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EE 177 Logic Circuits and Digital Electronics
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One-way Road Intersection Traffic Light
A Simple Logic Design
Mindanao State University
College of Engineering
Geraldine T. Casas, 2007-0057
Gayzel P. Dolor, 2008-0056
Mojahid A. Manapa-at, 2008-0296
Mary Rose G. Tabao, 2007-0015
EE 177 Logic Circuits and Digital Electronics
MTh 2:30-4:00
st
1 Semester, AY 2010-2011
August 28, 2011
Engr. Edilberto C. Vergara
Instructor III
Table of Contents
Abstract
1
Introduction
1
Design Specification
3
Design Procedure
5
Results
11
Conclusion
12
Appendices
Appendix A. Clock Design Procedure
Appendix B. Multisim™ Simulations Screenshots
References
I
V
VIII
i
Abstract
One of the vital inventions of mankind is the traffic lights which up to the present are
continuously still being modified for a more satisfactory result. Traffic lights are used to control
competing flows of traffic and serve as road signals to cars for a smooth and convenient travel. With the
aid of the obtained knowledge in logic circuits and digital electronics, a one-way traffic light design was
established.
The design was initiated by a definite goal which is to make a one-way traffic light mounted to
the road intersection with sensors used to detect the presence of vehicles leading to light transitions. After
which is the execution of logical operations of the design with the used of Moore model showing how and
when the network changes state. The now well established design was then tested on a MULTISIM where
the obtained result was analyzed. Modifications were done until the desired output, conforming to the
design specifications were attained. Armed with sufficient and efficient technical know-how, the Simple
Traffic Light Design was accomplished.
Introduction
During the horse and buggy days, traffic in big cities was often heavy. With the coming of
automobiles, the situation got even worse. Precisely, traffic on roads take place when a number of
pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances occurs either singly or
simultaneously while using the public way for purposes of travel.
Traffics are hindrance to those who are in a hurry and may also cause accidents. To avoid such
occurrence, traffic lights are adapted for street use. These are signaling devices positioned at road
intersections and other locations to control competing flows of traffic. They alternate the right-of-way of
road users by displaying lights of a standard color. They make use of three colors, each provides distinct
signal that a car must follow. There’s a green light allowing a car to proceed in the direction denoted, a
yellow light denoting prepare to stop short of the intersection, and a red light prohibiting it to proceed.
1
Time-controlled and sensor-controlled traffic lights have been implemented nowadays on
different types of road intersections such as cross intersections, T intersections and Y intersections. The
former operates on a timing mechanism that changes the lights after a given interval while the latter
senses the presence and absence of vehicles, and reacts accordingly. Sensor-controlled system reacts to
motion to trigger light changes. Putting it into operation in a one-way road intersection, sensors are
positioned on each lane at the cross-section to detect that a volume of cars has pulled up. These detectors
are driven by switches that cause light transitions.
To govern the actions of the traffic system, algorithms will be used. Specifically, Boolean logic
will be employed. It consists of binary variables and a set of logical operations. The said variables will be
used to represent state transitions. The logical operations will be executed using a Moore model.
Herewith, is a sequential circuit that will be composed of a state register driven by an input combinational
logic. It will also consist of an output combinational logic which is a function of state registers.
Aided with the right concepts and principles with the well-defined design parameters, the
researchers eagerly hope to come up with the desired output which on this particular case is the own
proposed one-way traffic light design.
2
R1
Y1
G1
TL1
Road 1
Design Specification
TL2
R2 Y2 G2
Y
Road 2
Road 2
Road 1
X
Figure 1. Road Layout
The main goal in this project is to design a one-way road intersection as shown in figure 1. That
is, a car from road 1 can go straight or turn left but is unable to turn right. Also, a car from road 2 can go
straight or turn right but is unable to turn left. Sensors (i.e., x and y) are placed on each side to determine
if a car is present at either road. These are used as inputs to the circuit being designed. Also, traffic lights
are positioned as shown (i.e., TL1 and TL2). Individual colors of the traffic lights are designated with
certain letter-number combination as shown. A third input to the circuit is a fifteen-second timer which
goes high if 15-seconds had passed and resets after such.
3
For a clear understanding of its behavior, refer to the diagram of Figure 2. As shown, there can
only be four states for which transitions occur. It follows a cyclical path and is unidirectional. Considering
G1R2 as the initial state, shifting to the next state Y1R2 is possible as long as the timer is high regardless
of the presence or absence of cars on road 1 and 2. Another case is when a car is present on road 2 and not
on road 1, transition happens. Otherwise, it will remain on this state.
TL1
TL2
TL1
TL2
TL1
TL2
TL1
TL2
Figure 2. State diagram
For the Y1R2 state, all conditions lead to the next state R1G2 after a certain time. Again,
transition from state R1G2 into R1Y2 occurs when the timer is high or when a car is present in road 1 and
not in road 2. After which, the state R1Y2 goes back to G1R2 state. These four states continue
transitioning for as long as the circuit runs.
4
Design Procedure
The design was started by designating a representation of each state as shown in Table 1. It is
comprised of four states. In the first state, green light of TL1 is ON together with the red light of TL2. For
Table 1. Representation of States
States
1
G1 R2
2
Y1 R2
3
R1 G2
4
R1 Y2
the second state, green light of TL1 switches yellow and TL2 holds on to red. Next, for the third state, the
conditions of state 1 are reversed. Now R1 and G2 are ON. R1 Then holds on while G2 switches to Y2
for the fourth state. After which, it will go back to state 1. Binary representation of the said states with
corresponding outputs is shown in Table 2.
Table 2. Binary Representation of States and Outputs
States
A
0
0
1
1
Outputs
B
0
1
0
1
R1
0
0
1
1
Y1
0
1
0
0
G1
1
0
0
0
R2
1
1
0
0
Y2
0
0
0
1
G2
0
0
1
0
The circuit has three inputs: , , and . These represent the road1 sensor, road2 sensor, and the
fifteen seconds timer, respectively. It was driven by a clock source with an interval of 3 seconds. This
clock source also drives a network in the circuit that performs 15 seconds time count which will control
the time interval of a green light in the ON state. Such network is a Negative Edge Triggered Four Bit
Binary Counter (sn74ls93). The binary counter was configured to count from binary 000 to 111. This was
done by using only the outputs
,
, and
with
the least significant bit. It is desired to have , the
15 seconds timer, be high (one) for binary counter value of 101. Truth table of which is shown in Table 3.
It is seen in the table that there are don’t care conditions for 110 and 111. It is because these values are
5
not needed in the timer. The timer will count from 000 to 101 and then go back to 000 by resetting the
counter. How will the counter reset will be discussed shortly.
Table 3. Timer Truth Table
Qd
0
0
0
0
1
1
1
1
Qc
0
0
1
1
0
0
1
1
Qb
0
1
0
1
0
1
0
1
t
0
0
0
0
0
1
X
X
The truth table of table 3 was mapped in a three-variable map shown in Table 4 and the
corresponding Boolean function was obtained.
Table 4. Timer Map
Qc
Qd
0
0
0
1
0 0
X X
Qb
The Boolean function for t is:
Eq. 1
Block diagram of the 15 seconds timer connected to the clock output is shown in Figure 2. Design
procedure for the LM555 and its operation are discussed in Appendix A. The output of the LM555 was
set as input B to the counter. This was necessary to generate an 8-bit count. Here, the reset inputs of the
counter were grounded for the circuit to be operational. The outputs B and D of the counter were ANDed
as based on Boolean function obtained in Eq. 1. The output of this AND is the input .
6
Figure 3. Clock and Fifteen-second Timer Diagram
The traffic light was designed using Moore model. Positive Edge Triggered D Flip-flops
(sn74ls74) were chosen as state registers. Figure 4 shows the Moore model design.
Inputs
Next State
Combination
al Logic
State
Registers
Output
Combination
al Logic
Outputs
Clk
Figure 4. Moore model design
The design for the next state combinational logic that will drive the state registers was then made.
Taking into account the transition conditions discussed in the Design Specification, a state table is
obtained as shown in Table 5. Present states A and B and inputs X, Y and t were mapped on a five
variable map and corresponding Boolean functions for A’ and B’ were obtained. Since D Flip-flops were
used, the inputs to the flip-flops are A’ and B’ themselves. Boolean function for A’ was simplified to a
single XOR operation while B’ was simplified so as to be implemented with all NAND.
7
Table 5. State Table with D Flip-flops
Present States
A
B
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
1
Inputs
Y
0
0
1
1
0
0
1
1
X
0
0
1
1
0
0
1
1
X
X
0
0
0
0
1
1
1
1
X
0
0
0
0
1
1
1
1
X
Next States
A’
B’
0
0
0
1
0
1
0
1
0
0
0
1
0
0
0
1
1
0
1
0
1
1
1
0
1
1
1
1
1
1
1
0
1
1
0
0
t
0
1
0
1
0
1
0
1
X
0
1
0
1
0
1
0
1
X
Flip-flop inputs
Da
Db
0
0
0
1
0
1
0
1
0
0
0
1
0
0
0
1
1
0
1
0
1
1
1
0
1
1
1
1
1
1
1
0
1
1
0
0
Map of A’ and B’ are shown in the figure below.
Table 6. Maps for a) A’ and b) B’
Y
BC\Yt
B
0
0
1
1
0
0
1
1
0
0
1
1
Y
0
0
1
1
X
B
1
1
0
0
t
A=0(A’)
1
1
0
0
1
1
0
0
1
1
0
0
X
0
0
0
0
X
t
A=1(A)
a)
Y
BC\Yt
B
0
0
0
0
1
1
0
0
1
1
0
0
Y
1
0
0
0
X
B
t
A=0(A’)
0
1
0
0
1
1
0
0
1
1
0
0
t
A=1(A)
b)
8
Obtained Boolean functions for A’ and B’ are:
Eq. 2
*(
) * ,(
)(
Eq. 3
)-++
Next is the design for the Output Combinational Logic. It was done by mapping truth tables of
R1, O1, G1, R2, O2, and G2 from Table 2 on two-variable maps as shown in Table 7.
Table 7. Maps for Output Combinational Logic
B
A
0
0
1
1
B
A
Map for R1
0
1
0
0
A
1
0
0
Map for R2
A
Map for Y1
B
1
B
1
0
0
0
Map for G1
B
A
0
0
0
1
Map for Y2
B
A
0
0
1
0
Map for G2
The outputs are functions of only the present states A and B. Output Boolean functions are:
Eq. 5
Eq. 6
Eq. 7
Eq. 8
Eq. 9
Eq. 10
9
Now that each networks for the individual part of the design are established, putting them into a
single diagram follows. Figure 5 shows the interconnection of the clock, fifteen seconds timer, next state
Figure 4. Final Circuit (One-way Road Intersection Traffic Light)
combinational logic circuit, state registers and finally, the output circuit. In addition to the aforementioned task of each part, note that the outputs of the flip-flops A and B were set as inputs to the input
combinational logic. X and Y inputs are connected into switches 1 and 2. Here, X and Y are high if
individual switches are closed, that is a car is present on the road. The output of the fifteen seconds timer
was connected to the input. It was desired that the Four Bit Binary Counter will reset if either Y1 or Y2
is high which was achieved by ORing them and connecting the output to the reset input of the counter.
10
Results
The final design was implemented in a simulation program NI Multisim™ where the behavior of
the circuit was then observed. Example simulation images are presented in Appendix B. The simulation
timing diagram is shown in Figure 10. By the moment the circuit runs, the fifteen seconds timer starts
automatically. Y1 and Y2 control the state of the timer by the way of resetting it. Since the timer is
←3sec→
C
t
(timer)
X
Y
A
B
R1
Y1
G1
R2
Y2
G2
Figure 5. Simulation Timing Diagram
negative edge-triggered, it changes state during the negative edge of the clock. States of inputs X and Y
are arbitrarily chosen all over the trace. These in turn determines the next state of the state registers A and
B which are considered initially reset. Considering every positive edge of the clock, the next states of the
registers are determined by taking into account the states of the registers and inputs at a time just before
11
the clock changes state. Note that the positive edges of the clock are considered because the registers used
are positive edge-triggered.
For the first positive edge, both registers and the three inputs are all reset. Thus referring to the
state table of Table 5, the registers remain in its state. At the second positive edge, inputs X, Y and t has
its values as 0, 1 and 0, respectively. Thus again referring to Table 5, registers A and B changes states to 0
and 1, respectively. Here, set states are Y1 and R2. Corresponding to that, the fifteen-second timer is
reset for the whole interval of the present state. It starts counting when the orange light in the previous
state goes low. The above mentioned process will continue following the states of X and Y as indicated.
CONCLUSION
The one-way traffic system was designed and developed successfully. The circuit’s behavior
affirms to the designers’ specification thus clearly implying that the sensors are synchronized with the
whole process of the traffic design and that the light transitions respond well with the sensors. The time of
occurrence of each state transition has met the expected time.
Minor discrepancies were encountered during the process which can be avoided by taking into
consideration every minute detail of the design specifically the interconnections of various networks.
The one-way traffic system is still subject to essential improvement since it was only
implemented for design purposes only and to stress out one of the vital applications of logic circuit and
digital electronics in the real world. It is not opt to be carried out for actual use considering that advanced
technologies are much preferred to use such as microcontrollers which operate more satisfactorily,
nowadays.
12
Appendix
Clock Design
Procedure
A
I
Figure A1. IC type LM555 timer connected as a clock pulse generator
IC type lm555 is a precision timer circuit whose internal logic is shown in Figure A1. This is an
IC timer unit that produces clock pulses at a given frequency. The circuit requires a connection of two
external resistors and two external capacitors. Figure A1 shows the external connections for astable
operation of the circuit. Capacitor
charges through resistors
biased and conducting. When the charging voltage across capacitor
and
when the transistor is forward
reaches 3.3 V, the threshold input at
pin 6 causes the flip-flop to reset and transistor turns on. When the discharging voltage reaches 1.7 V, the
trigger input at pin 2 causes the flip-flop to set and transistor turns off. Thus, the output continually
alternates between two voltage levels at the output of the flip-flop. The output remains high for a duration
equal to the charge time. This duration is determined from the equation:
(
)
Eq. A1
II
The Output remains low for a duration equal to the discharge time. This duration is determined
from the equation:
Eq. A2
It was desired to generate a square wave with a period of 3s. The frequency corresponding to that
is 0.333Hz. Also, the duty of the square wave was chosen to be 66.67%. That is the signal is high for twothirds a period and is low for one-thirds a period. Desired waveform is shown in Figure A2.
1s
3s
Figure A2. Output waveform for clock generator
Letting C be
and using Eq. A2,
(
)
Using this obtained value and Eq. A1,
(
)(
)
III
The final circuit is constructed as shown Figure A3.
Figure A3. Astable oscillator operating at a period of 3 seconds
This circuit was used in the design as an input clock source to the state registers and binary
counter.
IV
Multisim™ Simulations
Screenshots
Appendix
B
V
Figure B1. Oscilloscope connected to the output of the clock and output of the fifteen-second timer
Figure B2. Example trace for the oscilloscope of Figure B1
VI
Figure B3. Probes are used in deriving the states of Figure 5
VII
Figure B4. Traffic lights and switch sensors configured forming road intersection
VIII
References
Digital Design (4th Edition) - M. Morris Mano and Michael D. Ciletti
Design and Development of Sensor Based Traffic Light System - A. Albagul, M. Hrairi, Wahyudi and
M.F. Hidayathullah
Lecture 10: A Design Example - Traffic Lights Department of Computing, Imperial College London
DOC 112 Computer Hardware
http://en.wikipedia.org/w/index.php?title=Special:RecentChanges&feed=atom
http://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/question234.htm
IX