Digital Electronics

DIGITAL ELECTRONICS Logic Probe: A logic probe is used to determine the logic level 1(i.e +5 volts) or 0 (i.e o volts) of any point in a digital circuit. The anode and cathode of a LED can be identified using fig 1(a). Fig 1(b) shows the circuit diagram of a logic probe.

470ohm

Preparatory work: Set up a logic probe and use it to determine the pattern of interconnections among the connecting points of the Bread-Board. Seven Segment Display: Determine the type (common anode/cathode) and the pin connection of 7 segment display types. Description: The display consists of 8 LED shown as a,b,c,d,e,f,g and DP (decimal point) in fig 1.One terminal either anode or cathode of all LEDs are connected together and the common terminal is connected to pins 3 & 8.Thus there are two types of these displays namely, common anode and common cathode. The other terminal of each LED is connected to one of eight remaining pins. Experimental Procedure: 1. Insert the display on the bread board. With the AVO meter determine whether it is a common anode or a common cathode display. 2. For a common anode type of display connect pin3 or 8 to +5 volts supply.connect the negative terminal of another LED to the ground and the positive terminal to a 470 ohm resistor.As the other end of the resistor is connected to each pin of the display (excepting pin 3 and 8)one segment of the display will emit light. These shows that the cathode of that particular LED in the display is connected to the pin. 3.Step 2 is repeated for all pins(except 3 & 8) of the display and a table showing the pin connections to the different segment of the display is made. 4.For a common cathode type of display pin 3 & 8 is connected to the ground and the positive terminal of the LED used for testing is connected to +5 volts supply. 5.Stepa 2 & 3 are repeated with the changes mentioned in step 4. Experimental Results: 7 segment display type: Common terminal:Anode/Cathode Table showing pin connections

fig-1 10 9

8

7

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6

a-7

f-9

b-6

g-10 e-1

c-4 d-2

1

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View) DECODING BINARY INPUT FOR 7 SEGMENT DISPLAY Operate a 7 segment display with binary input using as suitable IC for decoding. Choice of the decoder: To operate a 7 segment display with input in binary (or any other) code we need a ROM which we call a decoder. For binary inputs such decoders are available as IC’s of type 7446,7447,7448,7449. Of these 7446 and 7447 are used with displays of common anode type since these IC’s have active low outputs.IC’s 7448 & 7449 have active high outputs and are used with common cathode LED’s. In our experiment IC 7447 with a common anode display is used. The pin connection of IC 7447 is shown in fig. Experimental Procedure: 1. Set up the circuit of fig 2 on the bread board. When any of the switches S1, S2, S3, S4 is open the corresponding input A, B, C, D is 1.When the switch is closed input is 0. 2. Record the display for complete set of inputs from 0000 to 1111. Experimental results: Input Output Binary DCBA Decimal Display 0000 0 0001 1 0010 2 0011 3 0100 4 0101 5 0110 6 0111 7 1000 8 1001 9 1010 10 1011 11 1100 12 1101 13 1110 14

1111

15

Clock Pulse Generator: Set up a clock pulse generator with friquency~1 kHz and duty cycle~0.6 using IC 555. Design formula: For the circuit of fig 1 (a) the output waveform is shown in fig 1 (b).The effect of the component values is given by the following relations. T1=0.693 C1 (R1+R2) sec-----------------------------1 T2=0.693 C1 R2 sec-----------------------------2 T=T1+T2=0.693 C1 (R1+2R2) sec -------------------3 f=1/T=1.44/C1 (R1+2 R2) Hz-------------------------4 Duty Cycle=D=T1/T= (R1+R2)/(R1+2 R2)----------5 Calculation: 1. using (5) find that R2=2R1 if D=0.6 2. Taking C1=0.1 mfd find R1 from (3) for the frequency f (1/T) of 1 kHz. 3. Now find R2. Experimental Procedure: 1. Set up the circuit if fig1 (a) on the bread board and observe the output waveform. 2. Trace the output waveform and determine the frequency, amplitude and the duty cycle. 3. Verify equation (4) using diff values of C1 and keeping R1 & R2 constant. 4. For C1=100 mfd, measure the frequency by a logic probe noting the number of times it glows in 30 seconds.