Download Logic Gates MES

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Potential dividers
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Using sensors in potential dividers
A potential divider uses two series resistors to reduce a high
input voltage (VIN) to a lower output voltage (VOUT).
By replacing either resistor with
an input device, the potential
divider can be used as an input
sensor for a control system.
Replacing R2 with a switch will
produce a simple pressure
sensor, while LDRs and
thermistors can be used to
produce light and temperature
sensors respectively.
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VIN
R1
VOUT
R2
0V
0V
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Light sensors
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Control systems
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What are logic gates?
Logic gates are electronic
switches that process information.
They are called gates because
they open to produce a high
output signal only when they
receive the correct combination
of input signals.
This chip contains four
NOT gates.
Most logic gates have multiple
inputs, which are used to
determine a single output.
A large number of logic gates
can be incorporated in one
electronic chip.
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Logic gate voltage
Digital systems have two states, ON and OFF. These simple
electronic states are represented in binary code: ON is
called logic 1; OFF is called logic 0.
Microchips contain logic gates, which use this binary
code to send and store information.
Microchips can be easily damaged
by high voltages. Due to this the
voltages for the two logic states are
standardized.
Logic 1 is 5 V, while logic 0 is 0 V.
Logic gates receive multiple signals in these two states.
Different combinations of signals lead to different outputs.
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Types of logic gate
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Logic gates
AND
• For the output to be high 1
• Both inputs must be high 1
OR
• For the output to be high 1
• One input must be high 1
NOT
• If the input is high 1
• The output is low 0
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NAND & NOR
NAND
• The opposite output from
an AND gate
• Output high if both A & B are not both
high
NOT
• Equivalent to a OR gate
• With the output inverted by a
NOT gate
Output is high if both A & B are NOT high
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Logic gates summary
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Truth tables for three inputs
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Three input truth tables question
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Truth tables for four inputs
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A burglar alarm
A burglar alarm is a great
example of a control system.
Can you identify the input
sensor, the processor and
the output?
input = movement sensor
processor = alarm unit
output = siren and lights
The alarm keeps sounding until the owner enters the code
into the control panel, regardless of the input it receives.
How does this happen?
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Making use of logic output
There are many different output
devices for an electronic system.
These include motors, buzzers
and lamps.
In a logic system both current and
voltage are very small to prevent
damaging the gates.
This limits the range of output devices which can be run on a
logic system.
In order to make full use of logic systems, we must find a
way to control a high voltage component using a low voltage
logic system: a relay allows this.
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Introduction to relays
A relay is an electromagnetic switch which allows one circuit
to control another circuit by turning it on and off.
A relay is used to isolate a high voltage circuit, controlling it
with a low voltage counterpart.
There are a number or possible reasons for this:
 To allow the use of a smaller, more practical switch.
 To isolate the user from a dangerous high voltage.
 To allow a logic system to control high voltage
components.
The starter motor in a car uses a
current of several hundred amps,
and is controlled by a relay, making
it safe and easy to operate.
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How does a relay work?
A relay uses an electromagnet to operate a switch.
As electricity flows in the input
circuit, the coil becomes an
electromagnet.
coil
steel
switch
input
output
The coil attracts the steel
switch in the output circuit,
closing the switch.
Electricity flows in the output
circuit driving the motor.
circuit symbol
for a relay
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Separating mains and logic voltage
Relays allow us to separate the output from an electronic
control system, of either 0 V or 5 V, from the mains voltage
of 230 V.
This allows electronic control
systems to turn on mains
appliances, which they would
otherwise be unable to run.
It also protects the user from the
dangerously high mains voltage.
Some electronic control systems do have their own output
device. This serves as an indicator, clearly showing the
user the logic state of the circuit.
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LEDs as indicators
LEDs are ideal indicators for a logic circuit – why is this?
 They require only a low current
and voltage to emit light.
 Their two states, on or off, match
the binary states of logic 0 and 1.
0-5 V
0V
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In real circuits, we must use
a protective resistor in series
with the LED.
This ensures a voltage of
around 0.7 V across the
LED, preventing the current
from becoming damaging.
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Outputs and relays summary
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Glossary
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Anagrams
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Multiple-choice quiz
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