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00 Table of contents
Chapter 1
Introduction
1.1 Problem statement
1.2 Objectives
1.3 Significance of project
Chapter 2
Literature Review
2.1The 555 Timer IC
*2.1.1 555 Timers in Astable Multivibrator Mode
2.2 NAND Gate Astable Multivibrator 2.3 CMOS Astable Multivibrator
2.4 OP- Amp Astable multivibrator
Chapter 3
Methodology
3.1 Design Methods
3.2 Modules design
3.3 Design structure 3.4 Electronic Component
*3.4.1 Transistor (2SC945)
*3.4.2 Electrolytic capacitor (22uf)
*3.4.3 LEDs
3.5 Hardware resources and procurement
3.6 Construction
List of figures
Fig 1.1 Example of astable multivibrator
Fig 2.1 Capacitance-Frequency graph
Fig 2.2 555 connected as an astable multivibrator
Fig 2.3 NAND gates in Astable Mode
Fig 2.4 output of the NAND gate.
Fig 2.5 CMOS in Astable mode
Fig-2.6 is the circuit of the 741 in astable mode
Fig2.7 Square wave output of the op-amp
CHAPTER1
1.0 Introduction:
Background
The first electronic flip-flop was invented in 1918 by William Eccles and Jordan. It was
initially called the Eccles–Jordan trigger circuit and consisted of two active elements
(vacuum tubes). Such circuits and their transistorized versions were common in
computers even after the introduction of integrated circuits, though flip-flops made from
logic gates are also common now.
Early flip-flops were known variously as trigger circuits or multivibrators. A
multivibrator is a two-state circuit; they come in several varieties, based on whether each
state is stable or not: an astable multivibrator is not stable in either state, so it acts as a
relaxation oscillator; a monostable multivibrator makes a pulse while in the unstable
state, then returns to the stable state, and is known as a one-shot; a bistable multivibrator
has two stable states, and this is the one usually known as a flip-flop. However, this
terminology has been somewhat variable, historically.
An astable multivibrator is also known as a free-running multivibrator. It is called free
running because it alternates between two different output voltage levels during the time
it is on. The output remains at each voltage level for a definite period of time. If you
looked at this output on an oscilloscope, you would see continuous square or rectangular
waveforms. The astable multivibrator has two outputs, but NO inputs.
A multivibrator is an electronic circuit used to implement a variety of simple two-state
systems such as oscillators, timers and flip-flops. An astable multivibrator has two states,
neither one stable. The circuit therefore behaves as an oscillator with the time spent in
each state controlled by the charging or discharging of a capacitor through a resistor. The
astable multivibrator may be created directly with transistors or with use of integrated
circuits such as operational amplifiers (op amps) or the 555 timers. A positive and
negative rail voltage, the output never able to exceed these rail voltages, powers most
operational amplifiers. Depending upon initial conditions, the op amp’s output will drive
to either positive or negative rail. Upon this occurrence, the capacitor will either charge
or discharge through the resistor R2, its voltage slowly rising or falling. As soon as the
voltage at the op amp’s inverting terminal reaches that at the non-inverting terminal (the
op amp’s output voltage divided by R1 and R2), the output will drive to the opposing rail
and this process will repeat with the capacitor discharging if it had previously charged
and vice versa.
Once the inverting terminal reaches the voltage of the non-inverting terminal the output
again drives to the opposing rail voltage and the cycle begins again. Thus, the astable
multivibrator creates a square wave with no inputs. Period of astable multivibrator
displayed. An astable multivibrator generates a string of pulses.
1.1 Problem Statement:
Most astable multivibrator fabricated on chips require more power to drive loads. This
could affect system performance since the generated heat has adverse effects on the ICs
and other surrounding components. Moreover these astable ICs have poor voltage
Figure 1.1: Astable Multivibrator
regulation and require input clock pulse to activate them. However, astable multivibrators
fabricated with discrete components have excellent voltage regulations and require no
clock activation.
1.2 Objective:
To design and construct a BJT astable multivibrator as a flasher.
1.3 Significance of project:
This project aims at developing an astable multivibrator that will:
* Provide less power to drive a load.
*Produce an accurate voltage regulation and no input clock pulse for its activation
CHAPTER 2
2.0 Literature Review.
2.1The 555 Timer IC
One of the most common linear integrated circuits is the 555 timers. SE 555/NE 555 IC
was first introduced in early 1970 by Signetics Corporation and was called "The IC Time
Machine" and was also the very first and only commercial timer IC available. It provided
circuit designers and hobby tinkerers with a relatively cheap, stable, and user-friendly
integrated circuit for both monostable and astable applications. Since this device was first
made commercially available, a myrad of novel and unique circuits have been developed
and presented in several trade, professional, and hobby publications.
The past ten years some manufacturers stopped making these timers because of
competition or other reasons. Yet other companies, like NTE (a subdivision of Philips)
picked up where some left off. Some typical application of the 555 timer is monostable
and astable multivibrator, DC-DC converter, digital logic probes, waveform generators,
analog frequency meter and tachometers, temperature measurement and control, infrared
transmitters, burglar toxic gas alarms, voltage regulators, etc. The 555 timers is a
monolithic timing circuit that is showing accurate and highly stable time delays and
oscillations.
The 555 timers are reliable, easy to use and economical. The 555 timer is available as 8-
pin metal can, 8-pin mini DIP or 14-pin DIP. The SE 555 is having large operating
temperature range (-55 oC to 125 oC) whereas other version of timer IC, NE 555 is having
small operating temperature (0 oC to 70 oC).
When the low signal input is applied to the reset terminal, the timer output remains low
regardless of the threshold voltage or the trigger voltage. Only when the high signal is
applied to the reset terminal, the timer's output changes according to threshold voltage
and trigger voltage. When the threshold voltage exceeds 2/3 of the supply voltage while
the timer output is high, the timer's internal discharge Tr. turns on, lowering the threshold
voltage to below 1/3 of the supply voltage. During this time, the timer output is
maintained low. Later, if a low signal is applied to the trigger voltage so that it becomes
1/3 of the supply voltage, the timer's internal discharge Tr. turns off, increasing the
threshold voltage and driving the timer output again at high.
2.1.1 555 Timers in Astable Multivibrator Mode
The 555 timers can generate a very wide frequency range, depending on the values of
R1, R2 and C. The following figure shows how to choose the timing resistors. The
designing equation is given as, charge time (output high): 0.693*(R1+R2)*CDischarge
time (output low): 0.693*(R2)*C, Period: 0.693*(R1+2*R2), Frequency: 1.44 /
((R1+2*R2)*C). Duty cycle: Time High / Time Low: (R1+R2) / R2With a 5-volt supply,
the resistors can range from 1KΩ (minimum value of R1 or R2) through 3.3MΩ
(maximum value of R1 and R2 in series)
Fig 2.1 Capacitance-Frequency graph
Credit: www.tele.pitt.edu/resources/lab_manuals/555Timer.pdf
FIG 2.2 555 connected as an astable multivibrator
Credit: www.tele.pitt.edu/resources/lab_manuals/555Timer.pdf
Best results are obtained with capacitors of 1000pF or larger, but smaller values can be
used with lower values of R1 and R2. The maximum operating frequency is around 1
MHz, but best operation is obtained below 300 kHz. The minimum operating frequency
is limited only by the size and leakage of the capacitor you use. For instance, a 10μF
capacitor and a 3.3 Ω resistor will give a time interval of 23.1 seconds if the leakage of
the capacitor is low enough. By making R2 large with respect to R1, we can get an
essentially symmetrical square-wave output.
For instance, if R1 is 1KΩ and R2 is 1MΩ, the difference in charging and discharging
resistance is only 0.1%, and good symmetry results. Any symmetry you want from 50%
through 99.9% can be obtained by a selection of the ratio of R1 and R2. Only a small
frequency variation occurs due to power supply variation but variation due to temperature
changes is large, so any precise instrumentation projects require more stable crystal
clock.
An astable timer operation is achieved by adding resistor RB to and configuring as
shown. In the astable operation, the trigger terminal and the threshold terminal are
connected so that a self-trigger is formed, operating as a multi vibrator. When the timer
output is high, its internal discharging Tr turns off and the VC1 increases by exponential
function with the time constant (RA+RB)*C. When the VC1, or the threshold voltage,
reaches 2Vcc/3, the comparator output on the trigger terminal becomes high, resetting the
F/F and causing the timer output to become low.
This in turn turns on the discharging Tr. and the C1 discharges through the discharging
channel formed by RB and the discharging Tr. When the VC1 falls below Vcc/3, the
comparator output on the trigger terminal becomes high and the timer output becomes
high again. The discharging Tr. turns off and the VC1 rises again. In the above process,
the section where the timer output is high is the time it takes for the VC1 to rise from
Vcc/3 to 2Vcc/3, and the section where the timer output is low is the time it takes for the
VC1 to drop from 2Vcc/3 to Vcc/3.
Important Features
The 555 timers basically operate in one of the two modes either as a monostable (one
shot) multivibrator or as an astable (free running) multivibrator. In the one-shot mode, the
555 acts like a monostable multivibrator. A monostable is said to have a single stable
state that is the off state. Whenever an input pulse triggers it, the monostable switches to
its temporary state. It remains in that state for a period of time determined by an RC
network.
It then returns to its stable state. In other words, the monostable circuit generates a single
pulse of fixed time duration each time it receives and input trigger pulse. Thus the name
one-shot, One-shot multivibrators are used for turning some circuit or external
component on or off for a specific length of time. It is also used to generate delays. When
multiple one-shots are cascaded, a variety of sequential timing pulses can be generated.
Those pulses will allow you to time and sequence a number of related operations.
The other basic operational mode of the 555 is as and astable multivibrator.
An astable multivibrator is simply and oscillator. The astable multivibrator
generates a continuous stream of rectangular off-on pulses that switch between two
voltage levels. The frequency of the pulses and their duty cycle are dependent upon the
RC network values.
The important features of the 555 timer are as follows:
(a) Can operate on +5V to +18V supply voltage.
(b) Having adjustable duty cycle.
(c) Timing from microseconds to hours.
(d) Producing high current output.
(e) Having capacity to source or sink current of 200 mA.
(f) Output can drive TTL.
(g) Having temperature stability of 50 ppm per oC change in temperature
or 0.005% per C.
(h) Is reliable, easy to use, and low cost.
The NE 555 timer is the bipolar version of timer. This primer is about this fantastic timer,
which is after 30 years still very popular and used in many schematics. Although these
days the CMOS version of this IC, like the Motorola MC1455, is mostly used, the regular
type is still available; however there have been many improvements and variations in the
circuitry. But all types are pin-for-pin plug compatible. This can operate over a supply
voltage range of +2V to +18V and has output current sinking and sourcing capabilities of
100 mA and 10 mA. Advantages of CMOS version timer are low power requirement and
very high input impedance.
2.2 NAND Gate Astable Multivibrators
Fig 2.3 NAND gates in Astable Mode
Credit: www.electronics-tutorials.ws/waveforms/bistable.html
The astable multivibrator circuit uses two CMOS NOT gates such as the CD4069 or the
74HC04 hex inverter ICs, or as in our simple circuit below a pair of CMOS NAND such
as the CD4011 or the 74LS132 and an RC timing network. The two NAND gates are
connected as inverting NOT gates.
Suppose that initially the output from the NAND gate U2 is HIGH at logic level "1", then
the input must therefore be LOW at logic level "0" (NAND gate principles) as will be the
output from the first NAND gate U1. Capacitor, C is connected between the output of the
second NAND gate U2 and its input via the timing resistor, R2. The capacitor now
charges up at a rate determined by the time constant of R2 and C.
As the capacitor, C charges up, the junction between the resistor R2 and the capacitor, C,
which is also connected to the input of the NAND gate U1 via the stabilizing resistor, R2
decreases until the lower threshold value of U1 is reached at which point U1 changes
state and the output of U1 now becomes HIGH. This causes NAND gate U2 to also
change state as its input has now changed from logic "0" to logic "1" resulting in the
output of NAND gate U2 becoming LOW, logic level "0".
Capacitor C is now reverse biased and discharges itself through the input of NAND gate
U1. Capacitor, C charges up again in the opposite direction determined by the time
constant of both R2 and C as before until it reaches the upper threshold value of NAND
gate U1. This causes U1 to change state and the cycle repeats itself over again.
Fig 2.4 Output of the NAND gate.
Credit: www.electronics-tutorials.ws/waveforms/bistable.html
2.5 CMOS Astable Multivibrator
Fig 2.5 CMOS in Astable mode
Credit: www.googleimages.com
The LM556 Dual timing circuit is a highly stable controller capable of producing
accurate time delays or oscillation. The 556 is a dual 555. An external resistor and
capacitor for each timing function provide timing. The two timers operate independently
of each other sharing only VCC and ground. The circuits may be triggered and reset on
falling waveforms. The output structures may sink or source 200mA.It results in
providing effective solutions for timing and pulse circuit applications.
Applications such as:
Pulse generation
Sequential timing
Time delay generation
Pulse width modulation
Pulse position modulation 0
Linear ramp generator
Operation
With the output high (+Vs.) the capacitor C1 is charged by current flowing through R1
and R2. The threshold and trigger inputs monitor the capacitor voltage and when it
reaches 2/3Vs (threshold voltage) the output becomes low and the discharge pin is
connected to 0V.
The capacitor now discharges with current flowing through R2 into the discharge pin.
When the voltage falls to 1/3Vs (trigger voltage) the output becomes high again and the
discharge pin is disconnected, allowing the capacitor to start charging again.
This cycle repeats continuously unless the reset input is connected to 0V which forces the
output low while reset is 0V.An astable can be used to provide the clock signal for
circuits such as counters.
A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher
frequency flashes are too fast to be seen clearly. Driving a loudspeaker or piezo
transducer with a low frequency of less than 20Hz will produce a series of 'clicks' (one
for each low/high transition) and this can be used to make a simple metronome.
An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from a
loudspeaker or piezo transducer. The sound is suitable for buzzes and beeps. The natural
(resonant) frequency of most piezo transducers is about 3kHz and this will make them
produce a particularly loud sound.
Duty cycle
The duty cycle of an astable circuit is the proportion of the complete cycle for which the
output is high (the mark time). It is usually given as a percentage.
For a standard 555/556-astable circuit the mark time (Tm) must be greater than the space
time (Ts), so the duty cycle must be at least 50%:
2.6 OP- Amp Astable multivibrator
The 741 chip: An important and very useful group of integrated circuits is the
“operational amplifier” or “op-amp” group. These devices have a very high gain, an
inverting input and a non-inverting input. There are many op-amps but we look at the
“741” which has an open-loop gain of 1000,000 times. All operational amplifiers work in
the same way in theory. The way they operate in a circuit is controlled by the external
components attached to them.
They can operate as inverting amplifier, a non-inverting amplifier (buffer), a comparator,
an Astable multivibrator and many more.
.
Credit: circuittoday.com
Fig-2.6 is the circuit of the 741 in astable mode
A capacitor C is connected to the inverting terminal (2) of the operational amplifier from
the ground. Similarly a resistance R1 is connected to the non-inverting terminal (3) of the
operational amplifier from the ground. The output terminal (6) of the amplifier is fed
back to inverting and non-inverting terminals of operational amplifier through resistors R
and R2 respectively. Here R2 is fixed resistor and R is variable resistor. To observe the
output waveform, the output terminal (6) is connected to CRO Y- Plates phase terminal
and the other terminal of CRO is grounded. The terminals (7) and (4) of the op. amp are
connected to +12 V and -12 V of the D.C. power supplies separately. The output terminal
(6) is also grounded through a series combination of two zener diodes connected in
reverse order as shown in the fig2.6.
Theory: - First the inverting terminal (2) is at zero potential (V2 = 0, the inverting
terminal 2 is virtually grounded) and the input at the non-inverting terminal (3) has some
potential V1 i.e. the voltage across R1. This occurs due to the power supply of the
operational amplifier.
This ‘+ ve’ voltage drives the output of operational amplifier into ‘+ ve’ saturation
voltage (+Vsat). This large saturation voltage is due to the high gain of the operational
amplifier i.e. the comparator character of the amplifier. When the + Vsat is fed back to
the inverting terminal (2) through the resistor R, the capacitor C gets charged and the
potential of the right side plate of the capacitor gradually rises (or) the V2 value rises
(Even though the inverting terminal 2 is virtually grounded but it is not mechanically
grounded). When V2 becomes slightly more than V1, the input (Vi = V1 – V2) becomes
‘–ve' and immediately this ‘–ve’ voltage drives the output of the operational amplifier in
to ‘–ve’ saturation voltage (- Vsat).
Now the capacitor discharges gradually. When V2 becomes less than V1 and (V1 – V2)
becomes ‘+ve’ and the output drives to +Vsat. The same process is repeated and the
output of the operational amplifier swings between two saturation voltages i.e. between +
Vsat and - Vsat.
The output eo of the operational amplifier is square wave. So, operational amplifier can
function as a square wave generator. The wave shape is as shown in Fig2.7.
Fig2.7 Square wave output of the op-amp
Credit: cicuirttoday.com
CHAPTER 3 METHODOLOGY
Overview
The purpose of this section is to outline and examine the design requirement and process
of Implementation based on the requirement analysis. This section will also give explicit
information about the system under development. The design stage produces a prototype
that includes performance, reliability, constraint and all relevant information about the
system.
A systematic examination and evaluation of data or information, by breaking it into its
component parts to uncover their interrelationships and also the breakdown of the topic
into simpler units in order to achieve a better understanding.
3.1 Design Methods
Approaches that will enhance the design of a two state device.
*Getting a suitable circuit diagram that will function as an astable multivibrator.
* Simulating to get the right components that will meet our design or requirement.
*Implementing our design
*Observing the output waveforms from generated by every module.
*Observing the final output waveforms from the structure.
3.2 Modules design
During this stage of the module the charging of the LED1 depends on the
transistor Q1 being connected to the negative plate of the C1. The period of the square
wave at the Q1 Outputs in this mode of operation is a function of the external
components employed.
3.2.1
During this stage of the module the charging of the LED2 depends on the
transistor Q2 being connected to the negative plate of the C2. The period of the square
wave at the Q1 Outputs in this mode of operation is a function of the external
Components employed
3.3 Design structure
R210k
R310k
R1390
R4390
C1
22µF
C2
22µFV112 V
LED1LED2
Q2
2SC945Q1
2SC945
36 re
d LE
Ds
40G
reen
LE
Ds
A high level on the ASTABLE input enables Astable operation. The
Period of the square wave at the Q1 and Q2 Outputs in this mode of operation
is a function of the external components employed. "True" input pulses on
the ASTABLE input or "Complement" pulses on the ASTABLE input allow
the circuit to be used as a getable multivibrator. The OSCILLATOR output
Period will be half of the Q terminal output in the astable mode. However, a
50% duty cycle is guaranteed at this output.
The characteristics of the two transistors are not exactly the same. When the circuit is
first switched on, the current through one transistor, say Q1, will increase faster than the
current through Q2.
Due to the rise of current through R1, the voltage across it will increase, causing the
collector voltage of Q1 to fall. This fall in voltage is coupled to the base of Q2. This
causes the collector current of Q2 to fall, and its collector voltage to rise, due to less
voltage being dropped across R4.
This rise in collector voltage is cross-coupled to the base of Q1, increasing the forward
bias of Q1 and increasing its collector current. Since the collector current was already
rising, its rise is aided by this rising forward bias. The effect is cumulative and Q1
becomes rapidly fully on and Q2 completely off.
The collector voltage of Q1 is now low, and that of Q2 is high. C1 now begins to charge
from the supply rail, via R2. As the voltage on the right hand side of C1 starts to rise, Q2
starts to conduct. Again we have the cumulative effect and Q2 rapidly comes on and Q1
goes off. The collector voltage of Tr1 is now high and that of Q2 low. It is now the turn
of C2 to charge from the supply via R3.
As the voltage on the left hand side of C2 begins to rise, the base voltage of Q1 increases,
turning it on and turning Q2 off.
3.4 Electronic Component
NAME COMPONENTUSED DESCRIPTION NO OF
COMPONENTS
REQUIRED
2SC945 2SC945 TRANSISTOR 2
RES 1,4
RES 2,3
RC390
RC 10K RESISTOR
2
2
CAP Electrolyte (22uf) CAPACITORS 2
VDC VDC DC VOLTAGE 1
LED (2mA) GREEN, RED LED 2
3.4.1 Transistor (2SC945)
A BJT transistor is an electronic device made by doped semiconductor material and can
be made use of in amplifying or switching functions. It is a three-terminal electronic
device. 2SC945 is an NPN bi-polar junction transistor. A transistor, stands for transfer of
resistance, is commonly used to amplify current. A small current at its base controls a
larger current at collector & emitter terminals. 2SC945 is mainly used for amplification
and switching purposes
The transistor terminals require a fixed DC voltage to operate in the desired region of its
characteristic curves. This is known as the biasing. For amplification applications, the
transistor is biased such that it is partly on for all input conditions. The input signal at
base is amplified and taken at the emitter
3.4.2 Electrolytic capacitor (22uf)
Electrolytic capacitors are polarized and they must be connected the correct way round, at
least one of their leads will be marked + or -. They are not damaged by heat when
soldering.
An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic
conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume
than other types.
They are often referred to in electronics usage simply as "electrolytic". They are used in
relatively high current and low frequency electrical circuits, particularly in power supply
filters, where they store charge needed to moderate output voltage and current
fluctuations in rectifier output. . Electrolytic capacitors also have relatively low
breakdown voltage, higher leakage current and inductance, poorer tolerances and
temperature range, and shorter lifetimes compared to other types of capacitors.
3.4.3 LEDs
An LED is often small in area (less than 1 mm2) and is easily populated onto printed
circuit boards, and integrated optical components may be used to shape its radiation
pattern. LEDs present many advantages over incandescent light sources including lower
energy consumption, longer lifetime, improved robustness, smaller size, faster switching,
and greater durability and reliability.
LEDs powerful enough for room lighting are relatively expensive and require more
precise current and heat management than compact fluorescent lamp sources of
comparable output. The low energy consumption, low maintenance and small size of
modern LEDs has led to uses as status indicators and displays on a variety of equipment
and installations. Their efficiency is not affected by shape and size, unlike fluorescent
light bulbs or tubes
3.5 Hardware resources and procurement
ITEM QTY UNIT PRICE TOTAL
AMOUNT
TRANSISTOR (2SC945) 2 50p Gh1
RESISTOR (390 ohms)
(10K ohms)
2
2
50p
50p
Gh1
Gh1
ELECTROLYTIC
CAPACITORS (22UF)2 50p Gh1
DC VOLTAGE 1 GH5 Gh5
LED 80 10 Gh10
BREAD BOARD 1 Gh2 Gh2
Total Ghc 21
3.6 Construction
After we have designed your circuit, perhaps even bread boarded a working
prototype, and now it's time to turn it into a nice Bread Circuit Board
design.
* The four resistors fit flat against the board. To make them sit neatly, bend the leads to
90° with a sharp bend and push them up to the board before soldering.
* The two 100u electrolytic are next. The positive hole is marked on the board for each
electro. This is the longer lead. The negative lead is marked on the component with a
black stripe.
*Fit the two NPN transistors. We have used 2SC 945 but any general-purpose
NPN low-power transistor will be suitable. They are pushed to the board.
*The red and green LEDs can be fitted to either position on the board. The short lead is
Cathode and this is the bar on the symbol.
* The project is now ready to turn on
390R
390R
10k
10k
22uf 22uf 40 green
CHAPTER 4 Result And Analysis
4.1 Simulation results on Q1
The entire normal range of silicon transistor operation involves a change in base-emitter
voltage of only about two-tenths of a volt. This is because the base-emitter diode is
forward biased. One of the constraints on transistor action is that this voltage remains at
about 0.6-0.7 volts.
Fig 4.1 Multimeter showing Emitter-Base Voltage (VEBO )
Multimeter showing a voltage reading of 0.73volts which means that the transistor is
operating at normal range.
4.1.2 Waveform signal of Q1
The graph below depicts the the 0.73v waveform signal on the oscilloscope.
Fig 4.2 Oscilloscope showing the signal of Q1
4.2 Simulation results on Q2
Multimeter showing a voltage reading of 0.73volts which means that the transistor is
operating between the normal range of 0.6-0.7 volts on Q2.
Fig 4.3 Multimeter showing Emitter-Base Voltage (VEBO )
4.2.1 Waveform signal of Q1
The graph below depicts the the 0.73v waveform signal on the oscilloscope.
Fig 4.4 Oscilloscope showing the signal of Q1
top related