School Science Lessons
39. "UNESCO Electronics Teacher's Guide", components, switches, logic circuits,
gates, bistables, drivers, pulser, counting
2012-05-05c SP
Please send comments to: J.Elfick@uq.edu.au
Table of contents
39.00 Electronics
All the experimental work below uses a 6 V supply from batteries but they
will also work from a 4.5 V supply.
39.1.0 Electronic Components
39.2.0 Switches
39.3.0 Logic circuits
39.4.0 Logic gates
39.5.0 Bistable circuits
39.6.0 Drivers
39.7.0 Coding
39.8.0 Pulser (pulse generator), the
astable and the clocked bistable
39.9.0 Counting circuits
38.1.18 Signal generator (commercial)
39.1.0 Electronic Components
39.1.1 Resistor and light-emitting diode (LED)
39.1.2 Brightness and current
39.1.3 LEDs in parallel
39.1.4 Current direction indicator
39.1.5 Light dependent resistor (LDR)
39.1.6 Simple burglar alarm using LDR
39.2.0 Switches
39.2.1 Manual control of a LED
39.2.2 Reed switch and magnet
39.2.3 Reed switch and coil, the reed relay
39.2.4 Reed relay controls a motor
39.2.5 Reed relay with a coil and contacts in parallel,
using a single power supply
39.2.6 Automatic washing line
39.2.7 Reversing an electric motor
39.3.0 Logic circuits
39.3.1 Simple AND circuit
39.3.2 Simple OR circuit
39.3.3 Relay as a NAND circuit
39.3.4 Simple burglar alarm
39.3.5 NAND circuit as an inverter
39.3.6 Make an AND circuit with two NAND circuits
39.3.7 Excess length detector
39.3.8 Two NAND circuits as a bistable (Flip-flop)
39.3.9 Latched burglar alarm
39.3.10 Use a bistable to control an electric
motor
39.4.0 Logic gates
39.4.01 Logic symbols
39.4.1.0 LED indicators
39.4.2.0 NAND gate truth table
39.4.2.1 NAND gate as an inverter (NOT circuit)
using joined inputs
39.4.2.2 NAND gate as an inverter (NOT circuit)
using separate inputs
39.4.2.3 Astable using two NAND gates connected
as an AND gate
39.4.2.4 AND gate from two NAND gates
39.4.2.5 OR gate using three NAND gates
39.4.2.6 NOR gate using four NAND gates
39.4.3.0 Applications using NAND gates
39.4.3.1 Burglar alarm (NAND application)
39.4.3.2 Length detector (AND application)
39.4.3.3 Automatic night light using two NAND
gates as inverters
39.4.3.4 Fire alarm using NAND gate as inverter,
and a thermistor
39.4.3.5 Safety circuit for a safe using three
input AND gate
39.4.3.6 Skittle alley winner indicator with
3-input NAND gate
39.4.3.7 Car doors warning light using 2-input
NAND gate
39.4.3.8 Light at the top of the stairs circuit
using OR gate
39.5.0 Bistable circuits
39.5.1.0 Bistable using two NAND gates
39.5.1.1 Two NAND gates as inverters
39.5.1.2 Two NAND gates with reversed conditions
39.5.1.3 Two NAN gates with positive feedback
link, bistable
39.5.1.4 Bistable with complimentary outputs,
Q and Q'
39.5.1.5 Bistable building block
39.5.1.6 Bistables and logic gates
39.5.2.0 Bistable applications using NAND gates
39.5.2.1 Latched burglar alarm
39.5.2.2 Latched fire alarm using a bistable
39.5.2.3 Simple stop-go traffic lights
39.5.2.4 Traffic lights operated by an SPST switch
39.5.2.5 Quiz master
39.6.0 Drivers
39.6.1.0 Loading an output
39.6.2.0 Using a NAND gate to switch an electric
motor on or off
39.6.2.1 Inverting driver amplifier
39.6.2.2 Driver amplifier with relay omitted
39.6.3.0 Applications involving the driver amplifier
and reed relay
39.6.3.1 Reversing an electric motor
39.6.3.2 Reversing an electric motor with a bistable
circuit
39.6.3.3 Automatic light
39.6.3.4 Motor vehicle moving backwards and forwards
between two light beams
39.7.0 Coding
39.7.1d Sending messages using a 4 bit binary
code
39.7.2 Seven segment LED display
39.7.3 Seven segment display with a decoder
39.7.4 Two-line to four-line decoder from NAND
gates
39.8.0 Pulser, the astable and the clocked bistable
39.8.1.0 Voltage outputs from the
pulser / astable module
39.8.2.0 Clocked bistable
39.8.3.1 Four 4 bit binary counters
from clocked bistables, binary up-counter
39.8.3.2 Four 4 bit binary counters
from clocked bistables, binary down-counter
39.9.0 Counting circuits
39.9.1.0 Four bit binary counter
integrated circuit
39.9.1.1 Linking the counter to
a decoder and seven segment display
39.9.2.0 Counters of different moduli
39.9.3.0 Switch contact bounce
39.9.3.1 Bistable circuit to eliminate
contact bounce
39.9.4.0 One bit memory from NAND
gates and an RS bistable
39.9.5.0 Linking two single digit
counters to make a dual decade counter
39.9.6.0 Applications using counting
circuits
39.9.6.1 Divide-by-N counting
39.9.6.2 Down-counting 1.
39.9.6.3 Down-counting 2.
39.9.6.4 Counting the swings of a pendulum
39.9.6.5 Controlling a motor 1.
39.9.6.6 Controlling a motor 2.
39.9.6.7 Reversing a motor at regular intervals
39.9.6.8 Flashing a lamp six times, six pips
39.9.6.9 Automatic light buoy
39.9.6.10 Electronic die
39.9.6.11 Traffic lights
39.9.6.12 Batch counting
39.9.6.13 Reaction time
Other topic headings
Timers
Position and Velocity Detectors
Sources of Sound
Sound Detectors
Circuits/Components/Instruments
Function Generators
Oscilloscopes
Advanced Instruments
Power Supplies
Light Sources
Light Paths Made Visible
Lasers
Microwave Apparatus
Computer Interface
Appendix A Bibliography
Appendix B Technical
See:
See: Interesting websites
See: Chapter 38 Electronics
1 Experiments
39.1.0 Electronic Components
39.1.1 Resistor and light-emitting diode (LED)
See diagram 39.1.1
Use a LED, a battery and a resistor.
The passage of current causes the emission of light. The intensity of
light depends on the size of the current. The direction in which current
will flow is the same as that of the arrowhead in the symbol. (2) Reverse
the connections so that the diode is the other way round. A LED will allow
current to pass in only one direction. (3) If you connect directly across
the battery you will destroy the LED, so insert a series resistor to limit
the current, e.g. 10 milliamperes.
39.1.2 Brightness and current
See diagram 39.1.2
Connect the circuit shown, using a LED module, a resistor and a battery.
Use a low value resistor and note how brightly the LED glows. Replace
the resistor first by the one of medium value, then by one of large value,
e.g. 27 k ohms, 2.7 k ohms and 270 ohms. Note what happens to the brightness
of the LED and how does the brightness of the LED depend on the current passing
through it.
39.1.3 LEDs in parallel
See diagram 39.1.3
Use one red LED, one green LED and a battery.
Both LEDs glow because Two LEDs of the same colour and resistance connected
in parallel will glow with the same brightness because the currents through
each are equal. A LED of different colour with the same value of series resistor
may not glow with the same brightness. Use a milliammeter to show that the
currents through red and green LEDs with equal series resistors are almost
the same. Variations in apparent brightness are due to differences in the
materials and to variations in the sensitivity of the eye to different colours.
39.1.4 Current direction indicator
See diagram 39.1.4
If you connect a battery one way round to the indicator, one LEDs should
glow. If you reverse the battery connections, the other LED will glow.
39.1.5 Light-dependent resistor (LDR)
See diagram 39.1.5
Use a light dependent resistor in series with a LED and a battery. If
you cover or uncover the LED, the current will flow in either direction as
with an ordinary resistor. The resistance of a LDR is very large in the dark,
e.g. a million ohms or more, but falls to a small value, e.g. two hundred
ohms, in bright light. Cadmium sulfide is commonly used for LDRs. A LDR is
not a photocell.
39.1.6 Simple burglar alarm using LDR
See diagram 39.1.6 Use and LDR, a buzzer and
a battery to construct a circuit that will sound an alarm when a light comes
on. The buzzer probably has a definite polarity.
39.2.0 Switches
The next 3 experiments show the progression from direct manual control
to magnetic control, and then to electromagnetic control.
39.2.1 Manual control of a LED
See diagram 39.2.1
Connect a battery, and LED and a push-button switch together in a series
circuit. The LED lights only when the push-button switch is pressed.
39.2.2 Reed switch and magnet
See diagram 39.2.2
Use a magnifying glass to see the two metal contacts inside the glass
envelope of the reed switch. These contacts are normally open. Connect the
switch in series with a buzzer and a battery. Bring a small bar magnet close
to the reed switch to see the contacts close. A reed switch has two metal
contacts, "reeds", inside a glass envelope filled with an inert gas to prevent
corrosion. The contacts consist of a ferrous metal so a magnet can magnetize
them. If you bring the magnet close to the switch, the metal strips become
magnetized and attract each other. When the contacts close, a current flows
through the buzzer. The magnet produces a force that closes the switch contacts.
The reed switch contacts are either open or closed so you call the reed switch
a single-pole single-throw, SPST, switch.
39.2.3 Reed switch and coil, the reed relay
See diagram 39.2.3
Using a reed relay with two separate battery supplies. Do not use either
of the diode connections to the coil of the relay. In this experiment switching
is controlled by the magnetic effect of a current-carrying coil that has a
magnetic effect, an electromagnetic relay. When the switch is closed, the
relay uses a change-over reed switch, a single-pole double-throw, SPDT, switch.
The reed C and contact A consist of a magnetic metal. Contact B is a nonmagnetic
metal. Contact C is normally in contact with B. However, when a magnetic
field magnetizes A and C, they are attracted together so that contact with
B is broken and C contacts A instead. When you remove the field, C springs
back to its original position. Change the switch connections at the relay
so that the LED is on until the push-button switch is pressed. If current
flows through the coil, the reed switch changes over and the LED lights.
Note that the flow of an electric current now determines the switching, and
although the two circuits are separate, what happens in one circuit is controlled
by what happens in the other circuit. The current through the coil, needed
to operate the switch, is usually smaller than the current allowed through
the switch contacts. So if the LED and resistor were replaced by an electric
motor that needed a large current to make it rotate, the electric motor could
be operated by a much smaller current through the coil circuit.
39.2.4 Reed relay controls a motor
See diagram 39.2.4
Use a reed switch, a motor and two batteries. When the LDR is covered,
its resistance is great, and not enough current flows through the relay coil
to operate the reed switch and switch on the motor. When the LDR is well illuminated
with a torch, its resistance decreases and the relay operates. Insert ammeters
in series with the coil and the motor to show that the current through the
motor, e.g. 0.2 A, is more than the current through the coil. So the flow
of a small current is used to control the flow of a much larger current.
If the reed relay is not used, the change in the resistance of the LDR is
not enough to allow the motor to operate.
39.2.5 Reed relay with a coil and contacts in parallel,
using a single power supply
See diagram 39.2.5
In 39.2.4 and 39.2.6, a small current in one circuit controlled a much
larger current in another circuit that was completely separate with its own
power supply. In this experiment, a current through one branch of a parallel
circuit, the coil branch, controls a much larger current through another
branch, the motor branch, using only one battery. Note what happens to the
motor when the switch is closed. In this circuit the relay coil and the relay
contacts are connected in parallel. Trace the closed current paths from the
battery, through each branch of the circuit and back to the battery again.
This circuit uses the idea of positive and negative supply rails.
39.2.6 Automatic washing line
See diagram 39.2.6
Use two batteries, reed relay, electric motor and the rain sensor to make
a circuit that switches on an electric motor when rain fails. Make a rain
sensor from a small piece of strip board on which are six copper strips are
connected. If a conducting solution bridges any of the strips, the two leads
are short circuited allowing current to flow through the relay coil. If the
experiment does not work with tap water add salt to the water. Rain water
may conduct electricity, but not sufficiently to operate the reed relay in
the circuit. The experiment will work with rain if the copper strips are covered
by dry absorbent paper soaked in a salt solution.
39.2.7 Reversing an electric motor
See diagram 39.2.7 Use a push-button switch,
the reed relay and three batteries to make a circuit so that the direction
of rotation of the motor is reversed when the switch is pressed. With a 3
volt motor only one battery may be used.
39.3.0 Logic circuits
39.3.1 Simple AND circuit
See diagram 39.3.1
Connect two push-button switch modules in series with a LED and a battery.
| S1 |
S2 |
LED |
| Not pressed |
Not pressed |
OFF |
| Pressed |
Not pressed |
OFF |
| Not pressed |
Pressed |
OFF |
| Pressed |
Pressed |
ON |
The table tells us that the LED is on when S1 AND S2 are pressed, so the
circuit is called an AND circuit. An AND circuit may be used in a motor car
if the ignition light shows that the car can be started only when the driver
has engaged his safety belt AND closed his door.
39.3.2 Simple OR circuit
See diagram 39.3.2
| S1 |
S2 |
LED |
| Not pressed |
Not pressed |
OFF |
| Pressed |
Not pressed |
ON |
| Not pressed |
Pressed |
ON |
| Pressed |
Pressed |
ON |
The truth table for an OR circuit tells us that the LED is on when either
S1 OR S2, or both, are pressed. In a burglar alarm pressure pads are open
SPST switches. A pad is placed under a carpet so that the pressure of the
burglar's foot closes the switch. The circuit could be used with two such
switches. An alarm sounds, if the buzzer replaces the LED, if entry were through
door 1 OR door 2.
39.3.3 Relay as a NAND circuit
See diagram 39.3.3
A logic circuit is a switching circuit in which the state of the output
at any instant depends on the present state of all the inputs. The output
is HIGH only for some input combinations. NAND is a contraction of "negative
AND". NAND circuits are commonly used in electronics. Construct the NAND relay
circuit with its coil and contacts in parallel with the supply, and its inputs
and outputs are either HIGH or LOW.
| INPUT B |
INPUT A |
OUTPUT |
| LOW |
LOW |
HIGH |
| LOW |
HIGH |
HIGH |
| HIGH |
LOW |
HIGH |
| HIGH |
HIGH |
LOW |
Using a reed relay, a LED and a battery, connect leads to the diode input
terminals of the relay, A and B. When you connect the other end of one of
these leads to the positive supply rail, the input is HIGH. If you connect
the lead to the negative supply rail, the input is LOW. The output is HIGH
if the LED is on and LOW if the LED is off. The output is HIGH when one or
both of the inputs are LOW.
In an AND circuit, its output is HIGH when input A and input B are HIGH,
see the AND circuit truth table below. In a NAND circuit, the output is LOW
when input A and input B are HIGH, see the NAND relay circuit truth table
below. The contacts at the output form an SPDT switch. You normally connect
the pole of this switch to the negative supply rail, i.e. the output is LOW.
This is the position at the output when no current flows through the relay
coil. When either or both inputs are taken LOW, current flows through the
coil and the output contact changes over. The output goes HIGH and the LED
module is connected directly to the positive supply rail. No current can flow
through an input if it is HIGH nor if the input is unconnected. So an unconnected
input behaves as though it were HIGH. You include diodes at the inputs because
without diodes and one input were taken HIGH and the other taken LOW, a
short circuit across the battery may occur.
AND circuits.
| INPUT B |
INPUT A |
OUTPUT |
| LOW |
LOW |
LOW |
| LOW |
HIGH |
LOW |
| HIGH |
LOW |
LOW |
| HIGH |
HIGH |
HIGH |
NAND relay circuit LOW
| INPUT B |
INPUT A |
OUTPUT |
| LOW HIGH |
LOW |
HIGH |
| LOW |
HIGH |
HIGH |
| HIGH |
LOW |
HIGH |
| HIGH |
HIGH |
LOW |
39.3.4 Simple burglar alarm
See diagram 39.3.4
Note what happens when switch A or switch B is pressed, or both are pressed
together. How would you modify this circuit so that the alarm sounds either
when a switch was closed or when a light shines on a LDR. This circuit is
more realistic if you use pressure pads instead of the switch modules, as
in 39.3.2. A light beam can trigger the alarm by replacing one switch module
by a LDR module. If you cover the LDR or the room is dark the LED will have
a very HIGH resistance and so little current flows through the relay coil.
When light fails on the LDR its resistance rapidly decreases and current can
flow through the coil, so the LDR operates the relay. The input is now LOW.
39.3.5 NAND circuit as an inverter
See diagram 39.3.5
Connect a LED to the output and a flying lead to one input of the NAND
circuit. Use the flying lead to take the input HIGH and LOW. When you use
one input of a NAND circuit, or you join the two inputs of a NAND circuit
as a single input, the NAND circuit behaves as an inverter.
| INPUT |
OUTPUT |
| HIGH |
LOW |
| LOW |
HIGH |
39.3.6 Making an AND circuit with two NAND circuits
See diagram 39.3.6
Using two NAND circuits (one as an inverter), a LED and a battery. The
inverter inverts the output of the NAND circuit. Compare the truth table
below with 39.3.3. Connect a flying lead to each input terminal of the first
NAND circuit. Remove the flying lead from input B and connect a push-button
switch between input B and the negative supply rail. Connect the flying lead
from input A to the positive supply rail, i.e. HIGH, then operate the switch
several times. Connect the flying lead from input A to the negative supply
rail, i.e. LOW, and operate the switch. When you connect the lead from input
A to the positive supply rail, HIGH, operating the switch connected to input
B causes the LED to go on or off. So the output follows the changes at input
B. When input A is LOW, the LED is off and operating the switch does not affect
it. The action of the circuit is like a gate and you call it a "gate". With
input A HIGH, the gate is opened and the signals pass through it. With input
A LOW, the gate is closed and the output is not affected by changes at input
B.
| INPUT B |
INPUT A |
OUTPUT |
| LOW |
LOW |
LOW |
| LOW |
HIGH |
LOW |
| HIGH |
LOW |
LOW |
| HIGH |
HIGH |
HIGH |
39.3.7 Excess length detector
See diagram 39.3.7
Use two NAND circuits (one connected as an inverter), two LDR modules
and a buzzer. Illuminate each LDR with a light beam. If light shines on an
LDR, its resistance is LOW. The inputs of the first NAND circuit are therefore
LOW, so its output must be HIGH. The buzzer sounds only when both the LDRs
are shaded. The second NAND circuit is an inverter, so its output is LOW and
the buzzer does not sound. If one LDR is covered, but not the other, one
input is still LOW, a current still flows in the coil, the output of the
first NAND circuit is still HIGH, the output of the inverter is LOW, and
nothing happens. However, if both LDRs are covered, the output from the first
NAND is LOW, so the output from the second is HIGH and the buzzer sounds.
If objects of greater length than the distance between the LDRs pass in front
of them, the buzzer sounds. The two relay modules, a NAND circuit followed
by an inverter, are together acting as an AND circuit. So, the output is HIGH,
the buzzer sounds, only when input 1 AND input 2 are HIGH.
39.3.8 Two NAND circuits as a bistable (Bistable
multibrator, Flip-flop)
See diagram 39.3.8
To show the circuit, press the RESET switch. The green LED is on and the
red LED is off. Press the SET switch. The red LED is on and the green LED
is off. Press the SET switch repeatedly. The red LED is still on and the green
LED is still off. Press the RESET switch again. The green LED is on and the
red LED is off. Press the RESET switch repeatedly. The green LED is still
on and the red LED is still off. When the RESET switch is pressed, the output
of NAND circuit 2 is HIGH and the output of NAND circuit 1 is LOW. When the
SET switch is pressed, the output of circuit 1 is HIGH and the output of
circuit 2 is LOW. The bistable circuit has two stable states. The first stable
state occurs when the output of NAND circuit 2 is HIGH and the output of
NAND circuit 1 is LOW, the RESET state. The second stable state occurs when
the output of NAND circuit 1 is HIGH and the output of NAND circuit 2 is
LOW, the SET state.
In a bistable circuit, after the SET switch has been pressed, repeated
depressions of this switch have no effect. Also, after the RESET switch has
been pressed, repeated pressing of this switch have no effect. The SET and
RESET inputs are normally unconnected, i.e. they "float HIGH". To change
from one stable state to the other the appropriate input must be taken LOW.
Follow the path of the current flow through the coils and the connections
between the two NAND circuits.
Note how the two NAND circuits are joined and share the same power rails.
The output C of the first circuit is connected to the input D of the second
circuit. Similarly the output F of the second circuit is connected to the
input A of the first circuit. A red LED is connected between C and M. If C
is HIGH, the red LED will light. If C is LOW, the red LED will not light.
A green LED is connected between F and N. If F is HIGH, the green LED will
light. If F is LOW, the green LED will not light. A bistable circuit has two
stable states. For the first stable state, if C is HIGH, the red LED will
light, D will be HIGH, F will be LOW so the green LED will not light. As
F is low, A is LOW, so C remains HIGH. For the second stable state, if C
is LOW, the red LED will not light, D will be LOW, F will then be HIGH so
the green LED will light. As F is high, A is HIGH, so C remains LOW. This
is the second stable state, and so you use the name "bistable" as it has two
stable states. Note that the output of the 1st module is connected to the
input of the 2nd module and the output of the 2nd module is connected
to the input of the 1st module.
1. To set the bistable in one stable state or the other, push-button switches
are connected between input B and the negative rail at P and between input
E and the negative rail at Q.
2. To set the first stable state, press the push-button switch at P, current
flows through the relay coil, C becomes HIGH and the red LED will light.
3. To set the second stable state, press the second push button at Q,
current flows through the relay coil F becomes HIGH, the green LED will light.
The circuit could be used to control of simple traffic lights so that the
red light and the green light cannot be turned on together.
39.3.9 Latched burglar alarm
Replace the red LED in the circuit of 39.3.8 with a buzzer. Remove the
green LED. If the buzzer sounds, press the RESET switch to silence it. The
SET switch is now the trip switch of the alarm. If it is pressed, the alarm
sounds and it cannot be turned off by releasing the trip switch or pressing
it again. You say the output is locked or latched because the bistable is
locked in its second stable state. The alarm can only be turned off by pressing
the RESET switch. This latched alarm is an improvement on the alarm constructed
in 39.3.4.
39.3.10 Use a bistable to control an electric motor
Remove the buzzer from the circuit of 39.3.9, or both LEDs from the circuit
of 39.3.8, and connect the motor between the outputs of the two NAND circuits
C and F in diagram 39.3.8. Pressing the SET and RESET switches alternately
changes the direction of rotation of the electric motor. The bistable circuit
here is used simply to reverse the polarity of the motor supply.
39.4.0 Electronic logic circuits, logic gates
See 39.4.0 Modules
Draw a circuit before connecting the parts. If their circuit does not
behave as expected, try to explain why this happens before making any changes
to the circuit. CMOS integrated circuits operate over the wide voltage range
of 3 V to 18 V and require little supply current unless driving external devices,
e.g. LEDs and 7-segment displays. Use dry cells that can drive the LEDs and
the 7-segment displays because the current required will rarely exceed 0.2
A. However, performance of CMOS integrated circuits can vary with the supply
voltage chosen. A supply voltage of 5 V or 6 V is recommended. Integrated
circuits can be damaged if supply polarity is reversed so connect a diode,
e.g. IN5401, between the power rails of each circuit so the diode conducts
if the supply connections are inadvertently crossed. The reverse voltage
is about 1 V. and current about 3 A. When connecting prepared circuits, link
the corresponding power rails of the circuits before connecting the circuits
to the supply.
39.4.01 Truth tables for logic gates
See diagram 39.4.01: Logic symbols
AND gate: Circuit has two or more inputs, and one output that is high if
all inputs are high.
NAND gate: Circuit has two or more inputs, and one output that is high
if one or more of the inputs are low, and low if all the inputs are high.
OR gate: Circuit has two or more inputs, and one output that is high if
one or more of the inputs are high.
NOR gate: Circuit has two or more inputs, and one output that is high
only if all inputs are low.
INVERTER (NOT gate): Circuit has one input, and one output that is high
if the input is low and low if the input is high.
| AND |
. |
. |
NAND |
. |
. |
| B |
A |
OUTPUT |
B |
A |
OUTPUT |
| LOW |
LOW |
LOW |
LOW |
LOW |
HIGH |
| LOW |
HIGH |
LOW |
LOW |
HIGH |
HIGH |
| HIGH |
LOW |
LOW |
HIGH |
LOW |
HIGH |
| HIGH |
HIGH |
HIGH |
HIGH |
HIGH |
LOW |
| . |
. |
. |
. |
. |
. |
| OR |
. |
. |
NOR |
. |
. |
| LOW |
LOW |
LOW |
LOW |
LOW |
HIGH |
| LOW |
HIGH |
HIGH |
LOW |
HIGH |
LOW |
| HIGH |
LOW |
HIGH |
HIGH |
LOW |
LOW |
| HIGH |
HIGH |
HIGH |
HIGH |
HIGH |
LOW |
| INVERTER (NOT gate) |
. |
| INPUT |
OUTPUT |
| HIGH |
LOW |
| LOW |
HIGH |
39.4.1.0 LED indicators
See diagram 39.4.1.0
Use a LED indicator module or different LEDs and power supply, connected
correctly. Plug a lead into the top input socket of the indicator module or
the first LED. Connect the other end of this flying lead to the positive power
supply rail or to the negative power supply rail. If the flying lead connects
to the positive rail, the input is said to be HIGH. If the flying lead connects
to the negative rail, the input is said to be LOW.
39.4.1.1 Indicator in a circuit diagram
See diagram 39.4.1.1
Note how an indicator is represented in the circuit diagram. The power
supply need not be drawn. Only the parts of a module in use need be shown,
so diagram 39.4.1.1 is the equivalent of diagram 39.4.1.0. The + and - signs
near the power rails show that a power supply is connected between them.
39.4.1.2 Control function in an AND gate
See diagram 39.4.1.2
The term gate refers to the switching circuit in which the state of the
output at any instant depends on the state of the inputs at that instant,
i.e. a logic circuit. In an AND gate one of its inputs performs a control
function when a succession of high and low pulses arriving at the other input.
If the control input is held low, the output will be held low if the state
of the other input is high or low, so the gate is closed. If the control input
is held high, the output state follows the state of the other input, so the
gate is open. A NAND gate is open when its control input is high as with
an AND gate, but the pulses arriving at the other input are inverted when
they arrive at the output. The OR gate and NOR gates are both open when their
control inputs are low. However, the output pulses are inverted with the NOR
gate.
39.4.2.0 NAND gate truth table
See diagram 39.4.2.0
Use a NAND gate, LED indicator and power supply. Each input A and B can
be connected to the positive power supply rail (high) or to the negative
power supply rail (low) using a flying lead. Note how the different LEDs light
when the input is connected high or low. The output is high if the LED is
lit and low if the LED is unlit. Connect the inputs high and low, complete
the truth table for a NAND gate. Note whether an unconnected input behaves
as if it is high or low.
39.4.2.1 NAND gate as an inverter (NOT circuit)
using joined inputs
See diagram 39.4.2.1
Note the inversion circle at the output of a gate with an inverting function.
The symbol for AND with an inversion circle becomes symbol for NAND ("not
and"). The symbol for OR with inversion circle becomes the symbol for NOR
("not or"). The NAND gates can be used to make NOT circuits, AND, OR and NOR
gates. The NAND gate used in these experiments has inputs that are high when
unconnected. Connect the two inputs of a NAND gate together as shown in the
diagram. The two joined inputs can now be thought of as a single input. This
single input can be taken high or low using the flying lead. By taking the
input high and then low, complete the truth table for the inverter. The inverter
is also called a NOT circuit.
39.4.2.2 NAND gate as an inverter (NOT circuit)
using separate inputs
See diagram 39.4.2.2
Instead of using a two input NAND gate as an inverter as in 39.4.2.1 by joining
the two inputs of the NAND gate, use one input of a NAND gate by joining
a flying lead to it and connect the other to the positive rail so it is permanently
high, or leave it leave it unconnected, i.e. "floating", as in diagram 39.4.2.2.1.
Note how the circuit behaves if the input is taken high and low. Note what
happens if one input is tied permanently low. However, for these experiments
show an inverter as in diagram 39.4.2.2.2.
39.4.2.3 An astable using two NAND gates connected
as an AND gate
See diagram 39.4.2.3
Use two NAND gates connected as an AND gate. When the control input is
high, pulses will pass through the AND gate and the two LEDs will flash on
and off together. If the control input is taken low, the LED at the output
will be permanently low. If a single NAND gate is used and the control input
is high, the output LED will flash on and off. However, it will be on when
the LED at the input is off, and vice versa. When the control input is low,
the output LED will be permanently on.
39.4.2.4 AND gate from two NAND gates
See diagram 39.4.2.4
Connect the circuit using two NAND gates. The second NAND gate is connected
as an inverter, i.e. a NOT circuit. Check that the circuit behaves as an AND
gate by taking the inputs high and low. Complete the truth table.
39.4.2.4.1 Power connections for AND gate from
two NAND gates
See diagram 39.4.2.4.1
By common convention draw the symbol without power supply connections
as in 39.4.2.2.2. However, the diagram shows how to include the supply lines
in the circuit diagrams to show the power supply connections to the electronic
devices inside the NAND gate.
39.4.2.5 OR gate using three NAND gates
See diagram 39.4.2.5
Note the truth table for an OR gate. Compare this with the truth table
for a NAND gate. If every high input to the NAND gate becomes a low and every
low input becomes a high, you have an OR gate. Design an OR gate using three
NAND gates. Build the circuit and check that it produces the correct truth
table.
39.4.2.6 NOR gate using four NAND gates
See diagram 39.4.2.6
Note the truth table for a NOR gate. Use four NAND gates to convert your
OR gate to a NOR gate. Check that your circuit produces the correct truth
the truth tables for a NOR gate.
39.4.3.0 Applications using NAND gates
Electronic circuits can do useful tasks and control devices, e.g. LEDs,
buzzers and motors. In these experiments integrated circuit NAND gates are
used instead of NAND circuit relays. However, the circuits are identical whether
relay or integrated circuit (IC) NAND gates are used. These applications use
illuminated LDRs to keep inputs low. So an LDR can act as a switch that is
open in the dark and closed when illuminated. Connect input devices such as
LDRs between gate inputs and the negative supply rail. Because an unconnected
input floats high, an LDR connected to the positive supply rail will not change
the logic level at the input when the illumination changes. When connected
to the negative supply rail, an LDR will cause a low level input when brightly
illuminated, low resistance, and a high level input when dark, high resistance.
A suitable LDR is the ORP 12. Buzzers are used because they are low current
devices that can operate over a voltage range of 3 V to 15 V and emit a constant
frequency audible tone.
39.4.3.1 Burglar alarm (NAND application)
See diagram 39.4.3.1 Use a single NAND gate,
an LDR, a push-button switch and a buzzer to make a simple burglar alarm.
The alarm should sound when the LDR is illuminated by the burglar's torch
or the switch is closed by the burglar's foot.
39.4.3.2 Length detector (AND application)
See diagram 39.4.3.2
Use two NAND gates (one gate connected as an inverter) with two LDRs and
a buzzer to construct a system that will sound an alarm when an object is
longer than a specified maximum length. Such a circuit might be used to reject
overlong objects passing along a conveyor belt in a factory.
The LDRs are positioned a distance x apart (where x is the specified maximum
length of the object) and illuminated by "pencil" torches. The alarm sounds
when both beams are interrupted.
39.4.3.3 Automatic night light (2 NAND gates as
inverters)
See diagram 39.4.3.3
Use two NAND gates (both connected as inverters), an LDR and a LED indicator
to make a circuit in which a light (the LED) will come on automatically in
the dark. This circuit could be used to turn on a light when it gets dark.
This application uses two NAND gate inverters to drive a LED indicator. The
circuit on the left is not very sensitive to changes of light intensity. This
is because the LDR and the internal circuitry of the NAND gate module effectively
form a potential divider between the supply rails. As R has a high resistance,
it needs a large change in the resistance of the LDR to change the voltage
level at the NAND gate input appreciably.
The second circuit allows the sensitivity to be changed. The variable
resistor should be set so that the LED is on the verge of switching on. Then
any darkening of the LDR will cause the LED to light. The value of the variable
resistance to use depends on ambient light conditions, but 10 k ohm should
suit most circumstances.
39.4.3.4 Fire alarm (NAND gate as inverter, and
thermistor)
See diagram 39.4.4
1. Use a NAND gate connected as an inverter, a thermistor, a variable
resistor and a buzzer to make a simple fire alarm. The alarm should sound
when the thermistor is heated. How would you adjust the circuit so that the
alarm sounded when the temperature reached a higher value?
It is important that students should have previously met the potential
divider and the thermistor if they are to solve this problem.
2. An inexpensive TH3 thermistor can be used for this fourth application.
Its resistance at room temperature is about 400 ohm, and decreases to about
20 ohm at 100oC. At room temperature, the 5 k ohm variable resistor,
i.e. a 5 k ohm potentiometer connected as a variable resistor, should be set
so that the buzzer is just turned off. Warming the thermistor with the hand
will then cause the buzzer to sound. To increase the temperature at which
the alarm responds, decrease the value of the variable resistance. Note that
a 2.5 k ohm variable resistor provides greater sensitivity.
39.4.3.5 Safety circuit for a safe using three
input AND gate
See diagram 39.4.5
A safety circuit for a large safe sounds an alarm only if the door is
closed but not locked. Closing the door opens a switch, while locking the
door closes another switch. What sort of logic gate is required to do this?
Set up the circuit and test it. Now adapt your circuit for a safe that has
two locks.
This project introduces the 3 input AND gate that will be used in several
projects later in the course. Note that 9 NAND gate 4 is omitted, the circuit
is a 3 input NAND gate.
39.4.3.6 Skittle alley winner indicator with 3-input
NAND gate
See diagram 39.4.3.6
In a fairground skittle alley, customers try to overturn three skittles.
Each skittle stands on a small switch that it keeps closed until it is overturned.
Design a circuit that lights a lamp only when a customer is successful. The
problem is similar to 39.4.3 (e) but it uses a 3 input AND gate. If gate 4
is omitted, the circuit is a 3 input NAND gate.
39.4.3.7 Car doors warning light using 2-input
NAND gate
See diagram 39.4.7
Design a circuit that will cause a warning lamp on the dashboard of a
two door car to light if either of the doors is not closed. Closing a door
closes
a switch.
39.4.3.8 Light at the top of the stairs circuit
using OR gate
See diagram 39.4.3.8
In the truth table, "0" stands for LOW, and "1" stands for HIGH. Complete
the truth table to show the voltage level at each of the lettered points for
each of the level combinations at A and B. Set up the circuit and check your
predictions for the output F. If you have made an error, use a flying lead
from another LED indicator to check your predictions for the level at each
point Q, D and E. Do this by touching the free end of the flying lead on
to each of those points in turn. The circuit is an exclusive OR gate. Note
that F is high when A or B is high, excluding the case when both are high.
It is a light at the top of the stairs circuit.
39.5.1.0 Bistable using two NAND gates
Use two NAND gates. Press RESET switch and note which LED is on and which
LED is off. Press and release the switch marked SET and note what happens.
Press and release the SET switch several times and note what happens.
Press and release RESET and note what happens.
In the circuit diagram the LED indicator connected to the output of NAND
gate 1 is not drawn beneath the LED indicator of gate 2, although that is
where it is placed on the indicator module. Circuit diagrams are easier to
understand if the parts are drawn in convenient places rather than as they
are placed on the modules. The positive power rail is drawn in the above diagram
although no leads are shown connected to it. Such a power rail may be omitted
from circuit diagrams but are included in these experiments. In this experiment
when the RESET switch is pressed, the output of NAND gate 2 goes high and
the output of NAND gate 1 is low. When the SET switch is pressed, the output
of NAND gate 1 goes high and the output of NAND gate 2 goes low. The bistable
circuit has two stable states. The first stable state exists when the output
of 2 is high and the output of 1 is low (the RESET state). The second stable
state exists when the output of 1 is high and the output of 2 is low (the
SET state). After the SET switch has been pressed, more depressions of this
switch have no effect. The same is true of the RESET switch. The SET and
RESET inputs are normally high. To change from one stable state to the other,
the appropriate input must be taken briefly low.
39.5.1.1 Two NAND gates as inverters
See diagram 39.5.1.1
Connect two NAND gates so that the output of one is joined to the input
of the other. If the second input of each gate is not used, both gates act
as inverters. If the input of the first gate is made high, its output will
be low. The input of the second gate is therefore low and its output high.
Connect the output of the second gate to the input of the first gate, see
broken line. The original high input to the first gate provided by the flying
lead is removed and the system is stable. The high output of the second gate
maintains the high input to the first gate.
39.5.1.2 Two NAND gates with reversed conditions
See diagram 39.5.1.2
A different situation arises if the flying lead to the first gate is originally
tied low. Since the input of the first gate is now low, the output of the
second gate is also low. Connect the output of the second gate to the input
of the first gate. Remove the original flying lead. The system is now stable.
However, the output conditions at the two gates are reversed compared with
39.5.1.2. The system consisting of two inverters can be connected to form
two stable states.
39.5.1.3 Two NAN gates with positive feedback link,
bistable
See diagram 39.5.1.3
The second, and so far unused, input of each NAND gate can be used to switch
between the two states. The diagram shows the initial stable state. The inputs
of the first NAND gate are both high, and its output is low. However, if
the SET switch is pressed, one input of the first NAND gate is taken low,
its output goes high. The output of the second gate therefore goes low and
this low state is fed back to the other input of the first gate. If the SET
switch is now released, the system is in the second completely stable state.
Note that pressing the SET switch a second time has no effect. The system
can only be switched back to its original state by pressing the RESET switch.
The line shown dotted in the earlier diagrams provides a positive feedback
link. In other words, the output of the system is fed back to provide an
input of the same polarity.
39.5.1.4 Bistable with complimentary outputs, Q
and Q'
See diagram 39.5.1.4
In electronics literature, the above bistable would usually be drawn as
in the diagram. The connections are identical, but the gates are shown one
beneath the other. The output of the top gate, with the SET input, is usually
labelled Q. The output of the other gate, with the RESET input, is labelled
Q'. The two inputs and called complementary outputs because each is the
inverse of the other.
39.5.1.5 Bistable building block
See diagram 39.5.1.5
Diagram 39.5.1.5 is too complicated to use in circuit diagrams so this
symbol is in common use. The circles on the S and R input lines show that
they are active low. The S and R inputs are normally high and must be taken
momentarily low to have an effect. This bistable building block, called an
RS bistable or as an RS flip-flop has the following properties: 1. There
are two stable states: SET state (0 high, b low) and RESET state (0 low,
U high). 2. Normally the system will be in one of its stable states. In both
states the S and R inputs will be high. 3. To change from the SET state to
the RESET state the R input must be taken briefly low, If R is taken low
again, nothing happens. 4. To change from the RESET state to the SET state
the S input must be taken briefly low. If S is taken low again, nothing
happens. This behaviour can be summarized in a truth table:
| S |
R |
Q |
Q' |
| HIGH |
HIGH |
same |
same |
| LOW |
HIGH |
HIGH |
LOW |
| HIGH |
LOW |
LOW |
HIGH |
| LOW |
LOW |
avoid |
avoid |
Note that the state with both S and R low is avoided or disallowed, since
both Q and Q' are then forced to go high simultaneously and the system will
be in neither of its stable states.
39.5.1.6 Bistables and logic gates
See diagram 39.5.1.6
Difference between a bistable and a logic gate. A logic gate requires
retention of its input signals to remain in a given state. The bistable remains
in a stable state when the inputs are removed (the particular state will
depend upon which input was last taken low).
39.5.2.0 Bistable applications using NAND gates
39.5.2.1 Latched burglar alarm
See diagram 39.5.2.1
A. Use two NAND gates, two push-button switches and a buzzer to make a
latched burglar alarm. One switch should correspond to the "trip" switch.
If this is closed (by the burglar's foot, perhaps) the alarm should sound
and stay on even when the switch is released or pressed again. The second
switch should correspond to the "reset" switch, and would be hidden away
in a place known only to the householder. Only when this switch is pressed
should the alarm be silenced.
Once you have built this circuit, convert it to an alarm that will come
on and stay on when a light, e.g. the burglar's torch, shines on an LDR.
Replace the trip switch by an LDR to make a light-activated latched alarm.
A variable resistor connected between the LDR and the positive supply rail
to form a potential divider, will allow the sensitivity to be adjusted, 39.4.3.3.
With CMOS modules, the output of NAND gate 1 may be unable to drive both the
buzzer and the input to NAND gate 2. If this is the case, possible courses
of action are as follows:
Use a LED indicator rather than a buzzer.
B. Try to "buffer" the buzzer by connecting
it to the output of another NAND gate whose inputs go to the output of NAND
2 in the circuit above. When the "trip" switch is pressed, the alarm sounds
and cannot be turned off by releasing the switch or by depressing it again.
You say that the output is latched. The bistable is now locked in its second
stable state (the state with the output of the NAND gate connected to the
buzzer high, and the output of the other NAND gate low). The alarm can only
be turned off by pressing the "reset" switch. In a practical system the trip
switch might be contained in a pressure pad of the type designed especially
for home security systems. Such a pad (effectively a normally open push-button
switch) is placed under a carpet near a door so that the switch is closed
by the pressure of the intruder's foot. Pressure pads of this kind are available.
39.5.2.2 Latched fire alarm using a bistable
See diagram 39.5.2.2
Build a fire alarm which, once triggered, will continue to sound until
it is reset. See 39.4.3.4 for information about a thermistor, and 39.5.2.1
for information about driving the buzzer.
39.5.2.3 Simple stop-go traffic lights
See diagram 39.5.2.3
Traffic in a one way street has to cross a bridge that can only have one
car on it at a time. The bridge is to be controlled by a set of stop-go lights,
activated by switches in the road, so that the lights go red when a car enters
the bridge and then green as the car leaves ft. Design a circuit to do this
job.
39.5.2.4 Traffic lights operated by an SPST switch
See diagram 39.5.2.4
A simple set of stop-go traffic lights is to be operated by an SPST switch
so that either the red light is on or the green light is on, but not both
together. When SET is high, RESET is low and the green LED is on. Since both
inputs to NAND gate 1 are high, the red LED is off. When SET is low, the red
LED is on, and since both inputs to gate 2 are now high, the green LED goes
off.
39.5.2.5 Quiz master
See diagram 39.5.2.5
A quiz master circuit is used to identify the first contestant to push
the answer button in a quiz game. Each contestant has a push-button and a
LED indicator light. The light of the first person to answer should come
on and stay on. Simultaneously all other lights should be prevented from
coming on. Use four NAND gates two connected as a bistable, a push-button
switch and a LED indicator. Complete the wiring of the four gates for one
contestant then wire for another contestant separately. Finally connect the
two sets. If the B inputs of the two quiz stations are taken low, the bistable
circuit formed by the two right hand gates will be reset. So the 5 outputs
at the stations are high and the LEDs are off. At the station of one contestant,
the A input will initially be high because it is connected to the output of
the other station, which has just been reset. The A input effectively "opens"
the NAND gate to which it is connected, so if this contestant is the first
to press his or her switch, the bistable is set by a low pulse and the LED
indicator comes on. At the same time 0 goes low, and since this output is
connected to the A input of the other station, the NAND gate controlled by
this A input is closed. This means that a low going pulse from the other contestant's
push-button can pass to his or her bistable. The bistable therefore remains
in the reset state with the indicator light off. You can build a simpler
quiz master station using fewer NAND gates but they may not have the required
properties of simultaneously latching the victor's indicator LED and permanently
disabling the switches of all other contestants.
39.6.0 Drivers
Drivers are essential to drive heavy current devices such as motors so
an operational approach to drivers follows.
39.6.1.0 Loading an output
See diagram 39.6.1
Set up the first circuit. Note what happens when you press the push-button.
When the switch is closed, note whether the output of the NAND gate high or
low. Replace the LED with an electric motor module as shown in the second
circuit. Note what happens when you press the push-button. To investigate
why the motor did not operate in (3), connect a LED in parallel with the motor
as shown in the third circuit. Note when you press the push-button. Note
whether the output of the NAND gate is still high as in or is it now low.
The experiment shows that a logic gate operates satisfactorily only when
small currents of a few milliamperes are used because a larger current for
the motor may change the logic level of the output. The next experiment shows
that this difficulty is overcome using a driver. Use a LED in series with
a 33 M resistor.
39.6.2.0 Using a NAND gate to switch an electric
motor on or off
See diagram 39.6.2
Use the Driver Amplifier module and the reed relay. Note what happens
when the switch is pressed. When the switch is pressed, note whether the
output of the NAND gate is high or low. The driver amplifier is a special
kind of inverter. When the switch is pressed, note whether its output is
high or low. The relay is connected between the positive supply line and
the output of the driver. The motor operates when a current flows through
the relay coil. When this happens note whether the output of the driver is
high or low. Replace the switch by an LDR. Note what happens to the motor
when the LDR is covered or uncovered.
39.6.2.1 Inverting driver amplifier
See diagram 39.6.2.1
The function of a driver is to provide an interface between a device such
as an integrated circuit that can control only a small current, and a device,
such as a relay or an electric motor, which usually requires a large current
for its operation. The simplest and cheapest inverting driver amplifier consists
of a single transistor and one resistor. A relay and its protective diode,
essential when switching inductive loads, would then be connected between
the positive supply line and the transistor collector in the way shown. When
the transistor input goes high, the collector current flows through the relay
coil into the transistor. Use two transistors connected as a Darlington pair.
The current gain of such a system is far higher than that of a single transistor,
and the base current needed to switch the collector current on is then easily
supplied by the high output of any integrated circuit.
Note that the circuit diagram shows the motor being driven by a separate
power supply. If separate supplies are not available, the motor may be driven
by the supply used for the modules, provided, of course, that this supply
can deliver the necessary current. It is a wise precaution to connect a 1000
muF capacitor between the power lines in those circumstances.
39.6.2.2 Driver amplifier with relay omitted
See diagram 39.6.2.2
The driver amplifier operates a relay that controls the motor. The same
job, of course, could be done in other ways. In diagram 39.6.2.3 the relay
is omitted entirely when the motor current does not exceed the maximum current
allowed by the driver. However, if the motor and the logic circuitry share
the same power supply this would disadvantage CMOS integrated circuits where
only a small battery is needed for the ICs. A motor would soon run this flat,
or might not operate at all. In diagram 39.6.2.4 the driver is omitted. Reed
relays are available which will operate satisfactorily off a low voltage supply
with a coil current of only a few milliamperes, and these can be operated
directly using the logic gate output. However, a disadvantage of this type
of relay is that the contact ratings are often small, and could possibly
overheat when used with a motor requiring a large current.
The current need of the device to be operated by the relay lies within
the current rating of the relay contacts.
39.6.3.0 Applications involving the driver amplifier
and reed relay
39.6.3.1 Reversing an electric motor
See diagram 39.6.3.1
Using a push-button switch, a NAND gate and the driver amplifier and reed
relay, set up a circuit so that the direction of rotation of a motor is reversed
when the switch is pressed. Three batteries will be needed, one for the modules
and two for the motor. The NAND gate is not necessary if the switch is connected
between the driver input and the positive supply rail.
39.6.3.2 Reversing an electric motor with a bistable
circuit
See diagram 39.6.3.2
Using two push-button switches, two NAND gates, a reed relay and the driver
module, set up a circuit that reverses the direction of rotation of a motor
when one switch is pressed and released. Three batteries will be needed, one
for the modules and two for the motor.
Use a low value resistor connected in series with the motor supply to prevent
a shot circuit of the motor supply. The resistor value must be low enough
not to interfere with the operation of the motor.
The two NAND gates form an RS bistable, The contact of the relay switch
will be in the lower position when 0 is low, and the motor will rotate in
a certain direction. When the bistable is switched to its other state, 0 goes
high, the relay operates and the switch goes to the higher position. This
reverses the polarity of the supply to the motor that now rotates in the
opposite sense. So a reversal occurs every time the bistable is switched.
39.6.3.3 Automatic light
See diagram 39.6.3.3
Use a driver module, a reed relay, an LDR and any necessary gates, to
make a circuit in which a filament lamp will come on automatically in the
dark.
Use a very bright lamp, e.g. 12 V 3 W lamp or 12 V 0.1 A MES bulb to show
that this circuit is controlling a much larger power than the circuit of 39.4.3
(c). If a variable resistor is used to adjust the sensitivity so that the
lamp is just on, and the lamp is moved to illuminate the LDR, the circuit
becomes astable because light from the lamp reduces the LDR's resistance.
This switches the lamp off. The fall in light intensity then causes the circuit
to switch the lamp on again.
39.6.3.4 Motor vehicle moving backwards and forwards
between two light beams
See diagram 39.6.3.4
Use the circuit of 39.6.3.2 with two inverters and two LDRs to make the
motor reverse every time a light beam is interrupted. The point about this
application is that, when illuminated, the resistance of the LDRs will be
low. So they cannot be connected directly to the SET/ RESET inputs of the
bistable, since these must both be high. The answer is to use inverters between
the LDRs and the SET/RESET inputs. The application has greatest impact if
a motor with wheels is available. Two long leads are attached to the motor
and connected to the appropriate points in the circuit. If the LDRs are set
up in the way shown above and illuminated by pencil torches, the vehicle should
oscillate happily backwards and forwards between the light beams. A 4.5 V
LEGO motor from technical set 107 can be fitted with wheels and works well.
A length of LEGO railway track along which the vehicle will run is also useful.
39.7.0 Coding
39.7.1 Sending messages using a 4 bit binary code
See diagram 39.7.1d | See 38.7.03.4: ASCII code
Connect a flying lead to each of the four inputs of the LED Indicator
module. The LEDs are turned on or off by taking the flying leads at the inputs
high or low. When an input is high and the corresponding LED is on, you will
let it represent the binary digit 1. When an input is low and the corresponding
LED is off, you will let it represent the binary digit 0. Note that the term
binary digit is usually shortened to bit. The four LEDs can therefore represent
a 4 bit binary pattern. The experiment shows that a 4 bit binary pattern can
represent non-numerical information if an agreed code is used, e.g. the 7
bit ASCII code (American Standard Code for Information Exchange). Invent a
4 bit binary code for sending messages. Write your code in a table. A few
words have been added to start off. How many words can you represent with
a 4 bit pattern? Give a copy of your code to another group, and then send
them a message using the LED indicators. Send the message, word by word, by
lighting the agreed 4 bit binary pattern for each word. Apart from a copy
of your code, what other information did you have to give the other group
before they were able to decode your message? When using any binary code,
numerical or non-numerical, know which is the LSB (least significant
bit) and which is the MSB (most significant bit). In these experiments the
LSB is shown on the right when binary patterns are written horizontally according
to the normal conventions for numbers, i.e. represents the number 13 in the
scale of 10 (decimal) notation.
| MSB |
- |
- |
LSB |
| 23 |
22 |
21 |
20 |
| 1 |
1 |
0 |
1 |
The binary digit 1 represents a high logic level, and the binary digit
0 represents a low logic level. Encoding means that information is put into
a binary pattern. For numerical information, encoding usually means changing
from a scale of ten, decimal, to a binary notation. In decoding the information
is extracted from the binary pattern. With numerical information, this means
changing from a binary to a scale of ten, decimal, representation.
39.7.2 Seven-segment LED display
See diagram 39.7.2
Use the seven-segment LED display on the Seven Segment Display module
for this experiment. The display has seven segments, labelled a to g in the
diagram. A single segment behaves like an ordinary LED, and lights when current
flows from positive (anode) to negative (cathode).
Inside this common cathode display, all the cathodes are connected and
to a single lead, the common cathode lead, which is connected to the negative
supply line.
Connect a flying lead to the point marked TEST and touch the other end
on to the pins marked a to g, one at a time. Note what happens. Copy the
tables below and complete them to show how you would display the digits 0
to 9. Use the Display/Decoder module with a flying lead. Insert a safety
resistor between the test point and the supply line to limit the current
and prevent any damage to the display in case of direct contact between the
flying lead and wires on the display side of the other resistors. For extra
safety feature locate the terminal pins on the decoder side of these resistors
to prevent damage to the display if any direct contact between a terminal
pin and the positive rail which would damage the module. To check answers
to the questions connect the terminal pins directly to the appropriate power
supply rail because the current flowing through each segment also flows through
the resistor in series with the test socket. Eventually the voltage drop
across this resistor is too large to allow sufficient current to flow to
light the segments clearly. However, do not make direct connection of the
decoder/driver outputs, the terminal pins, to the power rail.
39.7.3 Seven-segment display with a decoder
See diagram 39.7.3
Use the Seven Segment Display module and the four decoder inputs to light
the display. Use flying leads to take the decoder inputs (A, B, C and D) high
or low. Let a high input represent the binary digit 1, and let a low input
represent binary digit 0. Use the flying leads to complete the tables. When
the digit 1 is displayed, segments b and c of the display are In. Which outputs
of the decoder must then be low, and which outputs are high?
As there are four inputs (A, B, C and D) to the decoder, 24
or 16 separate messages can be transferred. When the binary number 0000 is
received, the decoder gives the message that a, b, c, d, e, f should light
and the number 0 is displayed. When the input 0001 is received the message
goes out that b and c should light and hence the number 1 displayed. With
a common cathode module, the inputs b and c are high, and all the others
low. The integrated circuit used in this experiment is called a BCD to seven
segment decoder/driver. (BCD stands for "Binary Coded Decimal"). The input
is a decimal digit in the binary code (i.e. 0000 to 1001) and the decoder
decodes the input to drive a seven segment display.
In practice, a BCD to seven segment display decoder may or may not respond
to the numbers beyond 1001 (that is, from 1010 to 1111) depending on the particular
integrated circuit used. For the recommended CMOS decoder/driver, the display
will be blank beyond 1001.
In digital electronics an N bit binary code is said to be completely decoded
if 2N distinct output signals are produced from N input signals.
In each case every possible binary pattern present on the input lines activates
a distinct output line. With two inputs there are four possible binary patterns
(00, 01, 10 and 11) and so four output lines are required. With three inputs,
eight output lines are required. With four inputs 16 output lines are required.
So N inputs require 2N outputs for complete decoding. In some computers
with 16 lines from the microprocessor to the computer memory each memory
location is identified by a different binary pattern present on these lines.
Also, a separate and unique signal must be generated for each location when
it is selected for the storage or retrieval of information. If you completely
decode the 16 memory input lines, you can generate 216 = 65536 distinct signals
to select memory locations. So some microcomputers have a memory size of 64
K where 1 K = 1024. So 64 K means that there are 64 × 1024 = 65536 memory
locations. The BCD to seven segment display decoder used earlier is not a
complete decoder since R has 4 inputs, but only 7, not 16, outputs so it
is a partial decoder.
39.7.4 Two-line to four-line decoder from NAND gates
See diagram 39.7.4
In diagram 39.7.4.1 two inputs labelled X and Y which are inverted by
NAND gates 1 and 2 to produce x' and y'. In 39.7.4.2 an AND gate is formed
from NAND gates 3 and 4, with two inputs A and B.
If X = 0 and Y = 0, how do you connect A and B to light the LED? How do
you connect A and B to light the LED K X = 0 and Y = 1? How do you do it
if X = 1 and Y = 0? How do you do it if X = 1 and Y = 1? Build the circuits
and check your predictions. Use your results from above to draw a circuit
diagram of a 2 line to 4 line decoder. Build the decoder with NAND gates.
NAND gates with 3 inputs are also available in integrated circuit form (the
output is low only when all 3 inputs are high). Draw a circuit diagram showing
how a 3 line to 8 line decoder could be built from 2 input and 3 input NAND
gates. A 3 line to 8 line decoder can be built using 2 input NAND gates only.
How many of these gates would be required? Gate 4 provides the high output
to light the LED and is not essential to the decoding function. Integrated
circuit decoders are often available with high or with low outputs and the
choice between these depends on the task the decoder is to perform. This 2
line to 4 line decoder uses ten NAND gates. To build the decoder with low
outputs only six NAND gates are required. Use voltmeters to detect the logic
levels. A 3 line to 8 line decoder would require three NAND gates to invert
the three inputs X1, Y1 and Z1 to produce X2, Y2 and Z2. Eight 3-input NAND
gates would then be required to produce a distinct low output for each of
the eight possible combinations of X, Y and Z at the decoder inputs. If high
outputs are preferred to light indicators, a further eight NAND gates connected
as inverters would be needed. Since three 2-input NAND gates are required
to produce one 3-input NAND gate, a total of [3 + (8 × 3) + 8] = 35
two input NAND gates would be required to produce a 3 line to 8 line decoder
with high outputs. Such decoders are available as single integrated circuits.