Download Input Components

Input Components
Moisture Sensors
Moisture sensors consist of a flat plate with two
conductors etched upon them. They can come in
different shapes. The type illustrated gives
maximum intimacy between conductors for a
given surface area. Another type takes the form of
a long strip with the conductors running parallel to
themselves. When moisture bridges the gap, the
resistance is lowered between the conductors. This
drop in resistance can trigger atransistor circuit,
which in turn could operate a relay.
Uses:- Plant watering in dry weather, sensing
when it rains, flood warning, sensing damp
conditions in electrical equipment etc.
Light DependentResistor (LDR)
The resistance of these devices decreases when
light falls on them. A typical example is the ORP
12 type which will vary its resistance from about 2
Megohms in the dark to 100 ohms innormal room
lighting.
LDRs are frequently used as sensors inlight
activated switch circuits, for example, to switch
on lights in the evening when it starts to get dark.
Thermistor
The resistance of these devicesdecreases when
they are heated (negative temperature coefficient
type).
Typical resistance values are 1 kilohm at 25°C
and 80 ohms at 100°C.
Thermistors are suitable for temperature
controlled switches and temperature measurement.
Control Components
Transistors
Transistors are used as electronic switches and
current amplifiers. They have three "legs" - base ,
collector and emitter. A small current to the
base switches on the transistor allowing a much
larger current to flow between collector and
emitter. The ratio of the collector current to the
base current is known as the "amplification" or
"gain" of the transistor
(hfe = Ic/Ib).
They come in various different shapes and sizes
and it is important to decide which type is
required at the outset.Factors to be considered are
voltage and power handling capacities, cost and
availability.
Transistors require a base voltage of at least
0.6V with respect to the emitter to "turn on".
(Darlington Pair transistors require 1.2 V). A
limiting resistor - typically 1K is required in the
base circuit of the transistor stage to prevent
overrunning and hence overheating and
destruction of the transistor. Transistors come in
NPN and PNP types. Itis important to use the
correct type when designing and building
circuits.
555
This chip is designed to function as a monostable
(one stable state) multivibrator or as an astable
(freerunning) multivibrator. It contains the
equivalent of approximately 20 transistors,
resistors, diodes etc. It is used for timing or
switching circuits which give pulsed or digital
outputs.
Data:
Pin 1 connected to the negative (-ve) supply
2 TRIGGER a voltage greater than 1/3 supply
volts enables the chip.
3 OUTPUT is equal to the supply voltswhen the
output is on (600mW max).
4 RESET when connected to positivesupply volts.
5 CONTROL OUTPUT applying a variable
voltage to this pin varies the frequency of the
output.
6 THRESHOLD a voltage higher than the
threshold voltage (2/3 supply volts)turns on the
output.
7 DISCHARGE when the output is turned on
the pin goes to 0v allowing discharge of
any connected capacitor.
8 positive supply voltage (+ve) (4.5v -16v)
Uses: Oscillators, triggers, inverters, timers
741 OPERATIONAL
AMPLIFIER (OP-AMP)
This chip is designed to accept two inputs on pins
2 and 3. The difference between the inputs is
amplified to give the output. If the voltage on pin
2 is greater than pin 3 the output is negative. If pin
3 is greater than pin 2 the output is positive. The
Op-Amp gives a voltage gain as opposed to a
transistor which gives a current gain.
Uses: Switching, amplifiers, oscillators
and analogue computing.
Data:
Pin
1 OFFSET NULL not normally used
2 inverting input
3 non inverting input
4 0V is connected to the zero supply
5 OFFSET NULL not normally used
6 OUTPUT
7 positive (+ve) supply voltage (+3V to+18V)
OFFSET NULL is used to balance output in
high gain applications.
Output Components
LEDs
Light Emitting Diodes (LEDs) come in different
colours, shapes, sizes and brightness. The more
common colours being red, green and yellow. They
are also available in bi and tri - colour versions
Round ones come in 3, 5 and 8mm sizes. There are
also rectangular, square and bar types. Flashing
types are available and seven segment displays are
made of a number of LEDs in a particular pattern.
LEDs are easily damaged by excess current
therefore it is normal to connect the LED to the
supply via a series current limiting resistor. The
value of this resistor is not critical but is usually
within the range 100 - 330R. Polarity must be
observed whenconnecting LEDs (it should be
noted that LEDs are damaged by reverse biased
voltages larger than about 6 volts). The Anode is
connected to the positive supply via the limiting
resistor. The Cathode is connected to the negative
supply.
Bulbs
Bulbs are used as a form of illumination indication
and the type commonly used has a filament coil
which glows white hot when a current passes
through it. The filament is in a space filled with
an inert gas to aid in light output andto prevent
oxidisation of thefilament.Bulbs consume more
power than LEDs but give a higher light output.
There are two common types of bulb usedin
schools MES and LES - which are smaller in
physical size than MES types. They may be
bayonet or screw cap but screw types are more
common.
Motors
A device for changing electrical energy into
rotational movement. The turning power or torque
of a motor can vary greatly. Generally speaking
small motors have low torque whereas large motors
usually have a high torque rating. Motors come in
ac and dc versions, dc types of between 3 - 12 volts
being most suitable for general use. The current
direction controls the direction of rotation for dc
motors.It is usually necessary to geardown a motor
as their speed of rotation typically 2400 rpm is too
great This can be achieved either mechanically or
electronically. DC motors have the advantage of
being relatively
inexpensive and can be obtained in high torque
versions which are most suitable for driving
models.
Solenoid
A solenoid is a coil of wire wrapped around a
former. The plunger is free to move and is attracted
into the coil when a current flows in the coil. When
the current is turned off, a spring pushes the
plunger out of the coil.Solenoids are used for
converting electrical energy into mechanical energy
(transducer) in the form of linear motion. They are
current and voltage rated and attention must be
taken to prevent these limits from being exceeded.
Uses include electrical door locks and model
animation.
Bells
These are electromagnetic devices with contacts
arranged so that once the coil is energized the
armature is attracted to it and the circuit is broken.
Once broken the coil is de-energized and the
armature springs back to complete the circuit once
more. Each time the armature is attracted to the
coil the hammer strikes the bell. This continues in
an oscillatory manner until the power switched off.
Bells normally work on low voltagetypically 6
volts. Bells are used as outputs for alarm circuits.
Loudspeaker
The loudspeaker is an electro-mechanical device
that turns alternating currents into audible sounds.
When a signal of sufficient amplitude is applied to
the coil it produces a magnetic field which attracts
or repels the permanent magnet. This produces
movement which is amplified by the diaphragm to
give sound. Speakers can reproduce sounds in the
audio spectrum between approximately 20 - 16,000
Hz. Small speakers handle high frequencies well,
whereas large speakers are mainly suited to low
frequencies. A combination
of two or more speakers can reproduce
frequencies over the whole audio range. Where hifi quality is not required, a small speaker will
normally suffice.
Buzzers
Buzzers are usually of the single tone variety and
are normally rated between 6 and 24 volts. They
are suitable for alarm circuits, test equipment,
timing and control. To acheive maximum sound
ouput from the device it should be firmly mounted.