Download 1.1.3 Simple Digital Outputs Word Document | GCE AS/A

Topic 1.1.3 – Simple digital outputs
Learning Objectives:
At the end of this topic you will be able to;
 Recall the use of an LED and resistor to indicate the output state of
a logic system;
 Understand that a logic gate output can be configured to either
source or sink current.
1
Module ET1
Introduction to Analogue and Digital Systems.
Simple Output Devices.
In topic 1.1.2 we examined how signals are processed for entry into a logic
system. In this unit we will be concerned with the output stage of the logic
system, since if we do not know when the output of a system is switched on,
there is no point in designing the system in the first place.
The output current from a standard logic gate is quite small (20mA) and
this is far too small to operate the majority of output devices like motors,
solenoids, relays and lamps. In order to operate these devices we need an
extra interface unit which is not covered until ET2, therefore none of these
devices will be discussed for the rest of this unit, nor will they be asked for
in the ET1 examination.
The only output device that we will consider is the Light Emitting Diode or
LED for short. This is a component that comes in various shapes, sizes and
colours as shown below:
2
Topic 1.1.3 – Simple digital outputs
The circuit symbol for an LED irrespective of its colour or shape is :
The LED gives off light when a small current 10mA flows through it. This
makes it perfectly suitable for use with logic gates as an output device. The
LED is normally used in the following way.
OR Gate
Output
0
1
Status of
LED
Off
On
The resistor is included to provide some protection to the LED and will be
discussed in more detail in ET2. You will not be asked to perform any
calculations involving this resistor in ET1 questions.
Now when the output of the OR gate shown above is at Logic 0, (0V) there is
no difference in voltage between the output of the logic gate and the 0V line,
so the LED will be off. However when the output of the OR gate reaches
Logic 1 (High Voltage) then there will be a difference in voltage between the
output of the logic gate and the 0V line, a current will therefore flow through
the resistor and LED, light will be produced indicating that the output is on.
Note: Any gate could have been used to drive the LED.
In this case we say that the output of the logic gate is ‘sourcing’ current, i.e.
current is flowing out of the logic gate into the resistor and LED, and then
down to the 0V line of the power supply.
There is another way in which we can use the LED to provide the opposite
function to that described above. This is less common but could appear on the
examination paper as an alternative circuit application.
3
Module ET1
Introduction to Analogue and Digital Systems.
Study the following circuit carefully:
OR Gate
Output
0
1
Status of
LED
On
Off
In this circuit the LED has been connected to the +9V supply. Let us consider
the two possible output conditions of the OR gate once again and see the
effect on the LED.
When the output of the OR gate is at Logic 0 (0V), there is a difference in
voltage between the power supply and the output of the logic gate, current
therefore flows through the resistor and LED and it lights up. When the
output of the logic gate is Logic 1 (9V) there is no difference in voltage
between the power supply and the output of the logic gate, therefore no
current will flow and the LED will be off. Note: Any gate could have been
used to drive the LED.
In this case we say that the output of the logic gate is ‘sinking’ current, i.e.
current is flowing into the logic gate from the power supply and through the
LED.
In many circuits that you will come across in the following chapters the
output device. i.e. the LED will be represented by a circle at the end of the
logic system to save complicating a logic diagram with resistors and LED
symbols, but this is just to save time. In any practical circuit you construct,
you will have to add the resistor and LED to your output to be able to see
when it is on.
So now we have the basics – lets start designing ! Next stop topic 1.2
4
Topic 1.1.3 – Simple digital outputs
Self Evaluation Review
Learning Objectives
My personal review of these objectives:



Recall the use of an LED and
resistor to indicate the output state
of a logic system;
Understand that a logic gate output
can be configured to either source
or sink current.
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
5