7-Segment Display _ Digital Integrated Circuits

7-segment displa PARTS AND MATERIALS 4511 BCD-to-7seg latch/decoder/driver (Radio Shack catalog # 900-4437) Common-cathode 7-segment LED display (Radio Shack catalog # 276-075) Eight-position DIP switch (Radio Shack catalog # 275-1301) Four 10 k resistors Seven 470 resistors One 6 volt battery Ca

ion! The 4511 IC is CMOS, and therefore sensitive to static electricity!

CROSS-REFERENCES Le

on In Elec

ic Ci c i , Volume 4, chapter 9: "Combinational Logic Functions"

LEARNING OBJECTIVES How Gain How How

to use the 4511 7-segment decoder/display driver IC familiarity with the BCD code to use 7-segment LED assemblies to create decimal digit displays to identify and use both "active-low" and "active-high" logic inputs

SCHEMATIC DIAGRAM

ILL

IN

A ION

C ION

This experiment is more of an introduction to the 4511 decoder/display driver IC than it is a lesson in how to "build up" a digital function from lower-level components. Since 7segment displays are ver common components of digital devices, it is good to be familiar with the "driving" circuits behind them, and the 4511 is a good example of a typical driver IC. Its operating principle is to input a four-bit BCD (Binary-Coded Decimal) value, and energize the proper output lines to form the corresponding decimal digit on the 7segment LED display. The BCD inputs are designated A, B, C, and D in order from leastsignificant to most-significant. Outputs are labeled a, b, c, d, e, f, and g, each letter corresponding to a standardized segment designation for 7-segment displays. Of course, since each LED segment requires its own dropping resistor, we must use seven 470 resistors placed in series between the 4511's output terminals and the corresponding terminals of the display unit. Most 7-segment displays also provide for a decimal point (sometimes two!), a separate LED and terminal designated for its operation. All LEDs inside the display unit are made common to each other on one side, either cathode or anode. The 4511 display driver IC requires a common-cathode 7-segment display unit, and so that is what is used here. After building the circuit and applying power, operate the four switches in a binary counting sequence (0000 to 1111), noting the 7-segment display. A 0000 input should result in a decimal "0" display, a 0001 input should result in a decimal "1" display, and so on through 1001 (decimal "9"). What happens for the binary numbers 1010 (10) through 1111 (15)? Read the datasheet on the 4511 IC and see what the manufacturer specifies for operation above an input value of 9. In the BCD code, there is no real meaning for 1010, 1011, 1100, 1101, 1110, or 1111. These are binary values beyond the range of a single decimal digit, and so have no function in a BCD system. The 4511 IC is built to recognize this, and output (or not output!) accordingly. Three inputs on the 4511 chip have been permanently connected to either Vdd or ground: the "Lamp Test," "Blanking Input," and "Latch Enable." To learn what these inputs do, remove the short jumpers connecting them to either power supply rail (one at a time!), and replace the short jumper with a longer one that can reach the other power supply rail. For example, remove the short jumper connecting the "Latch Enable" input

(pin #5) o g o nd, and eplace i i h a long j mpe i e ha can each all he a o he Vdd po e ppl ail. E pe imen i h making hi inp "high" and "lo ," ob e ing he e l on he 7- egmen di pla i che . Af e o ' e lea ned ha ppl ail enabling no mal ope a ion, (ei he "Lamp Te " o "Blanking Inp

a o al e he BCD code i h he fo inp he inp ' f nc ion i , connec i o he po e and p oceed o e pe imen i h he ne inp ").

Once again, he man fac e ' da a hee ill be info ma i e a o he p po e of each of he e h ee inp . No e ha he "Lamp Te " (LT) and "Blanking Inp " (BI) inp label a e i en i h boolean complemen a ion ba o e he abb e ia ion . Ba mbol de igna e he e inp a acti e-lo , meaning ha o m make each one "lo " in o de o in oke i pa ic la f nc ion. Making an ac i e-lo inp "high" place ha pa ic la inp in o a "pa i e" a e he e i f nc ion ill no be in oked. Con e el , he "La ch Enable" (LE) inp ha no complemen a ion ba i en o e i abb e ia ion, and co e pondingl i i ho n connec ed o g o nd ("lo ") in he chema ic o a o no in oke ha f nc ion. The "La ch Enable" inp i an acti e-high inp , hich mean i m be made "high" (connec ed o Vdd) in o de o in oke i f nc ion.