In the buzzer round of quiz contest,
the question is thrown to all the teams. The person who knows the answer hits
the buzzer first and then answers the question. Sometimes two or more players
hit the buzzer almost simultaneously and it is very difficult to detect which
of them pressed buzzer first. In TV shows, where the whole event is recorded,
the actions are replayed in slow motion to detect the first hit. Such slow
motions are possible only where huge funds are available to conduct the show.
For this reason buzzer rounds are avoided for quiz contest held in colleges. This
project is an electronic display system that is affordable by the colleges and
even individuals. The project is useful for 8-team quiz contest, although it
can be modified for more number of teams. This system is sensitive. The circuit
can detect and record first hit contestant among all the contestants that may
appear to be simultaneous. This device is capable of distinguishing between two
events with a time gap of microseconds. This device has provision of avoiding
key bouncing while the inputs are given. It is a microcontroller based project
which uses a monostable multivibrator as a timer. The inputs are given with the
help of 8 - SINGLE POLE DOUBLE THROW (SPDT) – single break switches. The output
is displayed on a 16x2 liquid crystal display (LCD). This project uses a
regulated 5V, 500mA power supply. 7805 three terminal voltage regulator is used
for voltage regulation. Bridge type full wave rectifier is used to rectify the
ac output of secondary of 230/9V step down transformer.
Circuit specifications
Total input time allowed (polling) =
15 sec
Microcontroller’s polling time for
each switch = 10-12 µsec
Electrical Specifications:
IC 89C51 and LCD operate on +5V DC
supply.
IC – 555 operates on +5V DC supply.
Step down Transformer specifications:
230
V primary.
9
V Secondary.
25:1 turns ratio.
500
mA current.
Power supply
The system uses +5V regulated power
supply. Step down transformer is used to step down mains voltage which has 25:1
primary and secondary turns ratio. Transformer can provide upto 500mA current.
Bridge type rectifier is used. Diodes 1N4007 form the bridge rectifier. Filter
capacitor is used for removing ripples present at the output of bridge
rectifier. IC 7805 provides regulated +5V DC output voltage.
Microcontroller (89C51)
The system uses 89C51 microcontroller
for controlling all the activities. It is the cheapest IC, within family of
8051 microcontroller series. It has flash memory. It doesn’t have In System
Programming (ISP) facility. It works on 5V supply. It poles all switches within
15 sec, and displays the result on LCD. Here we have not used internal software
delay which is not variable externally once the program is written. Polling
method is used instead of interrupting the microcontroller, by using IC – 555
as monostable multivibrator (Timer). This provides better way to display
numbers and avoids conflict with contestant after the end of 15 sec. If no one
presses the key then it displays ‘No - Enteries’.
Switches
The system uses 8 SPST switches. Each
switch is given to one team. SPST switches don’t provide bouncing when they
change state from ON to OFF and vice – versa. One end of the switch is
connected to +5V terminal of supply and the other end is connected to the
ground.
Start switch
It is push to on switch. It is
provided to anchor. When anchor presses this button trigger is given to
monostable multivibrator. At the same time LCD displays ‘WAIT’ label.
Monostable multivibrator and inverter
Monostable multivibrator acts as a
timer. IC 555 is used in monostable mode. It provides time delay of 15 sec.
This time delay is adjustable by varying a potentiometer. When ANCHOR presses
the START button a trigger is given to monostable, then monostable jumps to
HIGH STATE for 15 sec, which is inverted by 2N2222A transistor and output of
inverter is given to microcontroller’s pin number P1.2
LED shows status. When 15 sec are
going on GREEN - LED is on. In another condition by default RED - LED is on.
LCD (16X2)
LCD stands for Liquid Crystal Display.
Here we are using 16X2 LCD display. It can show 16 characters in 1st line and
16 characters in 2nd line. A 10kΩ potentiometer is used for contrast control.
It has three control lines connected to port 2 of micrconroller.8 Data lines
are used for transferring data from microcontroller to LCD. These data lines
are connected to port - 3 of microcontroller.
Module wise design
CIRCUIT DIAGRAM (POWER SUPPLY)
Power supply design
In many applications, it is necessary
to supply constant dc voltage. The conversion of ac to dc is achieved by
rectifier supplied with ac signal using step down transformer. Output of
rectifier contains pulsating dc which can be removed by use of filter circuit.
The IC78XX series of 3 terminal regulators are available with fixed positive
voltages. These IC’s has internal thermal shutdown and short circuit current
limiting.
LCD Display
The 2 line x 16 character LCD modules
are available from a wide range of manufacturers and should all be compatible
with the HD44780.
Pin No. Name Description
Pin no. 14 D7 Data bus line 7
(MSB)
Pin no. 13 D6 Data bus line 6
Pin no. 12 D5 Data bus line 5
Pin no. 11 D4 Data bus line 4
Pin no. 10 D3 Data bus line 3
Pin no. 9 D2 Data bus line
2
Pin no. 8 D1 Data bus line
1
Pin no. 7 D0 Data bus line
0 (LSB)
Pin no. 6 EN1 Enable signal
for row 0 and 1 (1stcontroller)
Pin no. 5 R/W 0 = Write to
LCD module
1 = Read from LCD module
Pin no. 4 RS 0 =
Instruction input
1 = Data input
Pin no. 3 VEE Contrast
adjust
Pin no. 1 VSS Power supply
(GND)
Pin no. 2 VCC Power supply
(+5V)
Pin no. 15 EN2 Enable signal
for row 2 and 3 (2ndcontroller)
Pin no. 16 NC Not Connected
We make no effort to place the Data
bus into reverse direction. Therefore we hard wire the R/W line of the LCD
panel, into write mode. This will cause no bus conflicts on the data lines. As
a result we cannot read back the LCD's internal Busy Flag which tells us if the
LCD has accepted and finished processing the last instruction. This problem is
overcome by inserting known delays into our program.
The 10kΩ Potentiometer controls the
contrast of the LCD panel. Nothing fancy here. As with all the examples, I've
left the power supply out. You can use a bench power supply set to 5Vor use a
onboard +5V regulator.
Mono - stable Multi - vibrator (Using
timer IC – 555)
The IC NE – 555 is used as a
Monostable multivibrator. It generates a
high(5V) pulse at pin no.3 of variable duration, which remains high for
the set duration, say TON(the duration being controlled by the timing components),
when a trigger pulse is applied at pin no.2.
R and C are the timing components which decide the duration of the low
going pulse. The trigger pin is held at 2/3 VCC and, when trigger is applied
the voltage at that pin is forced to (grounded). Once the trigger is applied,
it is not effective for the time period TON, thus not having any control on the
output and the output remains high (5V) for TON seconds.This high to
low output is an indication to the microcontroller, implying that the time period
of accepting inputs is over. Accordingly the controller accepts the inputs for
TON seconds only and then processes them to give the desired output.
Microcontroller 89c51
The 8051 is an 8-bit microcontroller.
It consists of CPU, two kinds of memory sections (data and program memory),
input/output ports, special function registers and control logic. The AT89C51
is a low-power, high-performance CMOS 8-bit microcomputer with 4K bytes of
Flash programmable and erasable read only memory (PEROM). The device is
manufactured using Atmel’s high-density nonvolatile memory technology and is compatible
with the industry-standard MCS-51 instruction set and pin out. The on-chip Flash
allows the program memory to be reprogrammed in-system or by a conventional nonvolatile
memory programmer. By combining a versatile 8-bit CPU with Flash on a
monolithic chip, the Atmel AT89C51 is a powerful microcomputer which provides a
highly-flexible and cost-effective solution to many embedded control
applications.
Features
• Compatible with MCS-51 Products.
• 4K Bytes of In-System Reprogrammable
Flash Memory.
Endurance: 1,000 Write/Erase Cycles.
• Fully Static Operation: 0 Hz to 24
MHz.
• Three-level Program Memory Lock.
• 128 x 8-bit Internal RAM.
• 32 Programmable I/O Lines.
• Two 16-bit Timer/Counters.
• Six Interrupt Sources.
• Programmable Serial Channel.
• Low-power Idle and Power-down Modes.
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