aerospace bsc module 4 assignment for reference only

8/8/2019 aerospace bsc module 4 assignment for reference only

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Module 4Assignment

James Lock


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Assignment objectives

In this assignment I intend to do the following three things1) Describe in detail the operation and typical uses of a 555 timer integrated circuit.

2) Give an example of a 555 timer in a practical circuit and explain its role.

3) Compare and contrast the operation, uses and characteristics of Bipolar Junction

Transistors and Field Effect Transistors.

The 555 Timer IC is an integrated circuit (chip) implementing a variety of a timer and

multivibrator applications.( refhttp://en.wikipedia.org/wiki/555_timer_IC).

Below is the Pin output of the 555 timer.

Pin Description

1 Negative supply

2 Trigger Input

3 Output Signal

4 Reset Input

5 Voltage Control

6 Threshold Input

7 Discharge Path

8 Positive Supply

The positive and negative supply pins are self-explanatory, although it's worth noting thatsome variants of the 555 won't function reliably or at all if there isn't a sizable decoupling

cap nearby. It's a good idea to put 10F across these pins in any design.

Pin 3 exposes the output signal, the timing signal generated by the IC. Most standard 555

timers can sink (connect to ground) and source (connect to the positive supply) loads up

to 200mA. Some are limited to 20mA and some will have different capabilities whensinking or sourcing current.

The reset input is used to reset the timer or prevent it from operating. A falling edge onthis pin forces the output signal low and the circuit remains disabled as long as the reset

input is low. Many applications don't require this function and will just tie this pin

directly to the positive supply. Occasionally a design will leave it floating (not connected

to anything) .
http://en.wikipedia.org/wiki/555_timer_IChttp://en.wikipedia.org/wiki/555_timer_IC


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The trigger and threshold inputs are responsible for producing the timing. When the

voltage level presented to the trigger input is less than 1/3 of the supply voltage then the

output signal will be driven high, and if the voltage level on the threshold input exceeds2/3 of the supply then the output is driven low again. Alone this may not be useful, but if

these pins are connected to a RC circuit then the exponential voltage curve of a charging

capacitor will set the output signal high for a reasonably precise and repeatable period oftime.

Pin 5 allows the threshold voltage which is normally 2/3 of the positive supply to be

adjusted. This pin connects directly into the comparator which means that the appliedvoltage directly effects the voltage level on the threshold input (pin 6) required to drive

the output low and therefore also effects the period of the output signal. If the pin is to

remain unused unnecessary noise into the comparator circuit may be introduced. To

avoid this it should be bypassed with a 10nF capacitor.

Finally, pin 7 is connected to ground whenever the output is low. the actual purpose of

this pin is to provide a discharge path for an RC circuit attached to the trigger and

threshold inputs. However it may also be used as a second output which is open-collectorand inverted from pin 3.

The 555 has three operating modes which are Monostable, Astable and Bistable.

Monostable.

Whilst in this function the 555 timer acts as a one shot ,switch debouncing application. A

diagramme of the 555 timer in monostable operation can be found below.

During this mode R1 will be between 10k and 14m, the timing cap C should be between100pF and 1000uF and the R2 prevents false triggering.


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The 555 timer will do nothing until a low pulse is applied toThe trigger pin ( number 2),the low pulse is usually provided by connecting to ground via

A switch or transistor. The width of the pulse received is determined by the time constant

Of a RC circuit ( a circuit consisting of a capacitor C and resistor R).The pulse will stop when the capacitor charge equals 2/3 of the supply voltage.

By adjusting the values of R and C it is possible to widen or shorten the the triggerPulse. Below is a diagramme showing the relationships of the trigger signal, the voltage

on C and the pulse width in monostable mode. Example uses for this kind of circuit

includes an electronic lock or light sensitive alarm.

Astable mode

A 555 in Astable mode is basically an oscillator,which chages states by itself according to

the support components installed in the circuit.

In astable mode, the '555 timer' outputs a continuous stream of rectangular pulses having

a specified frequency. Below is a circuit illustrating a 555 timer configured to function asan astable multivibrator.An astable multivibrator is a timing circuit which has unstable

low and high states,due to this the out put continuosly toggles between low and high

states. This in turn leads to the generation of a train of pulses. This type of circuit istherefore also known as a pulse generator circuit.


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In this circuit, capacitor C1 charges through R1 and R2, eventually building up enoughvoltage to trigger an internal comparator to toggle the output flip-flop. Once toggled, the

flip-flop discharges C1 through R2 into pin 7, which is the discharge pin. When C1's

voltage becomes low enough, another internal comparator is triggered to toggle theoutput flip-flop. This once again allows C1 to charge up through R1 and R2 and the cycle

starts all over again.( refwww.ECElab.com) . An astable can be used to provide the clock

signal for circuits such as counters.

Bistable mode

The circuit is called a bistable because it is stable in two states: output high and output

low. Because of this it is also known as a flip flop circuit.

Bistable mode is a less common configuration in 555 timer designs where the circuit hastwo stable states but doesn't actually producing any timing signals.(ref

www.ehobbycorner.com/pages/tut_timer.html)

Bistable mode can also be defined as a mode in a 555 timer which changes from 1 to 0 or

from 0 to1 when a current is applied. The timer will remain in this transitioned state until

a new input is received.
http://www.ecelab.com/http://www.ehobbycorner.com/pages/tut_timer.htmlhttp://www.ecelab.com/http://www.ehobbycorner.com/pages/tut_timer.html


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Below is a diagramme of a circuit in Bistable state.

(refwww.ehobbycorner.com/pages/tut_timer.html)

The "set" and "reset" inputs to this circuit are active low and must be held close to thepositive supply voltage when not asserted, with pull-up resistors if necessary. A falling

edge on the set input will cause the output signal to be driven high until another falling

edge on the reset input returns the output low. The circuit will remain in either stateindefinitely and is therefore bistable. The threshold input is connected to ground to ensure

that it can never reset the circuit as it would in a normal timing application. (refwww.ehobbycorner.com/pages/tut_timer.html)

Using the 555 timer in practical circuits

The output of the standard 555 timer is up to 200Ma, which is higher than most IC`s and

is sufficient to supply many output transducers including LED`s ( with resisters inseries),low current lamps, piezo transducers, loudspeakers (with capacitor in series) andwith protection from diodes relay coils and some motors. The output voltage does not

reach 0v and +Vs if large current is flowing in the circuit. To achieve this you need to

connect a transister.

The ability to both sink and source current means that two devises can be connected tothe output so that one is on when the output is low and the other is on when the output is

high. This arrangement could be used for flashing LED`s at a level crossing. Below is and

example figure of a speaker circuit using the 555 timer:

Further examples of uses for the 555 timer are as follows:


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LED`s

LEDs typically consume only 20mA, so they can be connected directly to the 555 Outputvia the usual current limiting resistor.

LAMPS

Small low voltage lamps may be connected directly to the 555 Output, although it is best

to switch them using a relay or transistor. You can of course control several lamps, andhigher voltage lamps, using an appropriate relay.

MOTORS AND ELECTROMAGNETS

These must be switched via a relay or transistor, because of the high current they

consume. The maximum current consumed by the relay coil itself is 200mA. Note that Vs

need not be the same voltage as that used to power the 555 - it can be the voltage that the

device requires, such as 12V for a motor, providing the limits of the relay or transistor arenot exceeded.

JOYSTICK INTERFACE CIRCUIT

The original IBM personal computer used a quad timer 558 in monostable (or "one-shot")

mode to interface up to two joysticks to the host computer. (refwww.wikipedia.org/555timer). Within the joystick interface circuit of the IBM PC, the

capacitator(C) of the RC network was generally a 10 nF capacitor. The resistor(R) of the

RC network is made up by thepotentiomenter inside the joystick with an externalresistor of 2.2 kilohms. The joystick potentiometer then acts as a variable resistor. when

moving the joystick, the resistance of the joystick increases from a small value up to

about 100 kilohms. The joystick operated at 5 V.Software running in the host computer started the process of determining the joystick

position by writing to a special address (ISA bus I/O address 201h). This would result in

a trigger signal to the quad timer, which would cause the capacitor (C) of the RC networkto begin charging and cause the quad timer to output a pulse. The width of the pulse was

determined by how long it took the C to charge up to 2/3 of 5 V (or about 3.33 V), which

was in turn determined by the joystick position.

Software running in the host computer measured the pulse width to determine thejoystick position. A wide pulse represented the full-right joystick position, for example,

while a narrow pulse represented the full-left joystick position(ref

www.wikipedia.org/555timer)The relationship between the trigger signal, capacitor voltage and the output pulse

representing joystick position is shown by the diagram in the monostable section of this

report.

Comparisons between the BJT and FET can be illustrated as follows.

A bipolar (junction) transistor (BJT) is used in amplifying or switching applications ,
http://wiki/Capacitorhttp://wiki/Resistorhttp://wiki/Potentiometerhttp://www.wikipedia.org/555timerhttp://wiki/Capacitorhttp://wiki/Resistorhttp://wiki/Potentiometerhttp://www.wikipedia.org/555timer


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it is a three-terminal electronic device constructed of doped semiconductor material .BJT

operation involves both electrons and holes. Charge flow in a BJT is due to bidirectional

diffusion of charge carriers across a junction between two regions of different chargeconcentrations (ref www.wikipedia.org). This operation is a contrast of the FET which is

a unipolar transistor. In FET operation by contrast The FET controls the flow ofelectrons

(orelectron holes) from the source to drain by affecting the size and shape of a"conductive channel" created and influenced by voltage (or lack of voltage) applied

across the gate and source terminals. (For ease of discussion, this assumes body and

source are connected). This conductive channel is the "stream" through which electronsflow from source to drain.

Additionally In FET operation only one type of carrier is used in charge flow due to drift,

(which is the average velocity that a particle, such as an electron, attains due to anelectric

field).

FET`s consist of agate,drain, andsource terminal that correspond roughly to the base,collector, and emitterof BJT`s Aside from the JFET, all FETs also have a fourth terminal

called the body, base, bulk, orsubstrate. This fourth terminal serves to Bias the transistorinto operation; it is rare to make non-trivial use of the body terminal in circuit designs,

but its presence is important when setting up the physical layout of an IC.

The names of the terminals refer to their uses or functions. The gate permits electrons to

flow through or blocks their passage by creating or removing a channel between the

source and drain. Electrons flow from the source terminal towards the drain terminal ifinfluenced by an applied voltage. The body refers to the bulk of the semiconductor in

which the gate, source and drain lie. Usually the body terminal is connected to the highest

or lowest voltage within the circuit, depending on type. The body terminal and the sourceterminal are sometimes connected together since the source is also sometimes connected

to the highest or lowest voltage within the circuit, however there are several uses of FETs

which do not have such a configuration, such as transmission gates and cascode circuits.(ref wikipedia.org/FET)

By design, most of the BJT collector current is due to the flow of charges injected from ahigh-concentration emitter into the base where they are minority carriers that diffuse

toward the collector (ref www.wikipedia.org), because of this BJT`s are classified as

minority carriers.

As mentioned previously the FET relies on an electric field which controls the shape andconductivity of a channel of a type of charge carrier in the semiconductor material. As

mention previously FETs are unipolar transistors which contrasts single-carrier-type

operation with the dual-carrier-type operation of BJT. The idea of the FET predates theBJT, but wasn`t implemented until after BJT`s due to the ease of manafacture of BJT`s

and the limitations of the semiconductor materials used.

Another contrasting factor between the two transistors can be illustrated by the fact thatThe Bipolar Junction Transistor (BJT) is an active device. Which means, it is a current

controlled valve. The base current (IB) controls the collector current (I

C).

The Field Effect Transistor (FET) is an active device. In simple terms, it is a voltagecontrolled valve. The gate-source voltage (V

GS) controls the drain current (I

D).
http://wiki/Diffusionhttp://wiki/Electronhttp://wiki/Electron_holehttp://wiki/Velocityhttp://wiki/Electric_fieldhttp://wiki/Electric_fieldhttp://wiki/JFEThttp://wiki/Biasing_(electronics)http://wiki/Integrated_circuithttp://wiki/Integrated_circuithttp://wiki/Transmission_gatehttp://wiki/Cascodehttp://wiki/Minority_carrierhttp://wiki/Electric_fieldhttp://wiki/Diffusionhttp://wiki/Electronhttp://wiki/Electron_holehttp://wiki/Velocityhttp://wiki/Electric_fieldhttp://wiki/JFEThttp://wiki/Biasing_(electronics)http://wiki/Integrated_circuithttp://wiki/Transmission_gatehttp://wiki/Cascodehttp://wiki/Minority_carrierhttp://wiki/Electric_field


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The FET is a three terminal device like the BJT, but operates by a different principle. The

three terminals are called the source, drain, and gate. The voltage applied to the gatecontrols the current flowing in the source-drain channel. No current flows through the

gate electrode, thus the gate is essentially insulated from the source-drain channel.

Because no current flows through the gate, the input impedance of the FET is extremelylarge (in the range of 10101015). The large input impedance of the FET makes them an

excellent choice for amplifier inputs.

The two common families of FETs, the junction FET (JFET) and the metal oxide

semiconductor FET (MOSFET) differ in the way the gate contact is made on the source-

drain channel.The JFET is the simplest form of FET, In the JFET gate-channel contact is a reverse

biased pn junction. The gate-channel junction of the JFET must always be reverse biased

otherwise it may behave as a diode. All JFETs are depletion mode devicesthey are on

when the gate bias is zero (VGS = 0).In the MOSFET the gate-channel contact is a metal electrode separated from the channel

by a thin layer of insulating oxide. MOSFETs have very good isolation between the gate

and the channel, but the thin oxide is easily damaged (punctured!) by static dischargethrough careless handling. MOSFETs are made in both depletion mode (on with zero

biased gate, VGS

= 0) and in enhancement mode (off with zero biased gate).

FET`s can perform the functions that the BJT can do with the exception that the biasconditions and characteristics are different. Therefore applications should be chosen in

accordance with the advantages and draw backs.

The characteristics of FET`s can be summarised as follows:

FET`s have a very high impedance,typically around 100 M ohms.

When used as a switch there is no offset voltage.

The FET is relatively independent of radiation ,where as the BJT is sensitive toradiation.

Intrinsic noise is relatively low compared to the BJT which makes them suitable for

input stage of low level amplification.

During operation the thermal stability of the FET is higher than the BJT.

Despite the advantages of the FET over the BJT it does have a major disadvantage as its

product of gain and bandwidth is smaller and it is easily damaged by static electricity.

The circuit symbol is shown below in Figure 1


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.

D = Drain

G = Gate

S = SourceG

+

-

+

-

ID

VDD

VGG

+

D

S

VGS

IG

VDS

IS

The characteristic curves of a FET look similar in shape to those of a BJT, but since the

FET has a different basis of operation, the curves are given in terms of other parameters.

This concludes the report on the 555 timer, BJT and FET. Below is a list of all external

references used.

Bibliography

http://www.antonine-education.co.uk/Electronics_AS/Electronics_Module_1/Topic_11/topic_11__555_timer_circuit.htmhttp://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/6.phphttp://www.electronics-lab.com/articles/basics/components/555astable.htmwww.eculabs.com

http://www.toolingu.com/definition-460350-34786-bistable.htmlwww.ehobbycorner.com/pages/tut_timer.htmlhttp://en.wikipedia.org/wiki/555_timer_IChttp://www.eleinmec.com/article.asp?http://www.kpsec.freeuk.com/555timer.htmhttp://web.iku.edu.tr/courses/ee/ee425/PDF/_EE%20425_Exp_7.pdf

aerospace bsc module 4 assignment for reference only

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