Electronics

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Electronics
Shatin Tsung Tsin Secondary School
By Mr. C.K. Yu
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Learning Approach (????)

• A black box approach (????)
• We dont care what happens inside the box and
  how it works, we do know the behaviour of the box
  and know when and how to use it.
• (e.g. we dont know how the refrigerator works,
  but we know the behaviour of the refrigerator and
  we do know when and how to use it.)

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Learning Approach

• Things to do when we have a new electronic
  device
• 1) Find out how it works, but not what happens
  inside.
• 2) Think how we may use this device.

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Electronics(???)
Electronic device Light Emitting diode (LED)
Current can flow in correct connection. At high
voltage (e.g. 5V), the LED comes on. We use
to represent this high voltage.
1
At low voltage (e.g. 0V), the LED comes off. We
use to represent this low voltage.
0
Remember voltage represents p.d.
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Electronics(???)
Electronic device Light Emitting diode (LED)
LED can be used to test whether the voltage is
high (5V) or low (0V) if one end is connected to
0V
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Digital Electronics

• Basic digital devices, logic gates, have two
  operation states,
• high state (or logic level 1) and
• low state (or logic level 0).
• We shall use 1 and 0 to represent the two
  logic states.

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NOT gate

• Logically, NOT means opposite.
• e.g, not True is False.
• b. The symbol of a NOT gate is .

c. The output, O, of a NOT gate is 1 (High) when
its input, A, is 0 (Low), and vice versa. d.
The rule of the logic operation can be found as
O
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NOT gate

• Truth table

A (input) O (output)


0
1
1
0
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OR gate

1. The output, O, of an OR gate is 1 (High) if any
   of its input (A/B) is/are 1 (high).
2. The rule of the logic operation of OR can be
   found as O A B (Note Result bigger than 1
   is still 1, high is still high)

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OR gate
c. Truth table
O A B
A(input) B (input) O (output)




0
0
0
1
0
1
1
0
1
1
1
1
There are four combinations for two inputs
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AND gate

1. The output ,O, of an AND gate is 1 (High) only if
   both inputs (A/B) are 1 (high).
2. The rule of the logic operation of AND can be
   found as O A x B (Note Result bigger than 0
   is 1)

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AND gate
c. Truth table
O A x B
A (input) B (input) O (output)




0
0
0
0
0
1
1
0
0
1
1
1
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NOR gate

1. The output ,O, of an NOR gate is 0 (High) if
   any of its input (A/B) is/are 1 (high).
2. The rule of the logic operation of OR can be
   found as O A B (Note Result bigger than 0
   is 1)

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NOR gate
c. Truth table
O A B
A(input) B (input) O (output)




0
1
0
0
0
1
1
0
0
1
1
0
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NAND gate

1. The output ,O, of an NAND gate is 0 (High) if
   its two inputs (A/B) are 1 (high).
2. The rule of the logic operation of AND can be
   found as O A x B (Note Result bigger than 0
   is 1)

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NAND gate
c. Truth table
O A x B
A (input) B (input) O (output)




0
1
0
1
0
1
1
0
1
1
1
0
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Use of Logic Gates
In the circuit of logic gate, connections to the
supply rails (dashed lines) are usually omitted.
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Use of Logic Gates
What will the output of the diagram be if the
input is connected to the supply rail of high
voltage, 5V?
LOW
0
Ans The output is _____voltage, ___V, or
logic state of ___ .
0
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Example
Connect the positive and negative supply rails to
5 V and 0 V respectively. Connect the output O of
the gate to an LED and plug a flying lead into
the input.
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Example

• Result
• Observation
• If the flying lead is not connected to anything,
  is the LED on? __________

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Example
2. What is the input of the NOT gate when the
flying lead is not connected to anything?
High/Low

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Example
3. While connecting the flying lead to the 5 V
supply rail to make the input high, the LED is
not lit. The output is HIGH/LOW or the output
state is ____ .

1
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Example
4. While connecting the flying lead to the 0 V
supply rail to make the input high, the LED is
lit. The output is HIGH/LOW or the output state
is ____ .

0
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Experiment session
P. 59 P. 60 and P.61 P.62
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Application of Logic Gates
Simple burglar alarm and Simple water heater
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Simple burglar alarm
LDR
What gate is being used?
NAND gate
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Simple burglar alarm
The buzzer sounds when the output of the logic
gate is ___ V or ___ (state).
5
1
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Simple burglar alarm
Under which situations will the output of the
logic gate is 5 V or 1?
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Simple burglar alarm
If the switch is not pressed, input A is a flying
lead. Its state is high (1).
If no light shines on the LDR, input B is also a
flying lead. its state is also high (1).
Then the output is __.
0
The buzzer will not/will sound.

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Simple burglar alarm
Under which situations will the output of the
logic gate is 5 V or 1?
Either input is LOW
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Simple water heater
If covered by water, input B becomes high
low resistance
low resistance
Thermistor
Resistance becomes high at low temperature, and
input A becomes high
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Simple water heater
The heater is on when water covers the contacts
and is at low temperature.
1
1
1
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Simple water heater
D
When water is at low temperature. VDE VEF
5V I (RDE REF) 5V If REFgtgtRDE then VEF
5V e.g. if REF1000 O, RDE5 O Then I
5V/1005O0.00498 A VEF I x REF 4.98 V The
potential at A is 4.98 V and A is at high (1)
state
E
F
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Simple water heater
D
When water covers the contacts, the resistance is
0 O. The input B is connected to the 5V supply
directly. The potential at B is 5 V and B is at
high (1) state
E
F
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Future thinking

• Should we add a switch to make sure that the
  water heater only works when the switch is also
  on?
• How should we design the logic circuit for this
  purpose?
• This question will not be tested in the final
  examination.

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The End

• Thank you very much