Download Working with the 555:

electronics Club
Introductory Lecture on Digital Electronics
url: www.iitk.ac.in/eclub
email: [email protected]
Basic Concepts:
Digital Electronics: Digital Electronics is defined in the dictionary as the
electronics applied to the processing binary numbers. Basically, this branch of
electronics has just two building blocks, or to say two building numbers, namely 0 &
1. It all depends whether the input or output is 1 or 0, in other terminology, low or
high respectively.
Logical High/ Low: As said above, in
digital electronics there are basically two states
– High or Low. High refers to the state when the
voltage is more than the reference voltage
(mostly ground) & Low refers the state when
the voltage is close to the reference voltage.
High is referred by 1 & Low is referred by 0.
High
v
o
l
t
a
g
e
Low
time
Number System: The two number system which are used most often in the
digital electronics are Binary No. System
only with the binary system.
Binary System: It consists of
just two numerals – 0 & 1.
Decimal numbers in binary
system using 4 bits &
hexadecimal system are:
& the hexadecimal System. Here we deal
Decimal
No.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Binary
Equivalent
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Hexadecimal
Equivalent
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
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Clock: Looking electronically, a clock is simply a square wave i.e. alternate
high & low states. Each alternate high-low forms a clock cycle with a specific
frequency & duty cycle. Frequency is the no of cycles completed in 1 sec &
duty cycle is the ratio of the time period of high state to the time period of the
low state
v
o
l
t
a
g
e
time
Gates & Truth Tables:
Gates are the logical units that perform
Boolean operations on one or more inputs. The common gates are NOT gate,
AND gate, OR gate, NAND gate, NOR gate, XOR gate & N-XOR gate. For the
function of the gates & their truth tables, refer the datasheets of the gates or
the presentation available on the club website.
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Now let us look into the IC’s & other electrical
components that are going to be used for the hands
on workshop:
But, first of all, let’s see what an IC is:
IC – integrated circuit – is a small chip which when given input, performs
some specific operation on the inputs & gives some output. It basically is
made of millions of transistors in such a way that the desired operation is
performed.
Eg: 555, 4029, 7447 etc
Resistors: These are the resistors whose value can be identified
from the colour code as given bel
Capacitors: We prefer to use electrolytic capacitors over ceramic
as they are more accurate. The important point to be noted is that
although the resistors, mica capacitors are non directional,
Electrolytic capacitors have to be used in the circuit with terminals
with the right polarity i.e. the terminal with marking of ground or
–ve is always the anode(it has a shorter leg). If these are connected
with opposite polarity, the capacitor won’t work & would infact burst!
Breadboards: These are the bases on which circuits
are built which already have certain holes
interconnected. How to use it & which holes are
interconnected would be explained in the workshop.
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Working with the 555
555 is used for producing a clock (square wave) at a
desired frequency. It can be used in various ways
like the astable mode, monostable mode etc. Here,
we deal with the astable operation of 555. Astable
mode ensures that the 555 is self-triggered & so, it
acts as a multi-vibrator.
Let us look into the working of 555 in astable mode:
These are the
connections
needed
to
make the 555 chip run in the ‘astable’ mode.
The pin numbers are given in circles.
Note the right-most side of the figure, and
consider all the connections with the pins as
‘open-circuits’. Ignore the rest of the circuit
for a while.
D1 - Circuit
Connections to be made
This over-familiar circuit will majestically
jump up. The Vc1 is the driving signal for
this 555, connected at PIN 6, which is the
THRESHOLD PIN. As soon as Vc1
reaches ⅔Vcc, then the output at PIN 3
goes low, and the capacitor starts
discharging, via PIN 7 with Rb as the
Resistor and ground as the other terminal.
(Refer to the first image to see how it
MIGHT happen.) When it reaches ⅓Vcc,
the output at PIN 3 goes high, and the
DISCHARGE PIN’s connection with
ground is broken. The capacitor again
starts charging, and the cycle is repeated.
You need not bother yourself with how
the
Circuit is broken or established.
This graph gives the OUTPUT at PIN 3
and the input voltage at PIN 7.
D3- Output
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D4- Internal Working of the 555
This is the internal working of the 555 timer. The chip derives its name from the three
R’s on the top of the image, they all are 5 kΩ. Hence, the name 555.
Now, let us see how we can set the 555 to work at the desired frequency by selecting
the right combination of resistances & capacitances. Using the convention as in D1,
From circuit analysis & mathematics, it can be obtained that:
Frequency = 1.44/{(RA + 2RB) * C1}
Also, 555 can produce waves with duty cycle else than the 50 % cycle. The desired
duty cycle can be worked out by using the result:
Duty Cycle = (RA + RB) * 100/ (RA + 2RB)
where duty cycle= Ratio of time period when the output is 1 to the time
period when the output is 0
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Working With 4029 Counter
The IC 4029 has multiple function , i.e. as
a counter as well as a memory
device(using the flip flops). Here in the
workshop, we are considering only the
counter function & how it works.
When connected in the way as described
below, the 4029 counts from 0 to 9 (or 0
to 15) & gives the output in BCD (Binary
Coded Decimal) with 4 bits.
Pins & their Connections:
Pin No.
1
Name
Parallel Load
2
3
Output Bit 3
Parallel bit 3
4
Parallel bit 0
5
Clock Enable (bar)
6
7
Output Bit 0
TC (bar)
8
V reference(Low)
9
10
11
12
Binary or hex count
UP or Down
Output Bit 1
Parallel bit 1
13
Parallel bit 2
14
15
Output bit 2
Clock Pulse
16
Voltage (high)
Function
When high, parallel bits are
corresponding output bits.
When low, counter.
Most significant bit
Bit copied to O3 when PL is
high
Bit copied to O0 when PL is
high
When grounded, the clock is
enabled
Least significant bit
Gives a low when one cycle
of count completes
To be connected to make the
chip work
To count upto 10 or 16
increase or decrease
Output bit
Bit copied to O1 when PL is
high
Bit copied to O2 when PL is
high
Output bit
Input-the clock pulse- each
high increments the counter
To be connected to make the
chip work
Connection
Ground
Output(goes to 7447)
ground
Ground
Ground
Output(goes to 7447)
Output – can be used for
subsequent clock for other
counter
Ground
ground
5v
Output(goes to 7447)
Ground
Ground
Output(goes to 7447)
Output of 555 i.e. pin 3
5V
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Working With BCD to 7 Segment Decoder
7447
The BCD to 7 Segment Decoder IC
7447 takes in the 4 output bits (O0,
O1, O2, O3 from the 4029 here) &
decodes it such that the corresponding
digit in decimal form can be shown on
the 7 Segment Display. The output
here are the pins numbered 9 to 15
named a to g as shown in the pin out
diagram. These 7 bits are to be
connected to the corresponding pins(to
be identified by the alphabets) via a
resistor, preferably a 220 Ω or 330Ω.
(Think about the reason resistor is
used. The pin connections in this chip
are:
Pin 1,2, 6,7 – input pins
Pin 8 – Ground
Pin 16 – 5 V
Pin 9 to 15 – To 7 Segment Display
Pin 3 – To test whether the 7 segment is defective. If it is high, all the 7 segments
must glow. It has to be connected to Ground.
Pin 4 , 5 – Ground
7 Segment Display
g
f
K
a
d
K
c
b
The 7 Segment Display consists of 7 LEDs which can be
glowed in some particular fashion using the 7 different
input pins. For example, if a, b, g, e, d glow, numeral 2 is
represented.
K stands for cathode, i.e. K has to be connected to 5 V. The
decimal is optional as it is used to glow the decimal point
in bottom right of the 7 segment display.
e
decimal
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Important Tips While Circuiting:
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Interconnect the horizontal lines & use them for ground & 5V purposes so that
ground & high can be provided to any pin in the circuit easily.
Always keep in mind, never ever short high with low.
Handle the chips carefully.
Always keep in mind to give the chip High & Low at the corresponding place
i.e. Vss(or ground)) & Vdd(or Vcc), else the chip won’t work.
Never leave any input of the chip in floating position i.e. not connected as that
can affect the output by introducing noise.
When using an LED, always use it along with a resistor of appropriate value.
Use the electrolytic capacitor with correct polarity.
Keep the wires small & tight for clean & strong circuiting.
Use some colour coding for easy identification of wires & connections during
debugging. For eg: Use red for high, green for ground & yellow, blue for
interconnections.
In digital electronics, keep an LED ready for testing & debugging purposes.
What I mean by this is, that the Led should be connected to a resistor & wire,
so that it is flexible.( believe me, there is going to be a great use of it).
When using resistors, do check that the legs do not touch each other.
Incase of any confusion, contact your friendly neighbourhood Secy or Coordy!
Things to Ponder:
 Why Is LED connected along with a resistor?
 Why did we connect each pin of 7 segment display through a resistor?
Is there any other way also that would use lesser no of resistors &
serve the same purpose?
 Is there a way that you can make your circuit safe against any crossconnection that takes place between high & low in your circuit?
 If you used 555 with a different duty cycle but same frequency, how
would it affect the counter?
 Can you think how you could use a 4029 as a memory device?(hint:
Parallel Load)
 Can you upgrade this counter of yours to multiple displays i.e. 2
Displays for seconds & 1 for min ( count upto 10 minutes only) working
like a clock i.e. after every 9 sec, the tens digit is incremented & after
59 seconds, sacs should go to 00 & minute is incremented? (hint:
Parallel load & TC)
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