating magnet is positioned on the door. When the door is closed the magnet holds the reeds apart and no current flows in the circuit.

Figure 97

When the door is opened the magnet moves away from the reed switch and the contacts close, allowing current to flow through the relay coil and sounding the alarm.

Figure 98

Problem!

If the door is closed by the intruder the alarm will stop sounding!! Not a very effective alarm!



Latching the relay will keep it energised and the alarm sounding even if the door is closed by the intruder. The modification to the circuit is shown below.

Connect pole to relay coil

This time when the door is opened the current flows through the relay

Figure 99

Latch wire coil, to the pole, through the latch wire and back to –ve terminal of battery sounding the alarm. The current is not flowing through the reed switch.

Figure 100

When the door is closes and the reeds open the current continues to flow through the latch circuit energising the relay and the alarm does not turn off.

So how can the circuit be turned off??

One option is to add a master switch to the circuit. Opening the master switch will deenergise the relay and turn off the alarm.

Figure 101



The second option is to add a reset switch on the latch connection. When activated it will stop current flowing through the relay and turn off the alarm.

Figure 102

Exercise 12:

A master switch or reset switch will stop the alarm from sounding when an intruder has closed the door in the alarm circuit shown in figure 100.

Why would it not be a good idea to use both options in the circuit?

Redraw the circuit in Circuit wizard and find out!



555 Integrated Circuit


• The 555 Integrated Chip timer circuits

• Monostable and Astable mode of operation

• Calculating the time delay

The 555 IC is an 8-Pin D.I.L (Dual in Line) integrated circuit or ‘chip’. It must be connected into a circuit in the correct way. The pins are not numbered on the body of the chip. To identify Pin 1, position the chip so the notch is at the top. Pin 1 is then located to the left of the dot on the surface of the chip.

Notch

Dot

Pin 1 to left of dot

Figure 102

Each pin on the 555 timer has a specific function. Pin1 is always connected to the negative terminal of the battery (0V). Pin 3 is the output. This can be: LED, Bulb, buzzer, relay etc. Pin 8 is always connected to the positive terminal of the battery

(+V). The symbol for a 555 timer shows the pins in a different position. This is simply to keep the wiring diagram as clear as possible.

Figure 104

As Pins 4 and 8 are connected to the +V they are at the top. Pins 1 and 5 are connected to the 0V so are positioned at the bottom.



The 555 timer can be made to operate in two modes: Monostable and Astable.

Monostable Mode

In monostable mode there is only one stable state. When the timing sequence is triggered the state changes but will always return to its original state until triggered again. The circuit below shows the 555 in monostable mode.

SPST is the master switch.

PTM switch is the trigger.

Figure 105

The time delay depends on the value of resistor R3 and capacitor C1.

The output is and LED.

Figure 106

Before the trigger is pressed the voltage across the trigger is high while the voltage across C1 and the output is low

(0V).

The LED is off.

This is the stable state.



When the PTM switch is pressed the voltage across it drops to 0V and triggers the time delay. The LED turns on and the capacitor begins to charge.

Figure 107

When the voltage across C1 reaches 2 /

3

of the applied voltage the LED turns off.

The time delay can be calculated as follows: T = 1.1 x R3 x C1 when resistance is in ohms and capacitance is in farads.

For the circuit above: Time delay = 1.1 x 56000 Ω x 0.001F

= 61.6 seconds

If the LED and resistor are connected as shown below the LED is on under normal conditions. This is the stable state. When triggered the LED goes off for the time period.

Figure 108



Astable Mode

There is no stable state in astable mode. The output keeps switching on and off continuously. The circuit below shows the 555 timer in astable mode.

The SPST switch turns the circuit on and off.

When the SPST switch is

Figure 109 closed the voltage across C1 is low (0V) and the voltage at pin3 is high (+V). The

LED turns on.

The capacitor begins to charge. When it reaches 2/3 of the supply voltage the voltage at pin 3 goes low

(0V) turning off the LED.

The capacitor begins to discharge until it reaches 1/3 of the applied voltage.

At 1/3 of the applied voltage the timer is triggered again and the LED turns on.

Figure 110

This cycle of turning the LED on and off continues until the SPST switch is opened.

The rate at which the LED flashes can be altered by changing the size of the capacitor or resistor.



A second LED and resistor can be added as shown. This will create a set of flashing lights. When the red LED is on the yellow one is off and vice versa.

Figure 111

In both monostable and astable mode a variable resistor can be used to change the time delay.



Logic

In this lesson you will learn:

• About digital signals

• How different logic gates process signals

• How to design circuits

A digital signal has two states: ON or OFF. ON means the signal has a high voltage level often referred to as HIGH and is represented by the number 1. Off means the signal has a low voltage level often referred to as LOW and is represented by the number 0.

Digital circuits are made from logic gates. Logic gates have one or more inputs. The input signal or signals is processed by the gate to give an output. The output depends on the logic gate used.

AND Gate

The AND gate has two inputs. The symbol is shown below.

Figure 112

An AND gate works in a similar way to two switches in series. In the series circuit the bulb will only light when both switches are closed. If SW1 or SW2 or both switches are open the bulb will not light.

Figure 113

SW1 SW2 Bulb

Open Open Off

Closed Open Off

Closed Closed On



In digital terms a closed switch represents a high voltage (1) and an open switch represents a low voltage (0). A and B represent the inputs and Q is the output. Table 8 shows how the working of an AND gate is digitally represented. The section in blue is called the AND gate Truth Table .

AND Gate A

0

0

B

0

1 0

Q

0

1

0 1 0

1 0 0

Table 8

1 1 1

OR Gate

The OR gate has two inputs and an output. The symbol is shown below.

Figure 114

An OR gate works in a similar way to two switches in parallel. In the parallel circuit the bulb will light when SW1 or

SW2 or both switches are closed. If SW1 and SW2 are open the bulb will not light. The bulb is off only when the two switches are open.

SW1 SW2 Bulb

Open Open Off

Closed Open On

Closed Closed On

Figure 115



Digital representation of an OR Gate. Truth Table shaded blue.

0 1

OR Gate A B

0 1

Q

0 0 0

0 1 1

1 0 1

1 1 1

Table 9

NOT Gate

The NOT gate has one input and one output. The symbol is shown below.

Figure 116

A NOT gate works in a similar way to a Push-to-Break switch. A PTB switch is normally closed (it has not been pressed, there is no input) and the bulb is on. When the switch is pressed (an input) the bulb is off. The output of a NOT gate is always the opposite of the input. A NOT gate inverts the input signal.

PTB Bulb

Normal position (no input) On

Activated position

(input = press of PTB)

Off

Figure 117



Digital representation of a NOT Gate. Truth Table shaded in blue.

0 1 0 1

NOT Gate A Q

0 1

1 0

Table 10

NAND Gate

A NAND gate has two inputs and one output.

Figure 118

The NAND Gate is a combination of the AND and NOT gate. The out put from the AND gate becomes the input for the NOT gate as shown below.

Figure 119

To find Q take the output from the AND gate and invert it for each arrangement of A and B.

A B Q

0 0 0 A Q

0 1 0

1 0 0

0 1

1 0

1 1 1 NOT Table

AND Truth Table

A B Q

0 0 1

0 1 1

1 0 1

1 1 0

NAND Gate Truth Table

Figure 120



Digital representation of a NAND Gate. Truth Table shaded in blue.

0 1 0 1

NAND Gate A B Q

0 0 1

0 1 1

1 0 1

1 1 0

Table 11

Using Logic Gates to form Digital Circuits

A digital circuit consists of three parts: inputs, process and output.

Input Process Output

Inputs are sensors that detect changes in the environment. Examples include: thermistor (heat), LDR (light), sound sensor, moisture sensor, and all types of switches. The signals are process by the logic gates used and send a signal to the output. The output takes the signal from the process and turns on or off the output device. Output devices include: bulbs, LEDs, motors, relays, buzzers etc.

Example:

1. A drilling machine has a master on/off switch and a safety switch. The machine is to work only when the master switch is on and the safety guard has activated the safety switch.

Sensors: SPST switch to turn on or off the machine and a microswitch to detect when the safety guard is in place.

SPST switch: on = 1 off = 0 Microswitch: closed (guard in place) = 1

open = 0 (guard not in place)

Off Open Off

Off Closed Off

On Open Off

On Closed On

Figure 121

0 0 0

0 1 0

1 0 0

1 1 1



An AND gate will give the desired output. The circuit is represented as a block diagram.

SPST

Microswitch

AND Gate Machine On

Figure 122

2. An alarm is required to sound when the door of a jewellery box is opened. The alarm should remain on until reset by the owner. The trigger for the alarm is a NC reed switch fixed to the inside of the jewellery box and a magnet on the door.

When the door is closed the reed switch is open (0) and when the door is opened the reed switch closes (1) setting off the alarm. The alarm is to remain on even when the door is closed again.

A latched circuit as shown below is required. The output is connected back to one of the inputs. The other input is connected to the reed switch.

Figure 123

How the circuit works.

Jewellery door closed and alarm is set

Jewellery door opened alarm sounds

When the alarm is set first the output is low. Both inputs are low. The alarm is off.

When the door is opened the switch sends a 1 to the OR gate making the output high. As the output is connected to input B it goes high.

Jewellery door closed alarm stays on

Switch input goes low but the input at

B remains high. As the OR gate requires only one input to be high to give a high output the alarm stays on.

Figure 124



Exercise 13:

1. Design a circuit that will turn on a light at night when someone rings the front door bell.

2. Design a circuit that will turn on a green house heater when it gets cold. It should also be possible to turn on the heater using a master switch.

3. Copy each of the logic circuits below and draw up a truth table for each of them.


JC Technology

Project Circuits

Steady Hand Game



When the hook touches the wire the buzzer will sound but stop when the hook is removed.

The LED indicates the circuit is working.

When the hook touches the wire the relay changes over and the buzzer sounds.

When the hook is removed the buzzer stays sounding as the relay is latched.

The PTB switch acts as a reset switch turning of the relay and buzzer.


Obstacle Avoiding Vehicle

When the SPST switch is closed MT1 and MT2 rotate in the same direction. If microswitch 1 hits an obstacle it stops current flowing to MT2 while MT1 continues to rotate spinning the vehicle until the obstacle is cleared. When the obstacle is cleared the two motors rotate again in the same direction moving the vehicle forward.

If microswitch 2 hits an obstacle it stops current flowing to MT1 while MT2 continues to rotate spinning the vehicle until the obstacle is cleared. When the obstacle is cleared the two motors rotate again in the same direction moving the vehicle forward.



Sensor Circuits

1. The circuit will turn on the LED in dark conditions. The variable resistor allows the level of light at which the LED comes on to be set.

Interchanging the variable resistor and

LDR will turn on the LED in light conditions.

The LED and 470 Ω resistor can be replaced by a 6V buzzer.

2. This circuit will turn on an alarm when in cold temperatures. The temperature at which the alarm sounds can be set by the variable resistor.

3. This circuit will turn on a bulb in dark conditions.



Motor Control

1. Forward and reverse control of a motor using a DPDTCO switch.

The motor shown is a 120:1 Inline motor and gearbox. It is fitted with a

3-12V DC motor.

2. Forward and reverse control of a motor with limit switches using a DPDTCO switch.

Any model of microswitch can be used: standard lever. Long lever, roller or button.



3. Relay Motor Control

Light Control

In daylight the motor will rotates until MS1 is activated. When it gets dark the motor rotates in the opposite direction returning MS1 to its normal position. The motor rotates until MS2 is activated when it stops.

MS1 and MS2 are microswitches.

BCX38B is a darlington pair transistor.

This circuit can be used to open and close curtains, blinds etc automatically in response to changing light levels.

Temperature Control

Changing the LDR for a thermistor (20k @ 25°) will allow the motor to be controlled automatically in response to changing temperature levels.

The sensors and variable resistor can be interchanged depending on circuit requirements.



Timing Circuits: Astable

1. This circuit will turn on the LED while the capacitor is charging and off when the capacitor is discharging.

Flashing sequence: on/off/on/off ……….

This circuit will turn the LED off while the capacitor is charging and on when the capacitor is discharging.

Flashing sequence:

Off/on/off/on ……….

Using two LEDs will cause them to flash alternatively.

The frequency of the flashing cycle can be changed by changing the variable resistor.



This circuit can be built as required on the T4 – 555 Astable PCB.

The variable resistor used is the preset model.

2. The circuit below will turn on and off two LEDs and a buzzer. It could be used as a warning/hazard alarm, reversing circuit for a vehicle or road crossing.



The circuit can be built using the T4 – 555 Generic PCB.

JC Technology

Outputs on a 555 Generic PCB include:

• Up to 8LEDs

• Buzzer

• Bulb



Timing Circuit: Monostable

The circuit below will turn on a light for the time period when the PTM switch is pressed. At the end of the time period the LED turns off.


basic electronics handout

Technology

Energy and Electricity

Contents

Work, Energy and Power

Electric Circuits

Resistors

Components

Switches

Transistor and Voltage Divider Circuit

Relay

555 Integrated Circuit

Logic

Project Circuits

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basic electronics handout

Technology

Energy and Electricity

Contents

Work, Energy and Power

Electric Circuits

Resistors

Components

Switches

Transistor and Voltage Divider Circuit

Relay

555 Integrated Circuit

Logic

Project Circuits

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