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Electronic
Analog Vs. Digital
Analog Vs. Digital
• Analog
– Continuous
– Can take on any values in a given range
– Very susceptible to noise
• Digital
– Discrete
– Can only take on certain values in a given
range
– Can be less susceptible to noise
Analog versus Digital
• Analog systems process time-varying signals
that can take on any value across a
continuous
range
of
voltages
(in
electrical/electronics systems).
• Digital systems process time-varying signals
that can take on only one of two discrete
values of voltages (in electrical/electronics
systems).
– Discrete values are called 1 and 0 (ON and OFF,
HIGH and LOW, TRUE and FALSE, etc.)
Benefits of Digital over Analog
•
•
•
•
•
•
•
Reproducibility
Not effected by noise means quality
Ease of design
Data protection
Programmable
Speed
Economy
Diodes
diodes
Diodes
symbol
•A diode is another semi-conductor device.
•A diode will only conduct electricity in one direction.
•They are useful for changing a.c. into d.c – this is
called rectification.
A diode is a component that
allows a current to flow in one
direction only.
It has a low resistance in one
direction and a very high
resistance in the other.
x
x
x
x
x
x
No current flows
x
x
If the voltage is reversed or
the diode is connected the
other way around, the high
resistance of the diode
‘blocks’ the flow of current.
x
Current / A
Current flows in the direction
with low resistance but is not
proportional to the voltage.
Voltage / V
Diodes
Anode
Wired towards
the positive
terminal
Cathode
Wired towards
the negative
terminal
Which way through ?
This is the direction
which current will
easily pass through a
diode.
Diode calculation
A diode has a current of 5.0 A
running through it and a
resistance of 5.0 .
What is the potential
difference across the diode?
V = IR
= 5.0 A x 5.0 
= 25 V
Example circuits
An electric current can pass through the diode on the
left, allowing the lamp to light. The diode on the right
stops the current and hence the lamp remains out.
Changing a.c. to d.c.
So that we can get an
output voltage.
The diode conducts
in this direction.
I
I
t
t
INPUT
The diode will not
conduct in this
direction.
OUTPUT
DIODE
This means that
there will be no
output voltage.
Half wave rectification
Smoothing
Although the above produces a direct
voltage and current, it is not the steady
sort of dc we get from a battery. To
‘smooth’ the voltage we add a capacitor.
The capacitor, labelled C, is placed in the
half-wave rectifier circuit as indicated
below:
The effect of the capacitor on the voltage across
R is represented below:
Light Emitting Diodes
•A LED is a type of diode designed
to emit light .
•The light can be visible such as a
laptop light.
•It can also be infra red such as on a
remote control.
•Here is its circuit diagram symbol.
Circuit symbol
for an LED
Quick graph quiz
?
Fixed
resistor at
constant
temperature
?
Filament
lamp
?
Diode
• Initially the voltage across the resistor rises to
its maximum, and the capacitor charges to its
maximum, the top plate becoming positive.
Without the capacitor, the voltage across R
then drops to zero and stays there for half a
cycle. But now, as soon as the voltage starts
to fall, the capacitor start to discharge through
the resistor – this maintains the voltage across
it close to its maximum until the next cycle
starts. Thus, the voltage across the resistor
and the current though it are smoother than
without the capacitor.
VIN
Potential Dividers
R1
VOUT
R2
0V
0V
The Potential Divider equation:
VOUT
VIN x
(R2)
(R1 + R2)
Some example questions
12V
50V
100 
100 
0V
0V
3V
75 
0V
VOUT
0V
1.5V
75 
25 
0V
10 
VOUT
50 
VOUT
0V
45 
0V
VOUT
0V
Practical applications
Here’s a potential
divider that is used to
control light-activated
switches…
Vin
VOUT
0V
When the light intensity on the LDR decreases its
resistance will ________. This causes VOUT to _______
so the processor and output will probably turn _____. The
variable resistor can be adjusted to change the ________
of the whole device.
Words – decrease, sensitivity, increase, off
Reed switch
Reed switch in a Burglar alarm
Transistors
A transistor acts like a switch:
Collector
Base
Emitter
When a SMALL current
flows through the baseemitter part of the
transistor a different
current is switched on
through the collectoremitter part.
It conducts between C and E when the
voltage between B and E is above +0.6V.
A Frost alarm
A light dependent switch
6V
Power supply
Output
device
0V
A light dependent switch
1) When the light on the LDR decreases its resistance _________,
which will decrease the ________ across the variable resistor
2) This will cause VOUT to ____. The____ gate will recognise this as a
“0” and convert it into a “1”, i.e. a current will flow into the resistor
3) The resistor limits the amount of current flowing into the
transistor, to avoid __________ it
4) When the transistor detects the current at its _____ it will
“switch __” the collector-emitter current
5) A small current will then flow through the _______
6) The relay will then switch on a _____ current in the output circuit
7) The “reversed biased” diode is also placed in the circuit to act as a
“_______” to prevent current flowing back into the transistor
when the relay is switched _____
Words – base, buffer, on, increases, damaging,
relay, off, larger, voltage, drop, NOT
A light dependent switch
We could modify this circuit (if we wanted to…)
1) Swap these two
around and the
output will now
switch on when it
becomes LIGHT, not
when it becomes dark
2) Adjust this
resistor to vary the
sensitivity
6V
0V
The Capacitor
A capacitor is a device that can store charge (it has a
“capacity”). It is basically made of two plates:
…or…
Charge builds up on these plates and the voltage
between them increases until it reaches the supply
voltage.
Charging and discharging a capacitor
P.d. across
capacitor
P.d.
Increase
resistance or
capacitance
Time
Time
P.d.
Increase
resistance or
capacitance
Time
Time
Time delay circuits
6V
Power supply
R
Output
device
0V
Time delay circuits
6V
Power supply
R
Output
device
“1”
0V
Time delay circuits
1) When the switch is closed the capacitor is being short circuited so
no charge builds up on it
2) This means that the input to the NOT gate is __, so the output is
1 and the output device is ___
3) When the switch is released the capacitor starts to ________ up
4) When the voltage across the capacitor reaches a certain level the
input to the NOT gate becomes __ so its output is 0
5) This means that the output device is now switched ___
6) To INCREASE the amount of time taken to switch the device off
you could:
1) Increase the _________ of the capacitor
2) _________ the resistance of the resistor R
Words – charge, 1, capacitance, increase, 0, off, on
Gates
• The most basic digital devices are called
gates.
• Gates got their name from their function of
allowing or blocking (gating) the flow of
digital information.
• A gate has one or more inputs and produces
an output depending on the input(s).
• A gate is called a combinational circuit.
• Three most important gates are: AND, OR,
NOT
LOGIC GATES
Logic generally has only 2 states, ON or
OFF, represented by 1 or 0. Logic gates
react to inputs in certain ways.
The AND gate will only switch on its output Q,
if Input A is ON and Input B is ON. This can be
shown in a Truth Table, 0=OFF and 1=ON.
INPUT A
OUTPUT Q
INPUT B
Symbol for AND gate
A
B
Q
0
0
0
0
1
0
1
0
0
1
1
1
Logic can be used to control devices according to certain conditions, such as “switch on a fan if it’s hot
AND the sun is out”. Look at the diagrams below.
If both inputs are OFF
the output is OFF
Even if one input is ON
the output is OFF
Only if A =1 and B =1
will the output switch on
MORE LOGIC GATES
Try and work out the truth tables for these gates. The rule will help you.
AND
A
OR
A
Q
B
Q
B
RULE: Q = 1 if A AND B =1
A
B
0
XOR
A
B
RULE: Q = 1 if A OR B =1
Q
Q
A
B
0
0
0
1
1
1
Q = 1 if A OR B =1, but NOT both
A
B
0
0
0
0
1
0
1
0
1
0
1
0
1
1
1
1
1
NAND
A
Q
Q
NOT
NOR
A
Q
B
Q
A
Q
B
RULE: Q = 0 if A AND B =1
A
B
0
Q
RULE: Q = 0 if A OR B =1
Q
RULE: Q = 0 if A =1
A
B
A
0
0
0
0
0
1
0
1
1
1
0
1
0
1
1
1
1
Q
LOGIC GATES
AND
A
OR
A
Q
B
Q
B
RULE: Q = 1 if A AND B =1
XOR
A
Q
B
RULE: Q = 1 if A OR B =1
Q = 1 if A OR B =1, but NOT both
A
B
Q
A
B
Q
A
B
Q
0
0
0
0
0
0
0
0
0
0
1
0
0
1
1
0
1
1
1
0
0
1
0
1
1
0
1
1
1
1
1
1
1
1
1
0
NAND
A
NOT
NOR
A
Q
B
Q
A
Q
B
RULE: Q = 0 if A AND B =1
RULE: Q = 0 if A OR B =1
RULE: Q = 0 if A =1
A
B
Q
A
B
Q
A
Q
0
0
1
0
0
1
0
1
0
1
1
0
1
0
1
0
1
0
1
1
0
0
1
1
0
1
1
0