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Topic 2.6.4 – Interfacing Outputs
Learning Objectives:
At the end of this topic you will be able to;
 select suitable comparators, transistors, and MOSFET’s for
connecting to signal sources and driving outputs such as lamps,
buzzers, loudspeakers, motors, solenoids and relays;
 state the need for diode protection for comparators, transistors, and
MOSFET’s.
1
Module ET2
Electronic Circuits and Components.
Interfacing Outputs.
In our previous topics we have discussed the use of op-amps, comparators,
transistors, and MOSFET’s in electronic switching circuits but limited the
output devices to LED’s or Lamps in order to concentrate the analysis of the
circuits on understanding the processes involved rather than introduce a wide
variety of output devices, to complicate matters further.
We have now examined all of the interfacing units needed for the AS course,
so it is now worth spending a little time looking at the typical range of output
devices you might come across in examination questions. For completeness we
will include all of the available output units as a reference point.
1.
LED’s – probably the most basic of output indicators. Provides light
output in a range of colours.
Symbol.
Picture:
Properties.
LED’s are notable for their high degree of efficiency, very long service
life and their impact resistance. Current limiting resistors are
necessary for safe operation at the low operating voltage, typically 2V
with a current requirement of 10-20mA. Damaged easily by reverse
voltages of more than 5V. The point-form light source enables precise
directing of the light. The radiated power decreases slightly as the
temperature increases. LED’s can be driven directly by logic gates, opamps, and comparators.
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Topic 2.6.4 – Interfacing Outputs
2.
Lamps – a simple indicator, now being phased out in many low power
applications by LED’s. Provides light output usually in white with a range
of colours being provided by the use of coloured glass or filters. A wide
range of output power is available from a few mW to many kW.
Symbol.
Pictures:
Properties.
Lamps are used for a number of general purpose applications, such as
panel indicators, to provide illumination for a room or street, to lighting
up a stage in the theatre, or in a searchlight scanning the skies as part
of a lighting display.
Most lamps cannot be driven directly by logic circuits, or op-amps. Some
small lamps needing just a few hundred mA could be driven by some
comparators, but they are usually found being used with transistor
switches, and MOSFET’s. Mains operated lamps would be connected via a
relay (see later).
Component catalogues will specify the operating voltage and current for
the lamp which will enable you to design a suitable driving circuit.
3
Module ET2
Electronic Circuits and Components.
3.
Buzzers / Sirens – a simple audible output device which will run off an
simple DC source.
Symbol.
Pictures: Buzzer (left) / Siren (right)
Properties.
Even low power buzzers requiring just 20-30mA can be quite loud, but
for alarm systems sirens are used which offer much greater volume up
to 115dB at a distance of 1m.
Some low power buzzers requiring less than 20mA can be driven directly
by logic circuits, or op-amps. Most low power buzzers needing just a few
hundred mA could be driven by some comparators. Usually they are
found being used with transistor switches, Sirens usually require the
use of a transistor switch or MOSFET. Mains operated sirens would be
connected via a relay (see later).
Component catalogues will specify the operating voltage and maximum
current rating for the buzzer / siren which will enable you to design a
suitable driving circuit.
4
Topic 2.6.4 – Interfacing Outputs
4.
Loudspeakers – another means of producing sound but only works with an
a.c. or pulsing signal – will not produce a sound with a constant d.c.
source.
Symbol.
Pictures:
Properties.
Loudspeakers work by using the electrical current to create a magnetic
field around a coil which reacts to the magnetic field of a permanent
magnet and causes movement of the paper cone, to set up a sound wave
in the air. In order to produce a sound this electrical current must be
changing otherwise the cone will stop moving, and therefore
loudspeakers are not suitable for use in a purely d.c. circuit.
An astable output may cause a sound to be heard as long as the
frequency of operation is high enough.
Most loudspeakers cannot be driven directly by logic circuits, or opamps or comparators. Loudspeakers are usually found in amplifier
circuits incorporating transistors and MOSFET’s but these are outside
the scope of the AS course and appear in module ET5, so you will not be
asked any questions about these circuits in this examination.
Component catalogues will specify the coil resistance and power for the
loudspeaker which will enable you to design a suitable driving circuit.
5
Module ET2
Electronic Circuits and Components.
5.
Motors – a simple device for creating movement as the output of an
electrical circuit, e.g. model car. Motors come in all different shapes and
sizes, with operating currents starting at 300mA running up to several
amps depending on the size of the motor.
Symbol.
Pictures: Small and Large motors
Properties.
Motors are used in a wide range of electronic products where motion is
the required result. The power of the motors varies considerably
depending on the application but they all require quite large currents to
make them work successfully, particularly at startup where the load
current can be very high.
Virtually all motors cannot be driven directly by logic circuits, op-amps
or comparators. Small scale motors requiring a current of less than 2 to
3 A can be driven by transistor switches but the large linear region and
the time taken to switch from cut off to saturation can be a problem
with transistor circuits. MOSFET switches are the primary source of
driver circuits for motors. Mains operated motors would be connected
via a relay (see later).
Component catalogues will specify the operating voltage and no load
current for motor which will enable you to design a suitable driving
circuit.
Motors cause large voltages to be generated when they are switched
off which may damage the transistor / MOSFET and therefore diode
protection must be included to protect the switching ciruit. (see later)
6
Topic 2.6.4 – Interfacing Outputs
6.
Solenoids – these are used primarily as locking mechanisms. The solenoid
consists of a large coil of wire which creates a magnetic field when
current flows through it. This attracts an iron core in the shape of a
bolt into the coil. When the current is switched off a spring usually
pushes the bolt back into place, unless the solenoid is mounted vertically
in which case gravity causes the bolt to fall back into place.
Symbol.
Pictures:
Solenoid
Properties.
Solenoids as well as being used electronic locks are used in cars to
engage the starter motor with the engine when the engine is first
started. Solenoids require large currents to be able to create a strong
enough magnetic field to attract and hold the iron bolt, and they tend to
be physically large.
Solenoids cannot be driven directly by logic circuits, op-amps,
comparators or transistor switches. They have to be operated by
MOSFET’s due to the very high current demand needed. Mains operated
solenoids would be connected via a relay (see later).
Component catalogues will specify the operating voltage and power for
the solenoid which will enable you to design a suitable driving circuit.
Solenoids cause large voltages to be generated when they are switched
off which may damage the MOSFET and therefore diode protection
must be included to protect the switching ciruit. (see later)
7
Module ET2
Electronic Circuits and Components.
7.
Relays – an electromechanical switch which uses a low voltage coil to
operate a pair of switch contacts that are electrically isolated from the
low voltage circuit. This allows mains voltage devices like sirens, motors
and solenoids to be controlled by a low voltage control circuit.
Symbol.
Relay
Picture:
Connections
to external
circuit
Properties.
Relays are used primarily to switch on mains powered appliances from
low voltage control circuits.
The contacts in the relay may be switching very large currents >10A and
the fact that the contacts are breaking this current may cause
electrical arcing to occur which will eventually cause the contacts to
wear out and the relay will need to be replaced. It is for this reason
that the MOSFET is now replacing relays for the control of large
currents.
Most relays cannot be driven directly by logic circuits, or op-amps.
Some small relays needing just a few hundred mA could be driven by
some comparators, but they are usually found being used with transistor
switches, and rarely with MOSFET’s.
Component catalogues will specify the operating voltage and coil
resistance for the relay which will enable you to design a suitable driving
circuit.
Relays cause large voltages to be generated when they are switched off
which may damage the transistor / MOSFET and therefore diode
protection must be included to protect the switching circuit. (see later)
8
Topic 2.6.4 – Interfacing Outputs
Protecting Devices from switch off voltages.
In the previous discussion about output devices we have mentioned that
certain output devices, namely motors, solenoids, and relays can damage
transistor switches or MOSFET’s when they switch off because they
generate a very high reverse voltage.
You may think that protecting the transistor / MOSFET would be very
complicated but in reality it is very easy, and uses a component with
which you are already familiar, the silicon diode introduced in topic 2.1.
The diode is connected into a circuit as follows:
9V
Protection
diode
Load
0V
The circuit has been shown here with a MOSFET, but it could just as
easily be a NPN transistor, there would be no change to position or
orientation of the protection diode. The load represents either a motor,
solenoid or relay.
The diode ensures that when the load is switched off, any high voltage
generated is clamped by the diode to a maximum of 0.7V, which causes no
damage to the transistor or MOSFET.
9
Module ET2
Electronic Circuits and Components.
Sometimes in the examination you are asked to show how relay can be
used with a sensing subsystem to operate a mains appliance. For example
turning on a mains heater in a greenhouse when the temperature drops below
a specified temperature. A possible solution is shown below.
9V
Protection
diode
Relay
Heater
240V
a.c.
0V
Can you remember why the Schmitt inverter is included in the above circuit ?
No examination questions have been set on this topic as the addition of a
protection diode, and use of a relay to switch on mains appliances are the only
new items not covered in previous questions. These additions would just be
one part of a longer question about transistors / MOSFET’s
You have now completed the ET2 syllabus and will shortly be taking your final
exam. Remember practice makes perfect, so keep reviewing the solutions to
the questions given and good luck in the examination.
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Topic 2.6.4 – Interfacing Outputs
Self Evaluation Review
Learning Objectives
My personal review of these objectives:



select suitable comparators,
transistors, and MOSFET’s for
connecting to signal sources and
driving outputs such as lamps,
buzzers, loudspeakers, motors,
solenoids and relays.
state the need for diode protection
for comparators, transistors, and
MOSFET’s.
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
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