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Transcript
Component Electronic Systems
Electricity
o Electricity is an important form of energy
o Our lives are full of devices that depend upon electricity and have electrical circuits
inside them to work.
o These circuits change electrical energy into other forms of energy such as heat,
light and sound.
Switch
Electric Circuits
An electric circuit is a closed loop or network made up
of electrical components such as batteries, bulbs, switches
and wires.
Battery
Lamp
Electric current
o Negatively charged particles flow through a conductor.
o Negatively charged particles are called electrons.
o Current is measured in amperes, usually referred to
e
le
c
tr
o
n
s
as ‘amps’ (A).
o Current is the rate of flow (speed) of electrons
through a circuit.
Voltage
o A battery or voltage supply (Vs) provides the
energy used to drive the electrons around the
circuit.
o The force which drives the electrons is called
the Electro Motive Force or EMF.
o Voltage is measured in volts (V).
Standard Grade Technological Studies: Component Electronics Revision Notes
1
Resistance
o All materials conduct electricity to some extent.
o Materials that conduct electricity well are called conductors.
o Poor conductors are called insulators.
o Metals are good conductors while rubber and glass are good insulators.
o A good conductor has very little resistance to the flow of electrical current.
o A good conductor lets currents flow through with very little EMF being applied.
o Resistance is a measure of how much voltage (energy) is required to make a current
flow.
e.g. 1 Volt will force a current of 1 Amp through a resistance of 1 Ohm.
o Resistance is measured in ohms ().
Electron flow  conventional current
o Early nineteenth century scientists decided the direction of conventional current flow.
o Conventional Current flows from the positive side of power supplies to the negative
side.
o Twentieth century scientists discovered electrons and the true direction of current flow
was proved.
o True Current flowed from the negative side of power supplies to the positive side
For convenience ‘conventional current’ is used in the circuits and calculations because:
o Conventional current flows from positive to negative and this is easy to understand.
o Symbols and other data based on conventional current
have become standard.
+
V
2
_
Conventional
Current
Standard Grade Technological Studies: Component Electronics Revision Notes
R
Batteries and voltage supplies
o Batteries and voltage supplies are the source
of power behind all electrical circuits.
o Without a power source, electrical circuits
will not work.
o In most electronic circuits power sources are
low-voltage. (up to 12V)
o Electronic components normally work on much lower voltages and so the circuits
must be designed carefully.
Symbols for batteries and voltage supplies:
Single battery or cell
Positive and negative side of the battery:
Multiple batteries or cells
-v
e
+v
e
6 volts
Voltage supply
Direct current (d.c.)
o Voltage supplied by batteries or low-voltage supplies is direct current (d.c.).
o The normal type of supply to low-voltage circuits is d.c.
o Alternating current (a.c.) supplies are high-voltage  usually 230 volts.
o The normal supply in homes and schools is a.c..
o Many portable electric power tools work from 110 volts for safety.
Resistors
o Resistors limit the amount of current flowing in
circuits or parts of circuits.
o Resistors are roughly cylindrical and have coloured stripes.
o Resistors have connection wires sticking out of each end.
o The stripes indicate the value of the resistors. The colours
represent numerical values according to a special code.
Standard Grade Technological Studies: Component Electronics Revision Notes
3
Resistor colour code
o Resistors are marked with a colour code.
o Each band that surrounds the body of the resistor helps identify the value (in ohms)
and the tolerance (in per cent).
o In most resistors only four colour bands are used.
The colour code chart for resistors:
First and second
colour band
Digit
Multiplier
Black
0
x1
Brown
1
x 10
Red
2
x 100
Orange
3
x 1000 or 1 K
Yellow
4
x 10 000 or 10 K
Green
5
x 100 000 or 100 K
Blue
6
x 1 000 000 or 1 M
Violet
7
Silver means divide by 100
Grey
8
Gold means divide by 10
Tolerances:
White
9

brown  1%

red  2%

gold  5%

silver  10%

none  20%
Standard values
o Resistors are supplied in a range of standard values: 1.0, 2.2, 3.3, 4.7, 5.6, 6.8, 7.5, 8.2
and 9.1.
o Standard values can then be multiplied by 10, 100, 1000, and so on.
o Typical values of resistors are 220 R, 100 K, 680 R, etc.
o Some other popular sizes are also available, such as 270 R and 390 R.
4
Standard Grade Technological Studies: Component Electronics Revision Notes
Resistor Colour Code layout for 4-band
Example
If the colours on the above resistor are:
4 Band Resistor Colour Code Layout
1st band  red
2nd band  violet
3rd band  brown
4th band  gold
1st band
1st digit
4th band
tolerance
Using the resistor coding table the value of
this resistor is 270  and its tolerance is 10
2nd band
2nd digit
per cent.
3rd band
multiplier
This is worked out as:
o ‘2’ for the red first band,
o ‘7’ for the violet second band and
o ‘times 10’ for the brown third band.
Symbol for resistance
The symbol for ohms is ‘’ it is often shown as a capital R; that is, 270 ohms can be
expressed as either 270  or 270 R.
Diodes
Current can pass this way only
Diodes are devices that allow
current to flow in one direction
only.
Current will flow through the
Anode
Cathode
Symbol for Diode
diode only when:
o The anode (positive side) is connected to the positive side of the circuit
o The cathode (negative side) is connected to the negative side of the circuit.
Standard Grade Technological Studies: Component Electronics Revision Notes
5
Light-emitting diodes
o A light-emitting diode is a special diode that gives
out light when current is flowing though it.
-ve
o LEDs are used as indicators to tell when a circuit is
working.
o The cathode (-ve) of an LED as it is the short leg and there is a ‘flat’ on the plastic
casing.
o As with the normal diode, the current can only pass one way.
Switches
o Switches are useful input devices (or transducers) that
have metal contacts inside them to allow current to pass
when then they are touching.
o The main types are  push-button, toggle, key, slide,
magnetic (reed) and tilt.
o Switches are ‘digital’ input devices as they can only be
on or off.
The switches shown are all single pole with single or double
throws. These are known as SPST and SPDT switches. The
symbols are shown below.
These can be wired using terminal combinations in another
switch called a double –pole Double-throw (DPDT)
6
Toggle
Slide
Key
Tilt
Rocker
Reed
Standard Grade Technological Studies: Component Electronics Revision Notes
Microswitches
o A small switch useful for detecting motion.
o Good as sensors and limit switches.
o Typical systems that use microswitches are traffic barriers and lift systems.
Description:
o Has a roller fixed to a lever that detects
movement and throws the switch.
o Has three terminals: common, normally open
(NO) and normally closed (NC).
Wiring:
o Microswitches can be wired in three ways.
1. Common (C) and NO: this is a normal
on/off switch.
2. Common (C) and NC: this allows current to flow when the switch is
not operated.
3. Common (C), NC and NO: when wired like this it acts as a changeover
switch.
These microswitches are single-pole double-throw (SPDT) switches.
Input transducers
Input transducers are devices that convert a change in physical conditions
(for example, temperature) into a change in resistance and/or voltage.
This can then be processed in an electrical network based on a voltage
divider circuit.
Strain gauge
Analogue input transducers
The two most common analogue input transducers are the thermistor and the lightdependent resistor (LDR).
Standard Grade Technological Studies: Component Electronics Revision Notes
7
Thermistor
A thermistor is a device whose resistance varies with temperature. It is a temperaturedependent resistor. There are two main types.
1. Negative temperature coefficient (t or NTC) – where resistance decreases as
temperature increases.
2. Positive temperature coefficient (+t or PTC) – where resistance increases as
temperature increases.
NTC thermistors are the most commonly used.
8
Standard Grade Technological Studies: Component Electronics Revision Notes
Strain gauges
Strain gauges are load sensors:

A length of resistance wire

When stretched their resistance changes.

Strain gauges are attached to structural members (beams, etc.)

As they are loaded, a reading on a voltmeter can be obtained.
Strain gauge
Light-dependent resistor (LDR)
The LDR (sometimes called a photoresistor) is a component whose:

Resistance depends on the amount of light falling on it.

Its resistance changes with light level.

In bright light its resistance is low (usually around 1 K).

In darkness its resistance is high (usually around 1 M).
The circuit symbol and typical characteristics of an LDR are shown above.
Standard Grade Technological Studies: Component Electronics Revision Notes
9
Variable resistor (potentiometer)
A potentiometer (variable resistor) can be used as a voltage or current control device.
It is used in voltage divider circuit to adjust the sensitivity of the input.
The spindle of the potentiometer is connected to the wiper, which is able to traverse the whole
of the resistive material. As the spindle rotates, a sliding contact puts more or less resistive
material in series with the circuit. In this way the resistance in a voltage divider circuit is
varied.
Miniature potentiometers
Modern circuits now use miniature potentiometers or variable resistors.
Examples of miniature potentiometers (not to scale)
10
Standard Grade Technological Studies: Component Electronics Revision Notes
Simple Circuits
Switch
I
Series circuits
The diagram shows a circuit diagram.
The components in this circuit are connected in series. This
means that they are connected up in a line, one after the
other (or end to end).
6V
LED
Series circuits and Kirchoff’s second law
When components are connected end to end (in series) to form a closed loop:
o The same current flows through all components
o The voltage is divided up amongst them.
Kirchoff’s 2nd Law:
6V
6V
6V
The sum of voltages dropped across each
component is equal to the total voltage supply
in the circuit.
VT = V1 + V2 + V3 + …
18 V
Parallel circuits
Parallel circuits are circuits where:
o There is more than one path for
electricity to flow along.
o Each branch receives the supply
voltage
o You can run a number of devices
from one supply voltage.
This arrangement ensures that:
o If one or two bulbs ‘blow’ then the rest
of them will continue to function
o You know which are faulty.
Note: In a series circuit if one bulb blew then
all the bulbs would go out and you would have
to test them all to see which one was faulty.
12 volts
Parallel circuits are normally set out so that you can easily see the parallel ‘branches’.
Standard Grade Technological Studies: Component Electronics Revision Notes
11
Practical Electronics
Circuits can be physically built on
o Prototype boards
o Strip board
o PCB (Printed Circuit Board)
o Computer Simulation
Prototype circuit boards
Prototype circuit boards (often called breadboards) They
have the advantage that:
o They are non-permanent
o Components can be moved and used again.
o Alterations or corrections are easy to make.
Once a circuit has been proved on a prototype circuit board it is usually built by a more
permanent method on strip board or printed circuit board (PCB).
LIGHT EMITTING DIODE
There are four mains sections of
connection holes.
o The two centre areas,
separated by a gutter, are
where most of the
components are placed.
o The two outer rows are
used for the power
connections.
VARIABLE RESISTOR
SLIDE SWITCH
WIRE LINK
1
5
10
15
20
25
555
TRANSISTOR
1
5
10
The diagram shows how some
common components can be
inserted.
The most complicated components
are usually connected over the
centre gutter.
This is especially true for transistors and integrated circuits (ICs).
ELECTROLYTIC
CAPACITOR
12
A
B
C
D
E
15
20
25
F
G
H
I
J
LDR
RESISTOR
Standard Grade Technological Studies: Component Electronics Revision Notes
Computer simulation
Series circuits can be built and simulated in a
computer programme such as Crocodile Clips.
The computer calculates circuit values for voltage,
resistance and current.
It makes the components appear to work or not
work according to these values.
LED circuit
As in the prototype circuit, when the switch is
‘pressed’ the LED should light.
Lamp circuit
As in the prototype circuit, the lamp should light
when the switch is pressed/moved and it will not
light when the diode is reversed.
Making Electrical Measurements
Digital multimeters
The digital multimeter is used to measure voltage, current and
resistance. It is very simple to use and easy to read.
On/Off
Amps 20mA
To measure d.c. voltage:
o
o
o
o
o
a.c. d.c.
Volts
10A 50V
10V
1V
2mA
connect the black lead to the ‘COM’ socket
connect the red lead to the ‘V’ socket
make sure that ‘d.c.’ is selected
move the dial into the voltage (volts) range
select a suitable range (always slightly higher than the
expected measurement)
100mV
200A
200k
20k
200
2k
Ohms
10A
mA
V
COM
Method:
1. Place the lead probes on the points at each
side of the place where the voltage is to be
measured.
2. Voltage is measured across components or
parts of circuits as shown in the circuit
diagram below.
V
6v
‘Across’ means in ‘parallel’
Standard Grade Technological Studies: Component Electronics Revision Notes
13
To measure direct current:






Connect the black lead to the ‘COM’ socket
Connect the red lead to the ‘mA’ socket
Make sure ‘d.c.’ is selected
Move the dial into the current (amps) range
Select a suitable range (always slightly higher
than the expected measurement)
Connect the probes to the wire in which the
current is to be measured.
6V
A
Method:
1. Current is measured through components or parts of circuits, as shown in the circuit
diagram below.
2. It is necessary to ‘break’ the circuit and connect the meter in series with the
components.
To measure resistance:
o
o
o
o
o
o
Connect the black lead to the ‘COM’ socket
Connect the red lead to the ‘V’ socket
Make sure ‘d.c.’ is selected
Move the dial into the resistance (ohms) range
Select the range (always slightly higher than the expected measurement)
Connect the probes to the ends of the component being measured.
Resistors connected in series
As resistors come in standard sizes, they are often
connected in series to obtain a specific size that is
otherwise unavailable.
R1
Resistance in Series and Kirchoff’s 2nd
Law:
The total resistance measured across both resistors
shown in the diagram will equal the sum of their
individual resistances.
10A
mA
V
COM
R2
Rtotal = R1 + R2
For three resistors in series
Rtotal = R1 + R2 + R3
14
Standard Grade Technological Studies: Component Electronics Revision Notes
Ohm’s law in Series Circuits
The current is proportional to the voltage applied.
e.g. If the Voltage increases the Current also increases.
This rule is known as Ohm’s law.
The rule applies to all metals, provided that their
temperature does not change.
This relationship gives rise to the Ohm’s law formula:
Current (A)
By applying a voltage to a circuit a current flows through the circuit.
Voltage (V)
R =
V
I
The formula can also be expressed as:
V=I x R
We can use the triangle trick to help transpose this formula. Cover up
the quantity that you are trying to find and the other two will be in the
form that is needed.
V
I
R
Ohm’s law in practice
For this exercise a simple series circuit is used.
The task is to calculate the resistance of the lamp
using Ohm’s law.
V
I
6
R =
0.06
6 volts
Lamp
R =
 R = 100 
Standard Grade Technological Studies: Component Electronics Revision Notes
Current 0.06 amps
15
Ohm’s Law with Resisters in Parallel Special cases:
Two resistors in parallel
There is a special rule that can be applied when adding two resistors in parallel only:
total resistance (RT) = product/sum.
R
R
1x
2
R
T=
R
+
R
1
2
More Than Two Resistors in Parallel
The following rule can be used to calculate the total resistance
Combined series and parallel circuits
It is possible and quite common, to have
series and parallel connections in the
same circuit.
You can see that R2 and R3 are connected in parallel and that R1 is connected in series with the
parallel combination.
16
Standard Grade Technological Studies: Component Electronics Revision Notes
Some points to remember with combined series and parallel circuits
o The voltage drop across R2 is the same as the voltage drop across R3
o The current through R2 added to the current through R3 is the same as the current
through R1
o The voltage drop across R1 added to the voltage drop across R2 (which is the same as
across R3) would equal the supply voltage Vs.
Power in electric circuits
o Electrical power is measured in watts (W).
o Electrical power can be converted into other forms of power using electric circuits.
E.g. the power used in overcoming electrical resistance can be converted into heat – this is the
principle of an electric fire.
o The power in an electric circuit depends both on the amount of current (I) flowing and
the voltage (V) applied.
o The formula for power in electric circuits is:
Power = Voltage x Current (watts)
P = V x I (W)
Standard Grade Technological Studies: Component Electronics Revision Notes
17
Voltage divider circuits
If two or more resistors are connected in
series the voltage drop over each resistor will
depend on the supply voltage and the ratio of
resistances.
the
Voltage divider circuits work on the basic principle that:
 If two resistors are connected in series across a supply, the voltage drop across
each of the resistors will be proportional to the value of the resistors.
The formulae shown on the left illustrate
how the calculations can be made to find
the voltage drop over each resistor.
 Note that the voltage drop over R2
will have the same value as Vout.
For the circuit shown above, two resistors connected in series.
 If you change the value of R1, the voltage drop across it will change.
 A corresponding voltage drop change will occur in R2.
 The sum of the voltage drops over R1 and R2 will equal the supply voltage.
In other words, the resistors divide the voltage up between them.
18
Standard Grade Technological Studies: Component Electronics Revision Notes
A voltage divider circuit can be represented in a number of different ways. Some of these are
shown below.
0V
0V
Simple switches can be used in voltage
divider circuits to give a digital signal (that
is definitely ON or OFF) to another part of a
circuit.
In the example on the right a normally
closed switch is used.
When the switch is pressed, the voltage
divider comes into use and power is
supplied to the LED to give a definitely ON
signal.
Digital switch types
A switch with its contacts apart when it is not operating is called normally open.
Double-pole switch symbols
Standard Grade Technological Studies: Component Electronics Revision Notes
19
Using Voltage Divider circuits With Transducers
The main purpose of the voltage divider circuits is to
sense and process inputs from analogue sensors.
VS = 9 volts
1
In this example the resistor R2 of the previous circuit
has been replaced by an NTC thermistor.
-t
VO
0 volts
Voltage Divider circuits using LDR’s
The LDR voltage divider circuit can be set up to detect when it is light or when
it is dark.
VS = 9 volts
VS = 9 volts
ORP12
10K
10K
ORP12
VO
VO
0 volts
Detects when dark
Detects when light
Adjusting Sensitivity in Potential Divider circuits
VS = 9 volts
With an analogue sensor it is normally desirable to adjust
the sensitivity of the circuit. Rather than using a fixed
resistor we can replace it with a variable resistor (or
potentiometer).
ORP12
47K
VO
0 volts
20
Standard Grade Technological Studies: Component Electronics Revision Notes
Practical Example: voltage divider circuits
The picture below shows a typical situation where a light sensor circuit could be useful.
Residents want an outside light to come on when it gets dark. They also want to be able to
adjust the sensitivity from summer to winter nights.
Transistors
There have been generations of electronic development:
Diode Valves (beginning of the twentieth century)


This was the first real electronic component and was to lead to the modern diode and
transistor.
A diode valve consisted of a heater inside a hollow rod that had been
coated with a substance which released electrons when heated. This
was surrounded by a thin metal cylinder, with all of this being
contained in a bulb-like glass container. When the rod was
heated, electrons were released but, as in any diode, the
electrons could only go in one direction.
Triode Valve




The triode valve allowed the current flow to be controlled.
These valves could act as electronic switches or amplifiers.
Radio and television were developed using these amplifier
In the 1940s the first computer was built using valves  it
contained over 20,000 valves and filled a large room.
valves.
Transistors (1947)
The transistor had many advantages over valves:

Small size

Efficiency

Durability

Cost.
Integrated Circuits (1958 ……)


Two transistors were fitted on a silicon chip.
The developments since then have been rapid and chips
now contain over a million transistors.
Standard Grade Technological Studies: Component Electronics Revision Notes
21
Transistors (bipolar)


The bipolar transistor is a
semiconductor device.
This means that it is sometimes a
good conductor of electricity and
sometimes a poor one.

A transistor is made up of three layers
of semiconductor materials that are
either ‘n type’ or ‘p type’.

There are two types of bipolar transistor available: pnp or npn.

The voltages and currents should be reversed for a pnp transistor.
Action of a Transistor:

A small voltage of about 0.6v is applied across the base and emitter.

The resistance between the collector and the emitter of the transistor drops from very
high to very low.

The transistor changes from being a very poor to a very good conductor.

When the voltage reaches 0.7 volts the transistor is fully ‘switched on’.

The transistor is said to be fully ‘saturated’.
General-purpose transistor
The BC 108 is common general-purpose transistor. The diagram below shows the position of
the legs when viewed from underneath the case.

The transistor has to be connected into circuits correctly.

The arrowhead on the emitter indicates the direction of ‘conventional’ current flow.
22
Standard Grade Technological Studies: Component Electronics Revision Notes
How does the transistor work?
Consider the circuit shown.

When the switch S1 is open, no current
can flow in any part of the circuit.

The transistor is acting like a barrier to
current flow (insulator).


9V
When switch S1 is closed, a very small
voltage is applied to the base of the
transistor.
When this happens the transistor allows current to flow through it and the bulb will
light; the transistor is said to ’switch on’.
Bipolar transistors amplify current. A small current flowing through the base
of a transistor causes a much larger current to flow from the collector to the
emitter.
The transistor as a switch
One of the main uses of a
transistor is that of a very
sensitive switch. The example
shows the transistor used as an
electronic switch activated by
changes in light. The transistor
switches on and in turn activates
the relay.
Standard Grade Technological Studies: Component Electronics Revision Notes
23
Transistor circuits calculations
+ 6 volts
The transistor circuit on the right is basically a
parallel circuit. If the circuit is rearranged slightly this
becomes obvious.
6V
60mA
Lamp
c
Ic
b
1K
BC 108
e
c
6 volts
T
x
e
0V
Ie
b
Rb
0V
Ib
The transistor (marked T) is at the junction of the parallel circuit.
Assume that no voltage drops across the collector/emitter in the transistor then
Vxe = 6 volts (in the bulb branch)
As the two branches between x and e are in parallel then Vxe across the resistor branch must
also be 6 volts.
VRb + Vbe = 6 volts
We know that Vbe must be 0.7 volts to switch the transistor on;
VRb = Vxe –Vbe
VRb = 6 – 0.7
VRb = 5.3 volts
It is now possible to calculate all other currents and resistance values in other components
which may be in the circuit.
Relays

Often considered to be output devices.

They are really output switches for electric or electronic
control circuits.

These output switches are used as inputs for other
circuits.
In practice you can hear relays clicking on and
off as the control circuit is switched on and off.

24
Standard Grade Technological Studies: Component Electronics Revision Notes
How the relay works

An electric current flows into the relay
coil


The coil becomes an electromagnet
The electromagnet attracts the
armature and moves the contacts.

This movement provides the
switching, just as the contacts in any other switch do.
The relay is very useful because it is a vital link between low energy microelectronics and
high-energy systems that require large amounts of current.
Relays  connections
The connections for a typical miniature relay are shown in the diagram below.

Connections 1 and 16 are those from the sensing or input circuit.

Connections 4 and 13 are the supply voltage to run the output.

Connections 6 and 11 are the normally closed output terminals.

Connections 8 and 9 are the normally open output terminals.
1 +
16 -
4
6
13
11
8
9
Standard Grade Technological Studies: Component Electronics Revision Notes
25
Relays and protective diodes

Relays have an electro magnetic coil
that is energised and de-energised as
the relay switches on and off.
During this process of switching, the
coil can generate a large reverse
voltage (called a back e.m.f.).


This reverse voltage can cause
considerable damage to
components, especially transistors.

Transistors and other sensitive components can be protected by the inclusion of a diode.

The diode provides a path for the current caused by the reverse voltage to escape.
A solenoid is another output transducer that has an electromagnetic coil inside.
Circuits containing a solenoid require a protective diode as well.
Relay in a Transistor Sensing Circuit
The circuit below shows a typical transistor temperature sensing circuit with a relay as an
output.
+ 6 volts
-t
Relays can also be used to switch on (and off) solenoidactuated pneumatics valves. This is a method of
controlling pneumatic circuits and systems with
microelectronics.
Relay
c
b
1K
BC 108
e
10K
0 volts
26
Standard Grade Technological Studies: Component Electronics Revision Notes
Frequency
Frequency is the regular rate at which a physical event repeats itself.
It may be the rate at which an LED flashes.
In electronic circuits the common events are:
o Flashing of optical devices (LEDs and lamps)
o Sounding of buzzers/speakers.
These outputs are driven by an electrical pulse from the electronic system or circuit.
Capacitors can be used to control the frequency of these events.
Capacitance
Capacitance in measured in farads, this is a very large quantity.
Because this is a very large measurement most capacitors are rated in F (microfarads)
or in nF (nanofarads).
Capacitor Construction
INSULATOR

Capacitors are electronic components that store
electricity for short periods of time within
electronic circuits or networks.

They are made from two metal plates or films
separated by an insulator.

In many capacitors, film is used so that the layers
of metal film and insulator can be wound into a
cylinder.
METAL PLATE
OR FILM

Capacitors are especially useful in timer circuits with the 555-timer chip.

There are two basic types of capacitor normally used in timer circuits:
o electrolytic and
ELECTROLYTIC
o polyester.
Electrolytic capacitors
AXIAL
CAPACITATOR
o These are polarity
conscious:
RADIAL
CAPACITATOR
o This means that they
must be connected ‘the right way round’
o The negative lead must be connected to zero volts
o The positive terminal towards the higher voltage side of the circuit
It is very dangerous to reverse connect capacitors.
Polyester Capacitors
Standard Grade Technological Studies: Component Electronics Revision Notes
27
o These are for small-value uses
o Can be connected without regard to polarity
POLYESTER
Capacitors and Resistor Networks
o Capacitors can be put in series with resistors in a
similar way as a voltage divider circuit.
o The output Vout will be low until the capacitor is fully
charged.
o Once full charge is achieved Vout will rise and a
current will flow.
o The time taken will depend on the value of the
resistor as it will slow down the current passing to the
capacitor.
o The discharge switch will discharge the capacitor when pressed and can be used to restart the charging process.
This effectively creates a time delay in
achieving an output. This can be seen in
the circuit on the right. The transistor will
not switch on until the capacitor is fully
charged. The timing of this will vary
depending on the value of the variable
resistor.
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Standard Grade Technological Studies: Component Electronics Revision Notes
Integrated Circuits: 555 timer
An integrated circuit (or IC) is simply an electronic package that
contains a number of components on a silicon ‘chip’.
The 555-timer IC that you are going to use is a very versatile chip that
has many applications.
The 555 chip has eight pins. The pin functions are
shown below.
1
8 +V (3 -15)
TRIGGER
2
7
OUTPUT
3
6
RESET
4
CONTROL
5 VOLTAGE
(LEAVE
UNCONNECTED)
NE555
0V
TIMING
PERIOD
CONTROL
Flashing LED using a 555 Timer Integrated Circuit
The circuit shown on the right uses capacitor
resistor network C1 and VR to control the
timing period of the 555 timer.
The capacitor will charge and discharge at a
predicted frequency set by the value of its
capacitance and the resistance of VR.
Standard Grade Technological Studies: Component Electronics Revision Notes
29