School Science Lessons
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38. Electronics
Table of contents
See: Electronics (Commercial)
38.4.0 Capacitors, charge and discharge
38.7.0 Computers
39.0.0 Electronics components
39.1 Electronics, pulser, binary
38.1 Electronics, switching circuits, motor vehicle ignition system
38.8.0 Gas discharge tubes, Cathode ray tubes
38.6.0 Radio waves
38.2.0 Semiconductors
38.3.0 Transistors
38.1.0 Valves

38.7.0 Computers
38.7.3 Addition of binary numbers
38.7.4 ASCII (American Standard Code for Information Exchange)
38.7.1 Bits and bytes, prefixes for multiples, kilobyte, K, megabyte, MB, binary counter
38.7.2 Decimal and binary codes for numbers 0 to 25, Powers of 2
38.7.01 Logic gates

38.8.0 Gas discharge tubes, Cathode ray tubes
See: Oscilloscope, (Commercial)
38.8.1 Gas discharge tubes, discharge tubes (Cathode ray tubes)
38.8.4 Cathode ray tube CRT, cathode ray oscilloscope CRO
38.8.3 Fluorescent lamp
38.8.2 Incandescent lamp

38.6.0 Radio waves
38.6.0 Radio waves, radio station
38.2.04 Capacitor (formerly "condenser"), capacitance in an AC circuit
38.1.03 Choke coils affects both AC and DC
38.1.04 Diode valve Diodes, laser diodes (See 7.)
38.6.1 Radio transmission and reception
38.1.09 Semiconductor instead of diode valve
38.6.2 Signal generator
38.1.0 Valves Variable capacitor

38.2.0 Semiconductors
See: Capacitors, (Commercial)
38.2.04 Capacitor, condenser, capacitance in an AC circuit Capacitor, Electrolytic capacitor Capacitor, Polyester capacitor Capacitor, Tantalum bead capacitor Capacitor, Variable capacitor
38.2.05 Earphones, crystal microphones
38.2.01 Electronics circuits
38.2.02 Electronics circuits, Soldering for electronic circuits
38.2.03 Resistors
38.2.1 P-type and N-type semiconductors Diodes, laser diodes
38.2.3 Diode as a half-wave rectifier Diode, Zener diode LED, Light-emitting diode LED, Seven-segment display LED, Test circuits using an LED Smoothing An AC to DC motor
38.2.4 Full-wave rectification

38.3.0 Transistors
38.3.1 Transistors NPN transistors PNP transistors
38.3.2 Transistor as a switch Transistor as switch with potential divider
38.3.3 Transistor as current amplifier
38.3.4 Transistor as voltage amplifier
38.3.5 LDR, Light dependent resistor, photocell, photoelectric sensor
38.3.7 SCR, Silicon-controlled rectifier, thyristor
38.3.8 LDD, Light-dependent diode

38.1.0 Valves
38.1.0 Valves
38.1.03 Choke coils affects both AC and DC
38.1.02 Condensers can pass alternating current, but not direct current
38.1.05 Diode in a battery charger
38.1.04 Diode valve
38.1.10 Discharge of capacitor using a neon tube
38.1.01 Discharge of condenser using high resistance
38.1.14 Rectifier, Bridge rectifier
38.1.07 Rectifier, Germanium diode as a rectifier
38.1.13 Rectifier, Simple chemical rectifier
38.1.09 Semiconductor instead of diode valve
38.1.15 Transformer
38.1.16 Triode circuits
38.1.06 Triode valve

38.1.00 Valves
See diagram 38.1.00: Electronics symbols - valves
(Comment: Transistors are seldom drawn this way nowadays.) (Comment: Valves require voltages that can be dangerous.
Nowadays valves are still used only in special applications, e.g. in high powered radio and television stations, older models of spot
welders, RF welding of plastic bags and microwave ovens.

38.1.01 Discharge of condenser using high resistance
See diagram 38.1.01
Use a 100 000 ohm resistor and capacitor with C = 1 muF to 100 muF.
For higher capacities use electrolytic condensers.
Charge the condenser by joining X to Z.
Then discharge the condenser by joining X to Y.
The movement of the pointer of the microammeter will show the rate of discharge.
The rate varies with the capacity of the condenser.
(Comment: muF is a most unusual notation).

38.1.02 Condensers can pass alternating current but not direct current
See diagram 38.1.02
Use the changes in brightness of the torch globe to show changes in current.
Vary the voltages from 4 to 6 volts by using a variable voltage power pack delivering both AC and DC.

38.1.03 Choke coils affects both AC and DC
See diagram 38.1.03: Passage of current through a choke coil
A choke is a high-inductance coil used to smooth variations or change the phase of the alternating current fed into it.
A choke is an inductor that slows changes of electric current and opposes the flow of this changing current.
An inductor has the same effect on an alternating current as a resistor has on a direct current.
In a radio circuit, use an inductor to oppose the flow of high or low frequency current and permit the passage of direct current only.
Use the primary coil of a soldering iron transformer.
The lamp glows brightly with DC but not with AC.

38.1.04 Diode valve
See: Thermionic Tubes, (Commercial)
See diagram 38.1.04
A radio valve has two electrodes, so it is called a diode.
A heated filament loses electrons by thermionic emission.
When the filament electrode is connected to the negative terminal of a variable voltage power pack to deliver both AC and DC.
Connect the other electrode, the anode, to the positive terminal, so that an electric current flows between the electrodes.
Connect the type AV 33 diode valve in the circuit.
The filament heated by a 4 volt battery supplies the electrons so it is called the cathode being negative with respect to the other
he anode.
If the cathode circuit is "broken" so that no electrons are emitted, there is in effect an open circuit in the valve so that current does not
flow from the anode battery.
If the anode circuit were " broken " instead of the cathode, the heated cathode would still emit electrons so that the filament battery
would very soon be useless although current would not flow from the anode battery.

38.1.05 Diode in a battery charger
See diagram 38.1.05
The diode valve in the circuit is acting as a rectifier.
Use a step down transformer to supply 12 volts.
Note the smaller secondary winding to supply heat to the filament of the diode.
The current in the secondary coils should alternate in the same way as in the primary coil.
So in secondary coil AB, during one half of the cycle electrons should move from A to B and during the other half of the cycle electrons
should move from B to A.
However, the diode acts as a valve and only allows electrons to pass during half the cycle from the cathode filament to the anode.

38.1.06 Triode valve
See: Thermionic Tubes, triode valve, (Commercial)
See diagram 38.1.06
A triode valve has three electrodes, cathode, anode and grid, i.e. a screen of wires between the cathode and anode.
The more negative the grid compared with the cathode, the smaller the anode current.
Use a type AV 25 triode valve with an anode that fluoresces when electrons hit it.
The diode valve should have a tungsten filament because it may be used in place of the AV 33 valve.
Vary the grid voltage from -100 volts to +100 volts.
Observe the size of the anode current.
The extent of the fluorescence has not only indicated it, but it can be measured by means of a milli-ammeter placed in the anode circuit.
Observe the effect of a bar magnet on the electron stream.
The electric current is a flow of electrons that can be stopped by applying a negative voltage to the grid.
Electrons travel from the cathode to the anode in straight lines so a sharp shadow of the grid can be formed on the anode.
The stream of electrons may be deflected by a magnet.

38.1.07 Germanium diode as a rectifier
See diagram 38.1.07
Connect a low voltage AC to DC voltmeter.
Use the secondary winding of either a bell transformer or a transformer designed for heating soldering irons is satisfactory.
Note any movement of the pointer of the voltmeter.
Switch off the AC supply then connect a germanium diode into he secondary circuit.
Note any movement of the pointer of the voltmeter.
Alternating current has been rectified by the germanium diode to produce a pulsating direct current.
(Comment: The rectification action is fine for single frequency application, e.g. power supplies.)

38.1.09 Semiconductor instead of diode valve
Use a semi-conductor as a detector instead of the diode valve
Use an aerial coil from a discarded radio receiver in place of the one described.
If the leads from the diode and the junction of the coil and condenser are connected to the amplifier of a projector with a jack plug, the
class can listen to local radio transmission.

38.1.10 Discharge of capacitor using a neon tube
See diagram 38.1.10
To show the ability of a capacitor to store electricity, use its discharge through a lamp.
A high resistance is connected in series with a source of potential difference and a neon lamp.
Connect each condenser across the neon lamp in turn.
The flashing of the lamp indicates the rate of discharge, which depends on the capacity of the condenser.

38.1.13 Simple chemical rectifier
See diagram 38.1.13: Chemical rectifier
Put aluminium and lead electrodes in a saturated solution of borax (sodium tetraborate, Na2B4O7.10H2O) in a glass container.
Connect to a low voltage AC supply from a transformer.
A direct current flows from the lead to the aluminium.
This is a half-wave rectifier and could be used as a battery charger.
(Comment: I am not sure of the safety of making lead-aluminium-borax diodes.)
Other rectifiers use copper and selenium oxide.

38.1.14 Bridge rectifier
Use four such cells as made in 38.1.13 can be used in a for a bridge rectifier.

38.1.15 Transformer
See diagram 38.1.15: Step-up transformer
1. Step up transformer: Supply 6 volts AC to DC and light a 12 volt lamp at AB.
2. Step down transformer: Supply 12 volts AC to AB, measuring the current in the AB turns.
Take off six volts AC from CD to light up a large 6 volt lamp.
Measure current with an ammeter.
If the voltage drops by half, the current doubles.
If the voltage doubles, the current halves.
(Comment: This last sentence is "sort of O.K." but is incorrect as an explanation of what happens if the voltage of a primary
transformer is dropped.)

38.1.16 Triode circuits
See: Thermionic Tubes, triode valve, (Commercial)
See diagram 38.1.16
Circuits linking the anode and cathode of a triode and a battery connection to the grid:
1. The grid is not connected but some anode current exists.
2. The grid is more negative than the cathode.
The anode current would be decreased.
3. The grid is more positive than the cathode.
The anode current would be increased.

38.2.00 Semiconductors
Semiconductors have a resistivity between conductors and insulators.
Their conductivity increases with temperature and is affected by impurities in their crystals.
They are used in solid state devices, e.g. diodes and transistors.
Silicon and Germanium, in group 4 of the periodic table, are commonly used.
38.2.1 P-type and N-type semiconductors.

38.2.01 Electronics circuits
See diagram 38.2.00 Electronics symbols
Hardwiring means connecting the component leads with wire if there are only few components in the circuit.
A bread board is a plastic sheet with holes for the component leads to be pushed in.
Strip board has holes in parallel copper tracks.
The components are soldered to the tracks to make circuits.
Printed circuit boards (PCBs) for special circuits are made from sheets of copper attached to flat material with the copper are etched
away to make tracks for the conductors between components instead of using wires.
Components are soldered to the tracks to make complicated circuits in a small space.
Earth or ground means direct connection to the earth or to a chassis or place of zero voltage.

38.2.02 Soldering for electronic circuits
Be careful!
The soldering bit becomes very hot!
Do not breath in the poisonous fumes from the solder!
After handling solder, wash the hands!

Use side cutters with blades placed at one side to crop the ends off component leads after the components have been soldered to a
piece of strip board or a printed circuit board (PCB), now called printed wiring boards because "PCB" also refers to carcinogenic
Use wire strippers on single core or stranded thin equipment wire when connecting off board components to the strip board or PCB,
or when constructing a prototype circuit on breadboard.
Use soldering irons with low wattage, preferable with silicone insulated cable fitted to prevent accidentally melting of the mains cable,
and with a 4 mm bit.
Use a soldering iron stand because the temperature of a soldering iron bit can reach 400C.
Use a desoldering pump as a solder sucker to remove excess solder from a joint.
Solder for electronics should be 22 swg 60% tin, 40% lead alloy, including a flux core.
The melted flux cleans the component leads and strip board or PCB tracks.
When first using a new soldering iron bit, "tin the bit" with solder by holding the solder against the bit as the iron heats up.
Let the solder melt onto the bit and wipe off excess solder with a wet sponge.
To solder a wire or component lead to strip board or a PCB, push the lead through the hole in the board with 5 mm of the lead
Place the tip of the soldering iron at an angle against both the track and the lead and allow both to heat up.
Feed the solder in around the track and it will run around the track and the lead if the track and the lead are hot enough.
Remove the soldering iron.
The solder joint should look shiny and have a pyramid shape.
A dull looking joint, "dry joint", may be a weak joint due to using too much solder.
If so remove the solder with the desoldering pump and try again.
Remove excess wire or lead with the side cutters.
To join two wires together, first "tin" to let solder melt onto each one wire, bring the tinned ends together, then heat again briefly.
Integrated circuits (ICs, or microchips) and transistors may be damaged by the heat from during soldering.
Integrated circuits should be inserted into an IC holder and inserted on the board when soldering is completed.
As transistors must be soldered directly to the board attach a crocodile clip to each lead before it is soldered to act as a "heat shunt" to
absorb the heat during soldering.
Heat-shrinkable sleeving is used around wires joined to off board components to give the joint extra mechanical strength.
(Comment: The by-products of soldering are lead and lead oxide particles.
Flux fumes used to be corrosive and could damage the nasal passages and eyes.
However, nowadays most fluxes are water based and relatively harmless but hey do not clean the surfaces so effectively so more
lead-tin is consumed in hand soldering.
Machine soldering is another matter.
Some solders are non-lead solders.
Other methods of connection include crimping, bolt and nut, and star washer, e.g. brass clamps of car battery terminals.)

38.2.03 Resistors
See diagram 38.2.01: Resistors colour code
A resistor in a circuit opposes the flow of electric current through it, allowing some current to flow, and changes the voltage drop
following Ohm's Law.
Resistor values include 1 k = 1 000 ohms and 1 M = 1 000 000 ohms.
Usually, combinations of 1 k and 10 k are used in series and parallel for school experiments.
The maximum error is called the tolerance value, usually 10% tolerance.
If the power rating in Watts (W) is exceeded, the resistor may be damaged.
For school experiments usually 0.6 W resistors are suitable.
A resistor colour code is used for fixed value they are so small.
The four band colour code uses three coloured bands for the resistor's value, and another coloured band for the resistor's tolerance.
The first two bands indicate the first two digits of the resistor's value.
The third band indicate the number of zeros to be placed after the first two digits.
The forth band indicates the resistor's tolerance as a percentage.
The five band colour code uses the same colours as the four band system, but an extra band indicates another digit, and gold and silver
bands indicate multipliers of 0.1 and 0.01.
The five band system can indicate values lower than 1.
The first three bands give the first three digits of the resistor's value.
The fourth band indicates the number of zeroes to be places after the first three digits.
The fifth band gives the resistor's tolerance as a percentage.
A polarized component must be connected in a certain way for the circuit to operate.
If reverse connected, damage may occur.
Resistors are not polarized so they can be placed either way in the circuit.
See: Potentiometers, (Commercial)
A potentiometer, variable resistor, is a fixed resistor with a slide arm to select different .resistance.
It has three terminals, one terminal to each end of the resistance and one to the slide arm.
Potentiometers may be polarized.
(Comment: It is almost impossible to get 10% tolerance resisters nowadays - most are 5% or better.
Other problems with resistors are the temperature effects and power dissipation.)

38.2.04 Capacitor (formerly "condenser"), capacitance in an AC circuit
See: Capacitors, (Commercial)
Capacitors are made of layers of flat conductor separated by layers of insulator(dielectric), e.g. vacuum, waxed paper, mica, glass,
plastic, air (for tuning high frequency oscillators).
The capacity to hold charge, capacitance, = ratio of total charge on the parallel plates / potential difference between the plates (volts).
Capacitors store electrical energy.
Capacitors store electric charge.
When the power is switched off, the computer indicator lights keep glowing for a while because electrical energy is stored in the
capacitors in the computer.
When charge builds upon a capacitor, a voltage develops across the capacitor, which opposes the charging current.
Voltage across the capacitor V = Q / C, where Q is the amount of charge in coulomb, and C is the capacitance in farad.
The current leads the voltage by a quarter of a cycle, i.e. 90o.
Capacitance, measured in farads, symbol F, measures a capacitor's ability to store charge.
Prefixes show the smaller values:
1. mu (Greek letter), micro, 10-6, millionth (microfarads F),
2. n, nano, 10-9, thousand millionth,
3. p, pico, 10-12, million millionth (picofarads pF).
So 1000 F = 1 F, 1000 nF = 1 F, 1000 pF = 1 nF.
Capacitors are polarized or unpolarized.
Unpolarized capacitors, for less than 1 F, and may be connected with either polarity.
They have high voltage ratings of 50 volts to 250 volts.
They have different types of labels, e.g. 0.1 = 0.1 F = 100 nF, 4 n7 = 4.7 nF.
Very small capacitors have a number code:
First number is 1st digit of the value.
Second number is 2nd digit of the value.
Third number is the number of zeros to give capacitance in pF, e.g. 102 = 1000 pF = 1 nF, 472 J = 4700 pF = 4.7 nF, J =
5% tolerance.
Capacitors are used in filter circuits because they pass AC (changing) signals, but they block DC (constant) signals.
The time constant measures the time for a capacitor to charge or discharge, with a certain resistor, so they are used with resistors in
timing circuits because it takes a known time for a capacitor to fill with charge.
(Comment: The usual range of capacitors nowadays is 0.47 F (microfarads) to 47 mF (millifarads).
Capacitors for more than 1F are not manufactured, except to special order.
0.5 F and 0.68 F capacitors are found in some computer motherboards instead of the battery that was used to keep the
CMOS-based BIOS memory alive.
These capacitors are also used in some modern portable radio, TV tuner memories and high powered audio amplifiers in sporty
motor vehicles. Electrolytic capacitor
Electrolytic capacitors are polarized capacitors, for more than 1 F.
They must be connected with correct polarity.
Axial electrolytic capacitors have leads are attached to each end.
Radial electrolytic capacitors have both leads are at the same end for use on printed circuit boards and to take less space.
The value of electrolytic capacitors are printed on them showing capacitance and voltage rating, e.g. 6 volts.
Electrolytic capacitors contain an electrolyte.
If the voltage rating is exceeded, the capacitor can be damaged.
The capacitor should have with a rating greater than the circuit's power supply voltage, e.g. 25 volts. Tantalum bead capacitor
Tantalum bead capacitors are used where a large capacitance, 0.1 muF to 100 muF, is needed in a small size and are usually polarized.
Use in place of electrolytic capacitors of the same values.
(Comment: Tantalum capacitors should NOT be used in place of electrolytics, because in electrolytics quite different design and
application rules.
Electrolytics operate best when close to their rated voltage but tantalums fail as a power of the applied voltage, so they are best used at
small proportions of the rated voltage.) Polyester capacitor
Polyester capacitors, called green caps, are used mainly in audio circuits.
They have capacitance up to 0.001 uF, are not polarized and may have a colour code like the resistor code. Variable capacitor
Variable capacitors can be varied by moving a rotating shaft.
Usually used with a tuning dial with the radio stations marked.
The smaller "trimmer capacitors" from 1.5 pf to 160 pF are used with larger fixed value capacitor.
The larger are used in transistor radios, used to tune different radio stations.
(Comment: The physical size is related to the applied voltage.
The capacitance is related to the operating frequency.
So large variable capacitors are used in broadcast transmitters.
Small variable capacitors are used in transistorized portable receivers.
High capacity is used for low frequency.
Small capacity is used for higher frequencies or as trimmers for larger capacitors.)

38.2.05 Earphones, crystal microphones
See: Potentiometers, (Commercial)
Crystal earphones and crystal microphones use piezoelectric crystals to convert electrical voltage into sound waves.
Crystal earphones have high impedance and are not polarized.
The two way behaviour of piezoelectric crystals allow the crystal earphones to be used as a crystal microphone.
Magnetic earphones and microphones use a coil, permanent magnet and diaphragm to convert electrical voltage into sound waves.
Magnetic earphones have low impedance and are not polarized.
Speak into a magnetic earphones to use it as a microphone.
Similarly the loudspeaker and horn type loudspeaker have a coil for a driver.
Volume control is usually by a potentiometer situated at the input to the main amplifying section of the circuit.

38.2.1 P-type and N-type semiconductors
All semiconductors, e.g. silicon, are insulators.
However, the silicon atom has four outer electrons so it can form bonds with electrons from four atoms around it to form silicon crystals.
Phosphorus has five outer electrons.
If some phosphorus is added to silicon crystals, called chip doping or metallizing doping, the phosphorus atoms each provide an extra
electron to make an N-type semiconductor with free electrons to conduct electricity.
Aluminium has three outer electrons.
If silicon crystals are doped with aluminium, each aluminium atom lacks one electron for bonding.
So it has a "positive hole" to make a P-type semiconductor that can accept electrons. Diodes, laser diodes
See diagram 38.2.2: P-N junction | See diagram 38.2.2a: P-N junction graphs
1. Wafers of P-type and N-type semiconductors can be combined in direct physical content to form a P-N junction diode or PN diode
that lets current flow through it in one direction as electrons move from N-type to P-type semiconductors.
A device with a single P-N junction has two electrodes forming a semiconductor diode.
It is represented in the diagram as 1. the cathode, the electrode connected to the N-type wafer as a straight line and 2. the anode, the
electrode connected to the P-type wafer, as an arrow.
Electrons flow easily from N to P in the opposite direction to the arrow.
Hardly any electrons can flow from P to N in the direction of the arrow.
2. When a PN diode is placed in a circuit, in series with a battery and resistor, conventional current flows from the positive terminal of
the battery, through P to N, to the negative terminal of the battery, called forward bias.
Connect the negative terminal of the battery to the cathode and the positive terminal to the anode.
3. With reverse bias, no conventional current flows across the junction, through N to P.
Connect the positive terminal of the battery to the cathode and the negative terminal to the anode.
The P-N junction acts like a thin dielectric layer, like an electrical insulator so no current can flow through the diode.
However, if the voltage is increased without limit, a threshold voltage is reached where the diode suddenly starts to conduct, called the
avalanche effect.
A P-N junction under reverse bias can act as a capacitor and this property is used in varactor diodes made from silicon or gallium
arsenide, GaAs.
4. A minimum voltage is needed to cause movement of electrons through the P-N junction, called the forward breakover voltage, and
usually between 0.3 V and 1V, e.g. 0.6 V in a common silicon diode.
5. The resistors shown in the diagram are called "current-limiting resistors" to control the current so that the diode cannot be
damaged by battery voltage exceeding the forward breakover value or the avalanche value.
5. As diodes let current flow only in one direction, they must be place correctly in a circuit.
It is difficult to understand diodes in terms of conventional current flow, so refer to electron flow and ignore any arrows on device
6. Power diodes or protection diodes protect the circuit against reversal of current flow.
7. Signal diodes, made of Germanium, detect radio frequency currents.
Signal diodes in radio receivers have very low voltage drop.
8. Diodes are frequently used as rectifiers to change alternating current to pulsating direct current or, with smoothing, to direct current,
e.g. IN4001.
They may be used simply as a reversed polarity protection device.
9. Laser diodes are of two types:
1. injection laser diodes (ILD) and
2. optically pumped laser diodes.
Laser diodes are used in telecommunications with fibre optics, bar code readers, red or green laser pointers for lecturers, CR-ROM
technology, image scanners, welding, pumping other lasers, surgery, dentistry. Zener diodes
The Zener diode lets current flow only in one direction, but only up to a certain voltage.
Then the reverse resistance drops and the diode conducts reverse current maintaining a constant voltage drop.
It can control the maximum voltage in the circuit.
So a 12 V Zener diode is similar to a junction diode in forward bias but shows a sharp increase in reverse current at 12 volts.
A Zener diode can be used for voltage reference or as a voltage regulator The end closer to the cathode is marked with a band.
(Comment: Zener diodes are a bit more complex than outlined here.
For example there are different thermal effects between Zener diodes and Avalanche diodes, although marketers bunch them together.
Semiconductors do come between conductors and insulators, but the thermal effect on resistance is quite different between conductors and semi-conductors.) Light-emitting diode, LED
See: LEDs (Commercial)
The LED is a P-N junction diode that emits light when the forward bias voltage across it exceeds a certain value and current flows.
The positive lead, anode, through which conventional current enters, may be longer and the cathode side might be cut flat or have a
coloured dot.
LEDs are damaged by reverse polarity or excessive forward voltage so they must be protected by a resistance in series to allow a
forward bias of about 2 volts.
LEDs are used for low voltage alpha-numeric displays in calculators, warning lamps, panel metres, and as indicator lights because they
consume much less electricity than an incandescent lamp and they rarely fail.
The usual LED has a red or orange 5 mm diameter dome shaped container.
Some LEDs are bicolour and tricolour.
A LED is a diode, so current flows in one direction only through it.
A forward biased LED lights with a voltage drop across it of 0.7 V.
Reversed bias current does not flow through the LED so it does not light, but reverse bias will not harm it.
If a LED needs a forward bias voltage of 2 V and current 20 mA, place a resistor in series with resistance,
R = voltage of power supply - 2 volts / 0.02 amps.
A domed LED has the longest lead as the positive lead, or anode, and the shortest lead as the negative lead, or cathode.
Also the plastic container nearest the cathode will be flattened slightly and if hold up to a light, the largest part inside will be nearest the
LED lamps are much cheaper but some people find difficulty in seeing small things under it, e.g. threading a needle.
Lifespan Comparison Between Halogen Downlights And Their L.E.D Counterparts
5 Watt LED lamp, 455 lumens, life span 50 000 hours, comparative cost = $16.95 AUD
20 Watt halogen lamp, 360 lumens, life span 1 500 hours, comparative cost = $66.0 AUD
5 Watt LED lamp, electricity used in 365 days = 14.6 kWh, cost per year = $4.57 AUD
20 Watt halogen lamp, electricity used in 365 days = 58.4 kWh, cost per year = $18.27 AUD. Seven-segment display
Arrange 7 separate LEDs in a figure 8 pattern to create 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and so converting electrical signals to visual
information. Test circuits using an LED
See diagram
The LED can be used in a circuit to give 2 kinds of information as follows:
1. Whether current is passing though the circuit, i.e. all components have current passing through them,
2. Whether more or less current is passing because the LED can glow brighter or less bright.

In the circuit include a resistor of about 400 ohms and a protection diode, e.g. IN4002:
1. Test an incandescent lamp.
If the LED glows then there is no break in the lamp filament.

2. Test a resistor, e.g. about 1000 ohms.
Note the brightness of the LED.
Add a second resistor in series.
Note the brightness of the LED.
Add a third resistor in series.
Note the brightness of the LED.
The light from the LED gets dimmer because the resistance gets greater, R = r1 + r2 + r3.

3. Test a resistor, e.g. about 1000 ohms.
Note the brightness of the LED.
Add a second resistor in parallel.
Note the brightness of the LED.
Add a third resistor in parallel.
Note the brightness of the LED.
The light from the LED gets brighter because the resistance becomes less, 1 / R = 1 / r1 + 1 / r2 + 1 / r3.

4. Test a diode by inserting in the circuit in the direction indicated.
The LED does not give out light.
(e) Test a diode by inserting in the circuit in the direction indicated.
The LED does give out light.

5. Test an electrolytic capacitor, e.g. 100 muF.
As the capacitor charges the LED glows but as the capacitor achieves full charge the LED brightness decreases until when fully charged
the LED stops glowing.
The LED glow might be a brief flash depending on the value in farads of the capacitor.

38.2.3 Diode as a half-wave rectifier
Construction and testing of a diode rectifier and smoothing capacitor circuit to convert AC to DC voltage
See diagram 38.2.3
A semiconductor diode, like a thermionic diode, rectifies, i.e. converts alternating current, AC to pulses of direct current, DC, called
half-wave rectification The diode removes the negative cycles of AC to give a varying one way direct voltage across the load requiring
DC supply.
The humps are smoothed by connecting an electrolytic capacitor across the load.
During the positive half cycles of the AC current flow through the load and also into the electrolytic capacitor.
During the negative half cycles of the AC current flow when the diode is reversed biased and not conducting current the electrolytic
capacitor partly discharges through the load maintaining the current through the load and a steadier voltage. Smoothing
See diagram
The humps in the DC are smoothed by connecting a large electrolytic capacitor across a resistance.
During the positive half cycles of the AC supply, the diode conducts current that flows through the resistor and also into the electrolytic
During the negative half cycles of the AC supply, when the diode is reverse biased and non-conducting, the electrolytic capacitor partly
discharges through the resistance.
The charge storing action of the electrolytic capacitor thus maintains current through the resistor and a steadier voltage across it. An AC to DC motor
Power a DC motor with an AC power supply using two diodes and an Single Pole Double Throw (SPDT) centre-off switch to switch
the motor on and off.
AC supply must be at least 1.5 times the voltage required by the DC motor because a half-wave rectifier wastes half the power.

38.2.4 Full-wave rectification
See diagram 38.2.4
Using both half cycles of the AC, the current in a bridge circuit follows the solid arrows when A is positive and B negative on the
positive half cycles, and follows the broken arrows when A is negative and B positive on the negative half cycles when the polarities are
During both half cycles current flows through the load in the same direction, and can later be smoothed.

38.3.1 Transistors
See diagram 38.3.1: 1. Test N-P-N, 2. Test P-N-P
A thin layer of P-type semiconductor between two layers of N-type semiconductor is used for an N-P-N, transistor.
A bipolar transistor ("transfer resistor") is a semiconductor sandwich with a very thin filling of either p-type or n-type material called the
The outside layers, called the collector and emitter, are the same but of a different material to the base.
The two kinds of transistor are N-P-N (or NPN) and P-N-P (or PNP).
The arrow on the emitter in the symbol indicates the direction of conventional current in the transistor and shows its type.
Whichever way the transistor is drawn the arrow points out for an NPN transistor and in for a PNP transistor.
Construction and testing of a simple NPN transistor or IC operational amplifier circuit to measure voltage gain.
simulation. NPN transistors
Silicon NPN types are most common.
The transistor has 3 leads:
1. the base lead or B to the middle P-type semiconductor,
2. the collector lead or C to the first N-type semiconductor,
3. the emitter lead or E from the second N-type semiconductor.
An NPN transistor has a positive voltage on its collector and a negative voltage on its emitter.
When a positive voltage to the emitter is applied to the base the transistor lets current to flow through the collector / emitter circuit.
Small current through the base circuit causes much greater current through the emitter / collector circuit.
This current gain measures how much the transistor can amplify.
The arrow in the circuit diagram for the emitter points outwards.
Transistors are fast switches.
They can be damaged if too hot and must have the correct direction of connection.
The emitter can be identified by a tag or by the shape of the case or by a dot.
The NPN transistor is connected to the positive lead from the battery and the emitter is connected to the negative lead from the battery.
The transistor may be protected by a 1 k ohm resistor in the base lead.
BC 108 and ZTX 300 are general purpose NPN silicon transistors. PNP transistors
The PNP transistor has the same function as the NPN transistor but it has a negative voltage on its collector and a positive voltage on
its emitter.
When a negative voltage with respect to the emitter applies to the base conventional current flows in the opposite direction through the
collector / emitter circuit.
The arrow points inwards on the circuit symbols for a PNP transistor.
OC71 is PNP Germanium transistor.

38.3.2 Transistor as switch
See diagram 38.3.2: Transistor as switch
If the E and C are connected in a circuit, electrons from the N-type E fill the positive holes in the P-type B so no current can flow.
However, when a small positive potential is applied to B, electrons can flow out of the holes so B can conduct current again.
So the transistor acts as a switch. Transistor as switch with potential divider
See diagram Transistor as switch with potential divider
The base voltage depends on voltage from the variable resistor / voltage of fixed resistor.

38.3.3 Transistor as current amplifier
See: Amplifier, "Scientrific", (Commercial)
See diagram 38.3.3: Transistor as current amplifier
A small current from the P-type B allows a large current to flow through the NPN transistor, called current amplification.
The ratio of collector current to base current, called current amplification factor, may be a "gain" of 100.
A certain current to B produces a larger replica current flowing into C from the circuit and out through E to the circuit.
A field-effect transistor is the most effective device in weak-signal amplifiers, used mostly in wireless receivers.
They are also used in acoustic pickups, audio tape players, compact disc players because they can react to very small input signals
and generate minimal internal noise while greatly increasing the signal voltage.
The sensitivity of a weak-signal amplifier is the ratio:
microvolts (10-6 volts), of signal input /  signal output, (usually 10:1)
Power amplification in wireless and broadcast transmitters use bipolar transistors, PNP and NPN, to amplify analog or digital signals.
However Hi-fi audio equipment may use the vacuum tube.

38.3.4 Transistor as voltage amplifier
When a resistance is place in the C circuit, changes in current caused by an input signal voltage produce changes in output signal voltage
across the resistance.
The ratio of input signal voltage to output signal voltage voltage, called voltage amplification factor, may be a "gain" of 100.

38.3.5 LDR, Light dependent resistor, photocell, photoelectric sensor
Semiconductor applications of LDR and LEDR = R1 + R + R3 + .
The resistance depends on the light intensity, e.g. in the dark state resistance is very high, in the light resistance is much lower.
The resistance changes gradually as the amount of light changes so it can be used to monitor light levels.
The LDR is not polarized.
A photoelectric sensor can be adjusted respond only to the pulse of a particular LED or a certain threshold
of light and not background light.
A photoelectric proximity switch has the light source and light sensor in the same unit.
The sensor picks up the pulse of the LED as it reflects off of the object being sensed.
A retroreflective photoelectric sensor has the light source is reflected off a reflector back at the source, which houses the sensor.
So anything crossing between the two objects that decrease light intensity causes the sensor to switch.
ORP 12 is a general purpose LDR.

38.3.7 SCR, Silicon controlled rectifier, thyristor
A thyristor is a three-terminal semiconductor rectifier made up of four layers, p-n-p-n, so that when the fourth layer is positive with
respect to the first layer, a voltage impulse applied to the third layer initiate a flow of current through the thyristor.
A thyristor is a silicon diode with third connection, the gate.
If forward biased, the diode does not conduct current until the a small gate current enters then the diode always conducts until the
battery is disconnected.
THY 1A / 50 is a general purpose thyristor.

38.3.8 Light dependent diode, LDD.

38.4.0 Capacitors, charge and discharge
See: Capacitor, (Commercial)
See diagram 38.4.1: Capacitors, charge and discharge
1. Capacitance of a capacitor = charge on each plate per voltage between plates, C = Q / V.
The SI unit of capacitance the farad, F, is equivalent to 1 coulomb per volt.
Use an ammeter with deflection in either direction, e.g. 10-0-10 mA, note deflection and whether capacitor keeps or loses its charge:
1. before flying lead connected,
2. flying lead connected for charging,
3. then flying lead disconnected,
4. flying lead connected again for charging,
5. then flying lead disconnected,
6. flying lead connected for discharging.

2. Capacitor charge and discharge with cathode ray oscilloscope, CRO
See diagram 38.4.2: Capacitors, charge and discharge
Set variable capacitor at about 50 muF and variable resistance at about 50 k ohms.
Set CRO at slowest time -base and sensitivity 1 V / cm.
Connect the earthed Y-input socket to low potential side of the capacitor.
When slow trace starts to cross screen, connect flying lead for charging to see trace showing voltage across capacitor changing with
time as capacitor charges, i.e. a graph of voltage against time.
Disconnect flying lead.
When slow trace starts to cross screen, connect flying lead for discharging.
See graph.
Disconnect flying lead.

3. Bulb dims as the capacitor charges.
See diagram 38.4.3: Capacitor and light bulb
A 5600 microfarad capacitor, a light bulb, and a 120 V dc supply in series show a long time constant where the bulb dims as the
capacitor charges.
Similarly when current is switched off to a device containing light bulbs.
The bulbs continue to grow after switching off because of current stored in the capacitors.

38.6.0 Radio waves
Light Modulation, Photo Phone Kit, amplitude modulation
1. Radio frequency, AC, signal pulse, Frequency bands VHF, Telecommunications, radio receivers, wireless, waveband carriers, wave
tuners, transmitters, receivers, modulator, UHF radio wave generator
2. Frequency modulated, FM, amplitude modulated, AM, signal
Aerial or antenna is a length of wire in the air for the purpose of receiving radio waves.
AM Amplitude Modulation is a type of radio communication where audio frequency signals vary the strength of a radio frequency
carrier wave, called modulation.
Amplify means to enlarge a small current but keep the original signal information.
The broadcast band is the part of the radio frequency spectrum for local radio stations, e.g. 530 to 1600 kHz.
The carrier Wave in AM radio wave is the part that does not have any signal information itself but carries the signal with it.
Feedback occurs when some or all of the output of a device (an amplifier, for example) can be fed back into the input.
Accidental feedback is unwanted, e.g. acoustic feedback from a speaker to a microphone with a resultant squeal).
Deliberate feedback includes the many types of circuits where feedback is used to correct operation.
Radio frequency is the part of the spectrum with frequencies from around 20 kHz and above.

38.6.1 Radio transmission and reception
See diagram 38.1.08
Heinrich Hertz is credited with being the first person to send and receive radio signals between 1885 and 1889.
However, some people say it had been done twice before, in 1866 by Mahlon Loomis and in 1879 by David E Hughes.
Closely wind one hundred turns of 28 S.W.G. enamelled wire on a cardboard tube of approximate diameter 5 cm.
This coil, called an inductor, is tuned with a variable condenser of maximum capacity 0.0005 microfarads.
Detect radio signals from the local transmitter by means of either the diode valve used previously or a germanium diode.
An oscillator circuit produces alternating current at any frequency, but usually audio or radio frequencies.
The function of the diode is to rectify the current oscillations, and to permit detection of the audio frequencies by the earpiece.
By varying the inductance or capacity of such a circuit it is possible to receive broadcasts of different radio frequencies, i.e. from
different radio stations, because the tuning circuit is made to resonate electrically with the frequency being received.
(Comment: The function of the diode in radio reception:
A diode is a non-linear device that has the property of being able to "mix" several frequencies.
In mathematical terms this is "multiplying".
The side band frequencies are being mixed with the carrier produced by the resonance of the LC circuit.
One of the results of mixing is the original intelligence and elimination of the carrier.)
Use a DC moving coil voltmeter with a full-scale deflection of 10 volts.
On connecting it to a suitable source of AC there should be no deflection of the pointer.
However, on connection to either a germanium or a silicon diode, a deflection should be observed because the diode, which is a
semi-conductor, allows electrons to flow in one direction only.
This is an example of half wave rectification as in 38.41.5.
Replace the voltmeter in the circuit with a cathode ray oscillograph.
Connect the leads to the vertical deflection plates to obtain patterns as in diagram 38.41.4.
Suitable diodes include OA 2 10.
Use a diode valve instead of a semi-conductor or use a triode or multigrid valve by joining the grid of the valve to the anode.
The joined elements then act as the anode.
Use a 12 AD 6 valve because it may be operated from a 12 volt supply.
Connect the anode to one end of the secondary coil as in diagram 41.4 and the cathode to the voltmeter.
Heat the cathode with car battery current and observe the voltmeter.

38.6.2 Signal generator
1. The IEC LB3759-001 Mini Lab Signal Generator is a multi-purpose and low cost instrument for the electronics laboratory.
The frequency range is limited, however it provides the technician or student with a very convenient multi-purpose instrument.
The Mini Lab includes two analogue meters scaled from 0 to 20, which can be selected to be either voltmeters or ammeters.
In either mode, each meter has 2 ranges.
Also includes a well- regulated power source from 1.5 to 12 V DC at 1 amp, a signal generator (1 Hz to 10 kHz) and an audio
amplifier with inbuilt speaker.
The signal generator output is selectable to be either sine or square, the output level is fixed at 10V p / p and the maximum permissible
current is 50 mA.
2. IEC signal generator, convenient signal source, output includes sine, triangular and square.
Supplies currents up to 0.5 amp directly to loads, so amplifiers not necessary.
Can directly load devices requiring heavy current, e.g. simple controls include frequency adjustment, range selection, waveform
selection, output amplitude from 0 to 10 V, and attenuation in sB to set the output as required.
Output sockets for both 600 ohm output impedance and very low output impedance.
Output current maximum 500 mA even after short circuit, 0.1 Hz - 100 Hz output.

38.8.1 Gas discharge tubes
See: Spectrophotometers (Commercial)
The gas discharge tube contains gas at low pressure.
The electric field from the potential difference between the cathode and anode causes ions and free electrons to accelerate towards
the electrodes and collide with particles to excite them and create more ions.
The decay of the excited particles releases characteristic light.
In an evacuated discharge tube electrons from the cathode can be deflected by the application of electric or magnetic fields so
electrons must be charged particles.
The light from discharge tubes containing pure elements can be analysed using a spectrophotometer into the lines of different colours
This information can lead to identification of elements in a sample.
By using a hydrogen discharge tube, the frequencies of visible light, UV radiation and infra-red radiation can be measured to explain
the energy transition level for the electron in the hydrogen atom in kJ mol-1.
The flow of electric charge through gas occurs in fluorescent light bulbs and lightning.

38.8.2 Incandescent lamp
See 4.65: Electric light bulb (incandescent filament lamp)
Hot solids or liquids emit wavelengths of radiation depending on the temperature as a continuous spectrum.
At lower temperatures they emit red wavelengths, so the metal appears to be "red hot".
At higher temperatures, they emit the full visible spectrum as white light, so the metal appears to be "white hot" or "incandescent".
The incandescent filament in an electric light globe, a filament lamp, is "white hot".

38.8.3 Fluorescent lamp
See diagram 28.199: Fluorescent lamp, low voltage light source
Materials may emit light and other radiation when illuminated by higher frequency radiation or by streams of electrons.
Electron tubes contain gas under reduced pressure that allow movement of electrons between electrodes, e.g. the now obsolete
thermionic valve.
A fluorescent lamp is a gas discharge tube containing mercury vapour at low pressure.
It emits light because the inside of the tube is coated with a fluorescent substance, phosphor.
Electric current passes through the mercury vapour and produces ultraviolet radiation that hits the phosphor and is converted to light.
In a tube, the gas discharge tube contains gas at low pressure that glows red.
In a cathode ray tube, or in an X-ray tube, electrons from a heated cathode hit a fluorescent screen to produce light.

38.8.4 Cathode ray tube CRT, cathode ray oscilloscope CRO
See: Oscilloscope, (Commercial)
| See diagram: 38.1.11: Cathode ray oscilloscope CRO
| See diagram 38.1.11a: Johnson cathode ray tube
| See diagram 38.1.11b: Cathode ray tube
| See diagram 38.1.12: Simple discharge tube
A cathode ray is a beam of electrons coming from the cathode (negative electrode), of a vacuum tube.
In a cathode ray tube, the cathode rays cause a luminous image on a fluorescent screen.
The CRO measures constant and varying voltage, both.
It consists of a cathode ray tube and control circuits.
An electrically heated cathode gives off negative charged electrons that are accelerated towards a positive anode by a large voltage.
Electrodes between the cathode and anode control the intensity and focussing of the electrons, previously called cathode rays, that
pass through a small hole in the anode.
The beam of electrons emerging from the anode passes through a vertical electric field between two horizontal Y-plates to enable the
electron beam to be deflected vertically.
When the top plate is positive and the bottom plate is negative, the negative electron beam will be deflected upwards.
The beam also passes through a horizontal electric field between two vertical X-plates.
Controls connected to these plates cause the electron beam to be deflected horizontally.
The beam then strikes a screen coated with a fluorescent material that gives out light when struck by electrons.

1. Plug in the cathode ray tube.
Deflect the beam of the CRT by holding a permanent magnet near the edge of the tube.
If the beam disappears, you are holding the magnet too close, OR,
deflect the beam by attaching a battery to the binding posts and adjusting the variable control knob, OR
connect the battery directly to the deflection plate terminals.

2. To make a CRO, seal platinum wire into wide bore glass tubing, the main tube is now strongly heated and the heated fused glass
pushed into the hole so that the end is completely sealed.
The closed end is now alternately heated and blown until it is neatly rounded.
3. Bar magnet near a black and white television screen
The field of the bar magnet interacts with the magnetic field due to the stream of cathode rays so that the picture moves.

38.7.01 Logic gates
Logic board, logic gates, NOT gate (inverter) NOR gate, OR gate, AND gate, NAND gate, EOR gate, logic gate combinations
Integrated circuit, IC, is a circuit integrated onto a chip of almost pure silicon and may contain thousands of transistors.
Digital integrated circuits process signals with great speed and accuracy, e.g. switch, add to or subtract, they are the basic units of
computers, measuring instruments, control equipment, entertainment devices.
TTL ICs (Transistor Transistor Logic) CMOS (Complementary Metal Oxide Semiconductor). Digital IC's are represented in circuit
See Logic Gate Symbols.

38.7.1 Bits and bytes, prefixes for multiples
A bit is the name given to a single piece of information representing a 0 or a 1. In 0 can be represented by a negative voltage and the
1 can be represented by a positive voltage.
In a computer, the Central Processing Unit (CPU) power is measured by how many instructions it can execute per second,
e.g. an 8 bit or 16 bit or 32 bit or 64 bit microprocessor can process 8 bits or 16 bits or 32 bits or 64 bits at a time.
With 8 bits, 256 different values (the numbers from 0 to 255) can be represented because 28 = 256.
A groups of 8 bits is called a byte or octet.
A bit (Binary digit) is a digit in the binary number system, shown as 0 or 1.
A binary number is represented by one or more bits.
The binary number system uses successive powers of 2, so in binary notation the decimal number 11 is represented as the binary
number 1011 (decimal numbers 8 + 0 + 2 +1).
An analogue signal continuously changing with time, e.g. voltage, pressure, velocity, temperature.
A digital signal takes a reading of a signal at regular time intervals so it is a sample of an analogue signal or a value of original data.
Digital electronic circuits operate on the presence or absence of highs and lows, a pulse =1 and no pulse = 0.
A binary signal is a digital signal that has only two finite number of values, normally two different voltage values represented by
High / Low, ON / OFF, True and False, High and Low, Logic HIGH (or logic 1) Logic LOW (or logic 0).

Prefixes for multiples of bits and bytes
1000, K, kilo | 10002, M, mega | 10003, G, giga | 10004, T, tera | 10005, P, peta | 10006, E, exa | 10007, Z, zetta | 10008, Y, yetta
1024, K, Kilo, Kilobyte
10242, M, Mega, Megabyte
10243, G giga, Gigabyte
10247, Z, Zettabyte.

38.7.2 Decimal and binary codes for numbers 0 to 25, Powers of 2
Table 38.7.2
n 0 1 2 3 4 5
2n 1 2 4 8 16 32
n 6 7 8 9 10 11
2n 64 128 256 512 1024 2048
Decimal to binary, e.g. decimal number 99
Highest power of 2 in 99 = 26 = 64, 99 - 64 = 35
Highest power of 2 in 35 = 25 = 32, 35 - 32 = 3
Highest power of 2 in 3 = 21 = 2, 3 - 2 = 1
Highest power of 2 in 1 = 20
So binary number = 1100011
(64 + 32 + 2 + 1 = 99)
Binary to decimal
e.g. 1 1 0 0 0 1 1 =
1 1 0 0 0 1 1
26 25 0 0 0 21 20
64 + 32 + 0 + 0 + 0 + 2 + 1
= 99 -- Most significant bit Least significant bit Highest weight Least weight
Binary Binary Binary Binary Binary
- 24 = 16 23 = 8 22 = 4 21 = 2 20 =1
0 0 0 0 0 0
1 0 0 0 0 1
2 0 0 0 1 0
3 0 0 0 1 1
4 0 0 1 0 0
5 0 0 1 0 1
6 0 0 1 1 0
7 0 0 1 1 1
8 0 1 0 0 0
9 0 1 0 0 1
10 0 1 0 1 0
11 0 1 0 1 1
12 0 1 1 0 0
13 0 1 1 0 1
14 0 1 1 1 0
15 0 1 1 1 1
16 1 0 0 0 0
17 1 0 0 0 1
18 1 0 0 1 0
19 1 0 0 1 1
20 1 0 1 0 0
21 1 0 1 0 1
22 1 0 1 1 0
23 1 0 1 1 1
24 1 1 0 0 0
25 1 1 0 0 1
31 1 1 1 1 1

38.7.3 Addition of binary numbers
0 + 0 = 0, i.e. decimal number 0
1 + 0 = 1, i.e. decimal number 1
1 + 1 = 10, i.e. decimal number 2
1 + 10 = 11, i.e. decimal number 3
1 + 11 = 100, i.e. decimal number 4
10 + 11 = 101, i.e. decimal number 5
11 + 11 = 110, i.e. decimal number 6
37 + 20 = 57.

38.7.4 ASCII (American Standard Code for Information Exchange)
The first computer was the "difference machine" consisting of brass gears invented by Charles Babbage (1791-1871).
It was never built because of the cost, but a replica works well.
Table 38.7.4
A 1100 0001 K 1101 0010 U 1110 0100 4 0011 0100
B 1100 0010 L 1101 0011 V 1110 0101 5 0011 0101
C 1100 0011 M 1101 0100 W 1110 0110 6 00110110
D 1100 0100 N 1101 0101 X 1110 0111 7 0011 0111
E 1100 0101 O 1101 0110 Y 1110 1000 8 0011 1000
F 1100 0110 P 1101 0111 Z 1110 1001 9 0011 1001
G 1100 0111 Q 1101 1000 0 0011 0000 -
0010 1110
H 1100 1000 R 1101 1001 1 00110001 -
0010 1100
I 1100 1001 S 1110 0010 2 00110010 ? 0011 1111
J 1101 0001 T 1110 0011 3 00110011 ! 0010 0001