Experiment 7
BCD to 7-SEGMENT DECODER/DRIVER Objectives: 1. To familiarize the student with the use of 7-segment LED displays. 2. To illustrate the basic functions of BCD to 7-segment decoders/drivers. 3. To demonstrate the usefulness of the ripple blanking feature of some BCD to 7-segment decoders. Introductory Information: Among the popular binary codes, BCD is most often used in counting circuits, particularly because it is based on the decimal counting system most people are familiar with. Special MSI integrated circuits have been manufactured that are capable of converting the 4-bit code to a seven segment output and driving the seven segments required to display a facsimile of the complete Arabic number system. Optoelectronic display devices have been combined in a single package so as to permit the display of human-readable characters. A 7-segment display arranged in an 8 figure can display the digits 0 to 9. A 5x9 dot matrix display gives better lowercase characters. An alphanumeric display similar to the 7-segment would consist of at least 16 segments. A BCD to 7-segment decoder is a combinational circuit that accepts a 4-bit BCD data and generates the appropriate outputs for selection of segments in a display indicator used for displaying the decimal digit. Display patterns for invalid BCD codes (1010 to 1111) are unique symbols for each 4 bit invalid combination to authenticate input conditions. Combinational circuits are important particularly for numeric displays that a wide variety is available from the TTL family. These include the 7446, 7447, 7448, 7449, 74247, 74248 and 74249 integrated circuits. Although each performs the same basic function, subtle variations exist, taking into account other considerations such as display type, character font, and loading or display size. These displays include common anode and common cathode LED’s, or incandescent indicators. In addition to the standard BCD inputs, some BCD to 7-segment decoders feature ripple blanking. This feature is used to blank out leading and trailing zeroes. This is done by connecting RBI of the most significant digit to ground. BI/RBO is then connected to RBI of the next lower stage. This is particularly useful in making display more readable. The BI/RBO and RBI pin also allows display expansion, leading/trailing zero suppression and lamp intensity SWIDLAB Experiment Manual Experiment No. 7: BCD to 7-Segment Decoder/Drivers Page 1 of 6
modulation. Another added feature would be the lamp test input (LT). This allows the user to check the driver and display sections by forcing each segment to be activated. Materials/Equipment: 1 1 1 1 1 1
7447 or 74247 integrated circuit (BCD to 7-segment decoder/driver) 7-segment single digit common-anode LED display 150-Ohms, 1/4-Watt resistor Breadboard with connecting wires Regulated DC power supply Voltmeter-Ohmmeter-Milliammeter (VOM)
Procedure: 1. Set the VOM to its transistor/diode test mode. If none is available, use the x10 resistance range. 2. Using the VOM, find a combination of two pins from the seven-segment display that lightUP an LED segment. The pin at which a positive potential was applied is the anode. A common anode display must have a positive potential in this terminal. If no segment illuminates for all possible combination of two pins from the 7-segment display, you may have a defective 7-segment display.
Note: VOM’s may behave differently if used as an ohmmeter to test a device. VOM’s from American manufacturers apply a positive potential at the red (+) terminal with respect to the black (-/COM) terminal. VOM’s from Japanese manufacturers apply a positive potential at the black (-/COM) terminal with respect to the red (+) terminal. FET-input multimeters may not have this diode test capability. By applying a forward bias, a semiconductor PN junction will conduct. This is indicated in the LED segments by emitting visible radiation, typically in the lower spectrum, i.e., infrared, red, yellow or green light.
3. Using the standard segment labels shown in Figure 1, identify the pin corresponding to the common, and the pins corresponding to the different segments in the display and record them on Table 1.
SWIDLAB Experiment Manual Experiment No. 7: BCD to 7-Segment Decoder/Drivers Page 2 of 6
Figure 1 Pin No. Common a b c d e f g h Table 1 4. Turn ON the power supply. Set the power supply to +5 volts DC. Turn OFF the power supply. 5. Wire the upper distribution strip of the breadboard to the +5 volts and the lower distribution strip to ground. 6. Wire the circuit shown in Figure 2 on the breadboard. Use the pin designations obtained for the 7-segment display in Table 1.
SWIDLAB Experiment Manual Experiment No. 7: BCD to 7-Segment Decoder/Drivers Page 3 of 6
Figure 2 7. Turn ON the power supply. Set the BCD inputs (A, B, C and D) from 0000 to 1111. For each setting of the BCD input, take note of the illuminated segments and the digit they display. Record your findings in Table 2. Turn OFF your power supply. BCD Input Digit displayed Illuminated Segments A B C D 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 Table 2 SWIDLAB Experiment Manual Experiment No. 7: BCD to 7-Segment Decoder/Drivers Page 4 of 6
8. Apply an input of 0000 into A, B, C, and D such that a zero is displayed. Turn ON the power supply. 9. Complete Table 3, varying signals LT (pin-3) and RBI (pin-5), leaving the BI/RBO (pin-4) at active HIGH. Turn OFF the power supply. LT 0 0 1 1
RBI 0 1 0 1
Digit displayed
Illuminated Segments
Table 3 10. Apply an input of 0000 into A, B, C, and D such that a zero is displayed. Turn ON the power supply. 11. Complete Table 4. Turn OFF the power supply. LT
RBI
0 0 0 0 1 1 1 1
0 0 1 1 0 0 1 1
BI/ RBO 0 1 0 1 0 1 0 1
Digit displayed
Illuminated Segments
Table 4 12. Apply a non-zero binary input into A, B, C, and D. Turn ON the power supply. 13. Complete Table 5. Turn OFF the power supply. LT
RBI
BI/ Digit displayed RBO 0 0 0 0 0 1 0 1 0 0 1 1 1 0 0 SWIDLAB Experiment Manual Experiment No. 7: BCD to 7-Segment Decoder/Drivers Page 5 of 6
Illuminated Segments
1 1 1
0 1 1
1 0 1 Table 5
Questions: 1. Derive the Karnaugh-map for the 7447 decoder (without the LT, BI/RBO, nor RBI inputs) and give the expressions using NAND gates. Obtain the internal equivalent circuit for the 7447. Compare your figure with that in the data sheets. 2. Enumerate the unique features of each BCD to 7-segment display decoder/driver. 3. Show how you would connect a 3-digit display such that any leading zeroes will be suppressed. 4. What is intensity modulation? What is the advantage of this method over using the potentiometer to adjust the brightness of the LED? 5. Is it possible to use seven individual resistors for better series limiting? How would you compute for the resistance?
References: Mano, Morris, Digital Design. Prentice-Hall, Inc. 1984 Taub, Herbert and Schilling, Donald. Digital Integrated Electronics. Mc Graw-Hill Book Company. 1977 Signetics Logic-TTL, Data Manual. Signetics Corporation. 1978
SWIDLAB Experiment Manual Experiment No. 7: BCD to 7-Segment Decoder/Drivers Page 6 of 6
