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PROVIDING A REFERENCE SIGNAL
The simplest form of reference signal is obtained
using a potential divider.
As we learned earlier the official name for
electrical pressure is voltage. However, voltage can also be called potential.
A battery has 9V of electrical pressure at the + ve terminal and 0V (no
electrical pressure) at the -
ve terminal.
We often say there is a potential difference
of 9V between the two terminals. If there is a potential difference
between two points in a circuit, current will flow. Potential Difference
is usually shortened to p.d.
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| The diagram on the right shows a two resistor potential
divider. The potential difference across the two resistors is +V. If we
measure the voltage at the mid point V out we
find that the two resistors divide the potential into two parts. If the
two resistors are equal then the potential is divided into two equal
parts. There is a general formula which can be used to determine the
output voltage. It is:
Where Vin is the supply voltage.
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Back
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| This is the simplest form of potential divider. It is
cheap but not easily adjusted. The only way to change the reference
signal is to change one or both resistors. In practice it is very
difficult to predict V out with
any accuracy due to the tolerance of the resistors. |
| This second circuit shows the use of a potentiometer.
A potentiometer is more expensive, but it is easy to
change the value of V out.
The sensitivity of the potentiometer will be low if it has a high
value of resistance and its sensitivity will high if it has a low value
of resistance.
This is because with a high value of potentiometer, even small
movements of the slider will cause large signal changes at the output. |
Back
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| This third circuit shows a combination of the previous
two and allows for an increased sensitivity. It also ensures that some
resistance is present when the potentiometer is set to the maximum or
minimum value.
With all potential dividers the current flowing
through the resistors should be as small as possible or the resistors
will heat up.
If a large current is taken from the potential
divider the signal will be reduced. For this reason, the signal produced
by potential dividers (particularly when used as input sensors) should be
fed into signal processing circuits with a high input impedance
(for the moment think of impedance as resistance). |
Back
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LIGHT SENSORS
The most common system for sensing light intensity
involves the light dependent resistor or LDR. The LDR can experience a wide range of resistance
change - from about 100ohm in very bright light to several megohms in
total darkness. The most common type of LDR is the ORP 12.
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| This is usually
incorporated in a potential divider as shown.
The use of a variable resistor in the other arm of
the potential divider allows the level of the signal to be adjusted.
The resistance of the LDR increases as the light
level decreases so in this case the signal level will fall as it gets
dark.
Back |
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| Changing the position of the LDR and variable
resistor allows the signal to change in the opposite direction. In this
arrangement the signal level will fall as it gets light.
The LDR is
relatively expensive, quite large and rather slow in its response to
rapid changes of light level. Its response to different wavelengths of
light is very similar to that of the human eye.
Back |
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| This Light sensor is based upon a photo
transistor.
The output is high when in the dark
In bright light a small base current is generated as
photons of light fall on the base region and the transistor switches on
Details of the cost of these transducers,
their size as well as their electrical characteristics and requirements
can be found in manufacturers catalogues and data sheets. A number of
these can be found in and around the technology department.
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TEMPERATURE
SENSORS
The most common temperature sensing system involves a
temperature dependent resistor (thermistor) used in a potential divider.
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| This is a very similar arrangement to light sensing
and, like the light sensor, the positions of the thermistor and variable
resistor can be reversed to produce a signal that moves in the opposite
direction when the same temperature change occurs.
The thermistors shown are negative temperature
coefficient devices (n.t.c.) and are by far the most common. Their
resistance decreases as the temperature increases. It is possible to buy
p.t.c. (positive temperature coefficient) devices.
There are many different types of thermistor - both
different shapes and sizes as well as different values of resistance over different temperature ranges. The manufacturers catalogues
and data sheets give more details. |
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The value of the variable resistor should be chosen
to ensure that the signal will have the correct range of values over the
required temperature range.
For example, let us suppose we wish to sense a
temperature at about 25 °C. The thermistor chosen has a resistance of
about 4.7 K ohm at this temperature. (The numerical coding on resistors
can be found in most electronic text books) If the signal we require is
to be about half the supply voltage then a fixed value resistor of 4.7 K would put the signal in approximately the right place.
A variable resistor of about twice this value, that
is, 10 K, would allow us to adjust this signal and we would be using
the variable resistor with its wiper in about the centre position which
is good practice.
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SOUND SENSORS
Sound sensing is achieved with a microphone. Some
microphones are active transducers whilst others are passive.
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The carbon microphone is a passive transducer and can
be connected into a simple potentiometer circuit.
Changes in sound levels produce resistance changes in
the carbon.
This causes small changes
in the output voltage which can then be amplified.
Back
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Crystal microphones, on the other hand, are active
transducers and generate e.m.f. when they receive sound.
This has a high output impedance and would normally
send its signal directly to an amplifier which is capable of amplifying
this signal, i.e. one with a high input impedance.
Back
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POSITION SENSORS
Position sensors are often known as limit switches
and produce a digital output. They can be light - operated, pressure
operated, (e.g. micro switches) or magnetically operated, and are usually
arranged in two configurations, depending on whether the signal change
required is: high - going (rising edge triggered) or low - going (falling
edge triggered) when the switch is closed.
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| One slightly different switch is the tilt switch which
is usually a sealed container with mercury making a connection between
two contacts when the switch is in an upright position. It is connected
in exactly the same way as the micro-switches above. |
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PRESSURE
SENSORS
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The simplest form of pressure sensor is given by some
form of switching arrangement which will give a two-state output. The
switch can be a simple push-to-make type or, for greater sensitivity, a
micro-switch may be employed.
There are some special arrangements, known as
pressure pads, which are used in intruder detection systems. Their
function is exactly the same as a push-to make switch. The arrangement
of components is shown in on the right.
The signal will rise if the switch is pressed. The
components can change position, if a low signal is required when the
switch is pressed.
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MAGNETIC SENSOR
A digital signal is usually obtained from a reed
switch arranged in a potential divider.
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In this arrangement the reeds
will close together when the field strength becomes large enough and the
signal will go from low to high.
Back
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The components can be exchanged to produce a signal
which will fall when the magnetic field strength is greater than the
predetermined value. This value is not adjustable.
Although reed switches are quite cheap they are also
quite large and fragile.
Back |
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MOISTURE SENSOR
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The moisture sensor is used to detect changes in
moisture levels. The resistance of a moisture sensor decreases as the level of moisture
increases.
The moisture sensor forms a potential divider whose output voltage is determined by the
level of moisture and the value of the variable resistor.
A 'high' voltage appears at the output
when the moisture content is high.
Back
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The moisture sensor is being used here
to detect changes in levels of 'dryness'. The resistance of a moisture sensor
increases as the level of moisture decreases.
The moisture sensor forms a potential divider whose output voltage is determined by the
level of 'dryness' and the value of the variable resistor.
A 'low' voltage appears at the output
when the moisture content is high.
Back
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ROTATION
SENSOR |
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The rotation sensor uses a potentiometer to measure rotation.
As the potentiometer dial is turned
clockwise and anti-clockwise, the resistance of the rotation sensor
increases and decreases. The value of the output voltage changes as the
potentiometer is rotated.
The voltage change can be calibrated
to give an indication of the degree of rotation
Back
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DEBOUNCED
SWITCH |
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Back
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Whenever a mechanical switch is pressed, the switch contacts will bounce, producing several very quick on and off signals.
Each of these signals would be counted by a counter
subsystem. To overcome this, a debounce circuit is used to produce a clean output signal from the switch.
A capacitor and a pair of schmitt triggers
with a feedback resistor are used to clean the signal. Schmitt triggers are logic gates that have both an upper and a lower threshold
level. This ability of the schmitt trigger to switch on and off at different voltage levels is known as
hysteresis.
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PULSE GENERATOR A
pulse generator provides an output signal that is successively high and then
low. Pulse generators can be built in a number of ways. Some of
these are shown below. |
Back
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The time between the signal going from high to low and going from high to low again
is controlled by the variable resistor
The pulse generator is an oscillator consisting of two inverters along with an electrolytic
capacitor. A potentiometer is used to control the pulse rate. The time period for one pulse when the potentiometer is on its maximum setting can be
calculated: The pulse generated has a mark: space ratio of 1:1, meaning that the high and low parts of the pulse last for the same length of time. |
|
THE 555 ASTABLE |
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The 555 astable, or oscillator, subsystem uses a 555 timer IC to provide an output signal that constantly switches between high and low
states. It is similar to a pulse subsystem but is able to provide more control over the output signal.
The 555 astable is based on the 555 timer IC. The time that the output signal is
high is known as the mark of the pulse. The time that the output signal is
low is known as the space of the pulse.
A mark/space ratio is used to show how much longer the mark time is compared to the space
time. |
Back
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The basic arrangement is shown above
right. R1, R2 and C1 are external components whose values fix the frequency
of the stream of continuous square wave pulses produced
automatically at the output (pin 3).
The period T of the square wave is
given by
T = 0.7 (R1+2R2)C1
Where again T is in seconds if R
is in Megohms and C is in microfarads.
The frequency f = 1/T
The duty cycle or mark to space
ratio is the ratio of the time the output is on divided by
the time the output is off. For a true square wave this is 1.
If you look carefully at the pin out diagram and circuit you will see
that the timing capacitor charges through both R1 and R2 and discharges
through R2 only. This means that a rectangular wave is produced unless
R1 is very small in comparison to R2. |
| THE
555 MONOSTABLE |
|
Back |
The 555 monostable subsystem provides an output signal that stays high for a period of time before returning to
low. It is able to provide a range of time delays up to about 20 minutes
with reasonable accuracy.
The 555 monostable is based on the 555 timer IC.
A single pulse is generated by the monostable when it is triggered by a negative-going input pulse, such as that produced by
the push switch connected to the trigger input. Once triggered the output remains high for
the timed period. This time period can be calculated using the formula:
T= 1.1 R x C
where R is in M ohms and C is in µF |
| AND
GATE |
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| INPUTS |
OUTPUT |
| A |
B |
Q |
| 0 |
0 |
0 |
| 0 |
1 |
0 |
| 1 |
0 |
0 |
| 1 |
1 |
1 |
|
When two input
signals are fed into a two input AND gate, the AND gate subsystem
provides an output signal that is high only if both input signals are
high. Otherwise, the output signal is low.
The diagram shows an AND logic gate: The output from the AND gate is
shown in the truth table above, with 0 meaning “low” and 1 meaning
“high”.
Back
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| COMPARATOR |
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The comparator subsystem provides an output signal that stays high while the input signal is higher than the reference signal or threshold. The output signal remains low otherwise.The comparator is particularly useful for providing a large change in signal when the input signal only changes slightly and for converting an analogue signal into a digital signal.
The schmitt inverter also allows you to convert an analogue signal into a digital signal. |
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The comparator circuit uses an operational amplifier, or opamp with
no feedback, to amplify the input signal.
This is compared
Operational amplifiers have two inputs, an inverting input ('-') and a non-inverting input
('+').
A potentiometer connected to the inverting input provides control over the reference signal voltage.
Back |
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| COUNTER |
A counter subsystem counts the number of signal pulses
applied to its clock input. A change from low to high at the input increases or decreases the count by
1.
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As you can see the counter circuit uses several integrated circuits (ICs) to count and display values.
In this circuit the input (clock) signal is sent to a 4029B counter IC which performs the actual counting. Outputs in the form of binary signals are then passed to
a 4511B decoder IC that works out which segments on the 7-segment display should be
lit. The display itself is made up of 7 LED (light-emitting diode) segments linked together via a common cathode connection. Each diode has its own current-limiting resistor.
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The output state of the counter can be shown in decimal
(0 - 9) |
or hexadecimal
(0 -9 then A - F)
on a 7-segment |
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| An output signal provides a carry signal that can be connected to another counter. This output signal goes high-low-high when the count reaches 9 (decimal), 15 (hexadecimal) or 0 (when the counter is counting downwards).
The 'clock' pulse can be generated from a pulse
generator for regular pulses or any suitable input unit
provided a debounced switch unit is used to
provide a “clean” signal to the clock input.
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| Four output signals labelled
1, 2, 4 and 8 give the binary output for the current
value. The table opposite shows which logic signals are produced at each
ouput when counting upwards in decimal.
A reset connection labelled R allows you to reset the count back to zero. The count will be reset when a high (logic 1) signal is sent to this connection. |
| Decimal |
Bit 1(1) |
Bit 2 (2) |
Bit 3 (4) |
Bit 4 (8) |
| 0 |
0 |
0 |
0 |
0 |
| 1 |
1 |
0 |
0 |
0 |
| 2 |
0 |
1 |
0 |
0 |
| 3 |
1 |
1 |
0 |
0 |
| 4 |
0 |
1 |
1 |
0 |
| 5 |
1 |
0 |
1 |
0 |
| 6 |
0 |
1 |
1 |
0 |
| 7 |
1 |
1 |
1 |
0 |
| 8 |
0 |
0 |
0 |
1 |
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| DELAY |
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This delay subsystem can produce a delay of
a few seconds when the input signal is taken high.
The timing period begins after the input signal has returned from high to
low. If the input signal changes from low to high during the timing period, the timing starts
again.
For a longer length of time delay, use a 555 monostable subsystem. |
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The delay circuit uses the ability of an electrolytic capacitor to store
charge. A high input signal causes the capacitor to discharge through the transistor. During this period, the output signal is forced high by the right-most
inverter.
Timing begins as soon as the input signal goes low. The transistor switches off causing the capacitor to charge through the variable
resistor. Once the capacitor has charged to about 70%, the right-most inverter will trigger the output back to
low.
Back |
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| INVERTER |
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The inverter subsystem, which mimics a NOT gate, provides an output signal which is opposite to the input signal.
When the input signal is low, the
resistor holds the output high. When the input signal is high, the
transistor switches on and the output signal goes low.
The schmitt inverter provides a similar action and can be connected directly to analogue signals.
Back |
| NAND
GATE |
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When two input signals are used with a NAND gate, the NAND gate subsystem provides an output signal that is low when both input signals are high. Otherwise, the output signal is
high.
NAND gates are versatile. Two NAND gates can be connected together to make an AND gate, three NAND gates to make an OR gate and four to make a NOR gate.
See Combinational Logic
Back |
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| LATCH |
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The positive latch is a resetable memory block.
It produces an output signal that goes high and remains high when the input signal has been high. Pressing the push switch resets the output signal back to
low.
The positive latch circuit uses a pair of cross-coupled NOR gates to provide a simple form of
memory. |
The negative latch is a resetable memory block.
It produces an output signal that goes high and remains high when the input signal has been low. Pressing the push switch resets the output signal back to
low.
The negative latch circuit uses a pair of cross-coupled NAND gates to provide a simple form of
memory. |
| This form of latch is often referred to as an RS bistable or flip-flop. RS stands for reset and set.
Back |
| NOR
GATE |
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When two input signals are used with a NOR gate, the NOR gate subsystem provides an output signal that is high when both input signals are low. Otherwise, the output signal is low.
Nor gates can also be used to build other types of
logic gates
Back |
| OR
GATE |
When two input signals are used with an OR gate, the OR gate subsystem provides an output signal that is high if either of the input signals are high. Otherwise, the output signal is low.
Back |
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| SCHMITT
INVERTER |
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The schmitt inverter subsystem provides an output signal that is opposite to the input
signal.
When the input signal goes above the upper threshold, the output signal goes high and remains high until the input signal falls below the lower
threshold.
This action provides a cleaner output signal and makes the schmitt inverter ideal for converting analogue signals into digital signals.
Back |
| XNOR
GATE |
| The exclusive-NOR gate (XNOR) subsystem provides an output signal that is high if either both input signals are high or both input signals are low. Otherwise, the output signal is low.
Back |
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| XOR
GATE |
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The exclusive-OR gate (XOR) subsystem provides an output signal that is low if either
both input signals are high or both input signals are low. Otherwise, the output signal is high.
Back |
| INVERTING
AMPLIFIER |
| The inverting amplifier is used to amplify and invert an analogue input
signal.
The signal is amplified relative to a reference signal, which is fixed
by the potential divider across the power supply voltage.
The inverting amplifier circuit uses an operational amplifier, or opamp, to amplify the
signal.
Operational amplifiers have two inputs, an inverting input ('-') and a non-inverting input ('+').The amount of amplification, or gain, can be varied by changing
R
Back |
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| NON-INVERTING
AMPLIFIER |
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The non-inverting amplifier is used to amplify an analogue input
signal.
The signal is amplified relative to a reference signal, which is fixed
by the two resistors.
The non-inverting amplifier circuit uses an operational amplifier, or opamp, to amplify the
signal.
The amount of amplification, or gain, can be varied by changing
R
Back |
| DIFFERENCE
AMPLIFIER |
| The difference amplifier is used to compare two input signals.
The difference amplifier circuit uses an operational amplifier, or opamp, to subtract
one input signal from the other to determine the difference between two
voltages.
The gain is usually set to 1 by making Rf
equal to R in
Back |
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| SUMMING
AMPLIFIER |
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The summing amplifier subsystem is used to combine two input signals
together.
The summing amplifier circuit uses an operational amplifier, or opamp, to combine and amplify the input
signals.
If the input and feedback resistors are equal then the
output voltage equals the voltage on input A + the voltage on input
B
(Vout = V1 + V2)
Back |
| OUTPUT
TRANSDUCERS
At the output of an
electronic system there must be some means of converting the electrical
signal back into some other form of energy. The device that performs
this conversion is known as an output transducer. Almost all
output devices require large currents to operate whilst most signals
have a low electrical current.
It is very often
necessary to amplify the current just before the output transducer.
Details of current amplification subsystems are shown below.
As well as amplifying
the current it may be necessary to code or decode the signal in some
way. A good example is the decoder-driver required for a seven-segment
light emitting diode (LED) display.
There are two
possibilities for passing current through output transducers. the
current may flow from the positive power supply, through the output
transducer and back into the driver, when its output is low. This is
known as sinking current. The other option is for
the current to come from the driver, when it has a high output, pass
through the output transducer and go on to the 0 V power supply rail.
This arrangement is known as sourcing current. both
options are represented diagrammatically below. |
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When a signal emerges
from a processing subsystem it has very little energy. It rarely has
enough energy to drive anything more than an LED. To be of any use the
power of the signal has to be boosted so that there is sufficient energy
to drive the required output device. Transducer drivers perform this
function. |
| TRANSISTOR
DRIVER |
|
The transistor driver is essentially
an electronic switch. It provides an output signal that is low when the
input signal is higher than 0.7 volts.
The output signal from the transistor
has a much larger current than the input signal, this increase in
current is known as the gain of the transistor. The transistor driver
acts as an inverter because the output signal is always the inverse of
the input signal. |
|
The
output device is connected between the collector and the positive supply
rail (the resistor is a load resistor). A base resistor Rb
must always be present to limit the currents Ib and Ic.
The base emitter junction is
effectively a forward biased p-n diode so the p.d. across it (Vbe)
cannot rise much above 0.6V. If the base resistor was left out any Vbe
above 0.6V would cause excessive currents to flow in Ib and Ic
and the transistor could be destroyed by overheating.
Back |
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| DARLINGTON
DRIVER |
|
The current gain
can be increased by using two transistors connected together as shown on
the right.
This arrangement
is known as a Darlington Pair. As well as increasing the
current gain it also increases the input impedance.
The gain of a darlington pair is found
by multiplying the gain of the individual transistors together.
Vbe is doubled to 1.4 volts
with this arrangement. Ic max is usually in the region of
1Amp
Back |
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| TRANSDUCER
DRIVER |
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If currents larger than 1Amp are
required a high power transducer driver can be built using a MOSFET
(metal oxide field effect transistor).
Like bipolar transistors FET's have
three legs but they are called the gate, source and drain.
MOSFET's have a very high input
impedance and require only a very small gate current to operate.
This arrangement would act as an
inverter.
Back |
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| LED |
|
Light emitting diodes or LEDs are polarized
devices and must be connected the right way round in a circuit. Since
the maximum voltage to be applied across an LED is about 2 V
they almost always have a resistor connected in series. The value of the
resistor is calculated from the voltage of the power supply and the
current required by the LED.
LEDs come in three main colours: red,
yellow and green. Blue LED's are available but are very expensive. There
can be bought in a wide range of sizes and shapes - look at suppliers’
catalogues for more details. |
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| BULBS |
|
Bulbs are relatively
easy to use. A bulb will be rated according to the maximum safe voltage
and the current that will flow at this voltage.
A bulb rated at
6 V, 0.06A will have a resistance of 100 ohms when it is working
at normal brightness.
Back |
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| BUZZERS
and LOUDSPEAKERS |
|
Both these devices
convert the electrical signal into sound. A loudspeaker may need to have
the signal filtered after it is amplified and before it is received. |
|
A loudspeaker must be driven
from an alternating signal. The sound it produces will be at the same
frequency as the alternating signal. The most common impedance values
for speakers are
8 ohm and 64 ohm and the power rating will range from 100 mW up to a 100
watts or more.
Most buzzers require a steady
signal to produce a note at a fixed, predetermined frequency. The sound
output power is often quoted in decibels for a certain distance from the
device. Buzzers contain an oscillator circuit inside them and usually
have the polarity marked on the terminals. They must be connected in the
circuit the correct way round or else the oscillator may be permanently
damaged. |
Back |
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| MOTOR |
|
A d.c. motor converts
the electrical signal into rotational kinetic energy. Details of the
construction of d.c. motors can be found in most Physics textbooks.
Before connecting these devices to the outputs from processing
subsystems (we call this interfacing) it is necessary to know the
working voltage and the maximum current drawn by the motor. This will
enable the correct choice of driver to be made. |
|
Some d.c. motors tend to be
"noisy" - this is particularly true of cheap motors. The noise
referred to is not sound but electrical noise. Inside the motor there is
a part called the commutator which rotates against conductors
called brushes.
The commutator is made up of a series
of separate sectors with insulation between them and, as the brushes
pass from one sector to another, there is a switching of current. |
Back |
|
This rapid switching causes voltage
spikes to appear on the power lines and this can disturb the working of
the rest of the circuit. If electronic methods of suppression (usually a
resistor in series with the motor and a capacitor across the terminals
of the motor) are not successful, it may be necessary to use a separate
power supply for the motor and switch it on through a relay. |
| RELAY |
|
|
The relay subsystem is an electrically-operated switch. The relay switches when the signal coming into the driver is
high. It should be connected to a darlington or
transducer driver.
Relays use an electromagnetic coil to move the poles of a switch when powered. There are three pairs of connections known as common, normally open and normally
closed.
Back |
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The centre connection is the common (CO) connection and is connected to either
of the outputs depending on the state of the relay.
When not switched, the centre terminal is connected to the normally closed (NC)
terminal.
When switched, the centre terminal block is connected to the normally open (NO) upper terminal block.
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SOLENOID |
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A solenoid consists of
a coil of wire around a ferrous core. When a current flows through the
coil, it becomes a magnet, causing the iron core to move. It converts
the electrical signal into linear kinetic energy. A flywheel diode needs
to be incorporated into the subsystem. |
|
This is
because the solenoid is an inductive device, and when the magnetic field
is turned off it generates a large back e.m.f. which would
cause considerable damage to unprotected components in the driving part
of the system. The flywheel diode allows the energy to be dissipated by
providing an alternative path for the current. The circuit is shown on
the right.
Back |
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