electronics Club

Introductory Lecture on Digital Electronics

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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.

Decimal
No. / Binary
Equivalent / Hexadecimal
Equivalent
0 / 0000 / 0
1 / 0001 / 1
2 / 0010 / 2
3 / 0011 / 3
4 / 0100 / 4
5 / 0101 / 5
6 / 0110 / 6
7 / 0111 / 7
8 / 1000 / 8
9 / 1001 / 9
10 / 1010 / A
11 / 1011 / B
12 / 1100 / C
13 / 1101 / D
14 / 1110 / E
15 / 1111 / F

Number System:The two number system which are used most often in the digital electronics are Binary No. System & the hexadecimal System. Here we deal only with the binary system.

BinarySystem:It consists of just two numerals – 0 & 1. Decimal numbers in binary system using 4 bits & hexadecimal system are:

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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

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 polarityi.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.

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

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

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. / Name / Function / Connection
1 / Parallel Load / When high, parallel bits are corresponding output bits. When low, counter. / Ground
2 / Output Bit 3 / Most significant bit / Output(goes to 7447)
3 / Parallel bit 3 / Bit copied to O3 when PL is high / ground
4 / Parallel bit 0 / Bit copied to O0 when PL is high / Ground
5 / Clock Enable (bar) / When grounded, the clock is enabled / Ground
6 / Output Bit 0 / Least significant bit / Output(goes to 7447)
7 / TC (bar) / Gives a low when one cycle of count completes / Output – can be used for subsequent clock for other counter
8 / V reference(Low) / To be connected to make the chip work / Ground
9 / Binary or hex count / To count upto 10 or 16 / ground
10 / UP or Down / increase or decrease / 5 v
11 / Output Bit 1 / Output bit / Output(goes to 7447)
12 / Parallel bit 1 / Bit copied to O1 when PL is high / Ground
13 / Parallel bit 2 / Bit copied to O2 when PL is high / Ground
14 / Output bit 2 / Output bit / Output(goes to 7447)
15 / Clock Pulse / Input-the clock pulse- each high increments the counter / Output of 555 i.e. pin 3
16 / Voltage (high) / To be connected to make the chip work / 5 V

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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

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.

Important Tips While Circuiting:

  • 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 cross-connection 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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