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The Seven-Segment LED Driver

Introduction

In the last experiment (The Seven-Segment LED), we discovered that the 7-segment LED is capable of displaying different digits, according to which of the segments are turned on or off. However, it would be clumsy and inconvenient to have to build the necessary logic gating structure to turn individual segments on and off to display chosen digits. It's not all that complicated, involving a set of seven combinational logic circuits (one for each LED segment), all having the same four input lines (a BCD number). To set up such a circuit for a single digit would not be all that bad. But how about a 6- or 8-digit display? The required logic structure expands rapidly.

The demand for decimal digit displays is nevertheless quite large. Therefore, it becomes worthwhile to incorporate the required gating functions in a single IC. This has been accomplished in both TTL and CMOS technology, as well as in specialized MSI and LSI devices.

For the purpose of this experiment, we have selected the CMOS type 4511 IC. It is specifically designed to be able to drive common-cathode LED display devices, and has other useful features that will be covered during this experiment.



Schematic Diagram

Block diagram of the 4511 7-segment decoder IC

The 4511 is more than just a 7-segment decoder/driver IC. It incorporates an input latch so that it can hold and display a steady digit as part of a multi-digit count, while a new count is being accumulated in the background. In some applications, such as a digital clock, this is unnecessary. In others, however, such as a voltmeter or frequency counter, the latch allows the display to be updated only at the end of each counting cycle, instead of displaying the ongoing count. The LE input shown to the right is the Latch Enable. A new number can only be accepted by this IC when the LE input is a logic 1.

The main circuitry of this IC is the Decoder section. This section consists of combinational logic circuits to accept a four-bit BCD (Binary Coded Decimal) input and generate seven output signals to control the individual segments of a 7-segment display device.

There are many possible variations in decoder circuits. Some are simplified so that non-decimal inputs (10 through 15) show up as incomplete digits or odd patterns. This is common in TTL devices of this type (7446, 7447, and 7448). Others are designed to display hexadecimal digits, which include the letters A through F along with digits 0 through 9. Specialized codes are also used in some situations. The 4511 device which we are using here is designed to blank the display for illegal input codes. Thus, only valid BCD inputs will generate an output on the display.

The decoder section also has two additional inputs. Lamp Test (LT') turns all segments on so you can verify at once that all display segments are working, or identify display units that need to be replaced. This input is normally left at logic 1. The Blanking (BL') input is just the reverse; it forces the entire display off. This is used in many cases to blank out leading or trailing zeros from a long display. LT' will override BL' so you can test even blanked-out display digits.


Schematic diagram of the 4511 driver outputs

The driver section of the 4511 deserves special note. This is one of the few CMOS ICs that deliberately incorporates bipolar transistors in order to permit substantial output current. As shown in the schematic diagram to the right, an NPN transistor is wired as an emitter follower. This permits each segment output to provide as much as 25 mA source current to the display. That is more than enough to directly drive the segments of a common-cathode LED display unit. That, in fact, is one of the reasons for choosing this particular IC for use in this experiment.

The same IC can also directly drive some other types of seven-segment displays, such as fluorescent displays or incandescent types. Common-anode LED displays require an external set of driver transistors for the segments, as the MOS transistor in the NPN emitter circuit cannot pass enough current to satisfactorily drive an LED segment.

Liquid Crystal displays (LCD) require a few extra factors for proper operation, and are best driven from ICs designed for that purpose, such as the 4054, 4055, and 4056, or the newer 4543.



Parts List

To construct and test the seven-segment LED decoder/driver circuit on your breadboard, you will need your breadboard system with the 7-segment display and its grounding cathode jumper still in place from the previous experiment, plus the following experimental parts:



Constructing the Circuit

Make sure the right end of your breadboard socket is clear of all jumpers and components. You'll need practically the whole space shown in the assembly diagram for this project. Also, make sure that you have your power and ground connections in place, to your test bed circuitry. Then refer to the image and text below and install the parts as shown.

Note: You may find it necessary or desirable to substitute a different LED display unit in this experiment. If so, you are responsible for identifying the pin connections to the individual segments and adjusting your placement of the 1K resistors accordingly. The assembly instructions include the required segment identifications. You do need to use a common-cathode display unit in this experiment.



Circuit Assembly

Start assembly procedure






















Starting the Assembly

You should still have your TIL322A 7-segment LED display and its cathode grounding jumper in place as shown to the right. If not, install them now. The remainder of the right end of your breadboard socket should be clear of all components. You'll need all of the space shown in the assembly diagram for this experiment.

Click on the `Start' button below to begin. If at any time you wish to start this procedure over again from the beginning, click the `Restart' button that will replace the `Start' button.

0.3" Black Jumper

Locate or prepare a 0.3" black jumper. Install this jumper on your breadboard socket, in the location indicated in the assembly diagram to the right.

Click on the image of the jumper you just installed to continue.

0.5" Red Jumper

Locate or prepare a 0.5" red jumper, and install it as indicated in the assembly diagram.

Again, click on the image of the jumper you just installed to continue.

0.1" Bare Jumper

Locate or prepare a 0.1" bare jumper, and install it in the location indicated to the right.

As before, click on the image of the jumper you just installed to continue.

0.6" Orange Jumper

Locate or prepare a 0.6" orange jumper, and install it as indicated in the assembly diagram.

As usual, click on the image of the jumper you just installed to continue.

0.4" Orange Jumper

Locate or prepare a 0.4" orange jumper, and install it in the location indicated to the right. This jumper connects to segment "c" of your LED display.

Again, click on the image of the jumper you just installed to continue.

4511 7-Segment Decoder/Driver IC

Locate a CMOS type 4511 7-segment decoder/driver IC. This IC is housed in a 16-pin DIP, and will fit nicely across the center channel of your breadboard socket.

Make sure all 16 pins are straight, and place the IC gently in the location shown in the assembly diagram. Gently press the IC downwards, making sure that all pins are being pushed into their respective contact holes. If the IC resists being pressed down, pick it up and recheck the pins. Then try inserting the IC again. Make sure the notch indicating pin 1 is oriented to the left as shown.

Click on the image of the IC you just installed to continue.

10K, ¼-Watt Resistor

Locate a 10K, ¼-watt resistor (brown-black-orange) and form its leads to a spacing of 0.5". Install this resistor in the location indicated in the assembly diagram.

Click on the image of the resistor you just installed to continue.

10K, ¼-Watt Resistor

Locate another 10K, ¼-watt resistor (brown-black-orange) and form its leads to a spacing of 0.5". Install this resistor in the location indicated to the right.

Again, click on the image of the resistor you just installed to continue.

10K, ¼-Watt Resistor

Locate a third 10K, ¼-watt resistor (brown-black-orange) and form its leads to a spacing of 0.5". Install this resistor in the location indicated in the assembly diagram.

As before, click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate a 1K, ¼-watt resistor (brown-black-red). Form its leads to a spacing of 0.4" and clip the ends to a length of ¼". Then install this resistor in the location shown to the right.

Again, click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate another 1K, ¼-watt resistor (brown-black-red). Form its leads to a spacing of 0.4" and clip the ends to a length of ¼" as before. Then install this resistor in the location shown.

As usual, click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate another 1K, ¼-watt resistor (brown-black-red). Form its leads to a spacing of 0.4" and clip the ends to a length of ¼". Then install this resistor in the location shown.

Again, click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate another 1K, ¼-watt resistor (brown-black-red). Form its leads to a spacing of 0.8" and clip the ends to a length of ¼". Then install this resistor in the location shown. This resistor will connect to segment "f" of your LED display.

As before, click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate another 1K, ¼-watt resistor (brown-black-red). Form its leads to a spacing of 0.8" and clip the ends to a length of ½" so this resistor can cross over the top of the 4511 IC and LED display. Then install this resistor in the location shown. This resistor will connect to segment "a" of your LED display.

Once more, click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate a final 1K, ¼-watt resistor (brown-black-red). Form its leads to a spacing of 0.8" and clip the ends to a length of ½" so this resistor will be held above the red jumper you installed earlier. Then install this resistor in the location shown. This resistor will connect to segment "b" of your LED display.

Again, click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate a final 1K, ¼-watt resistor (brown-black-red). Form its leads to a spacing of 1.0" and clip the ends to a length of ½" so this resistor can cross over the top of a resistor you installed earlier. Then install this resistor in the location shown. This resistor will connect to segment "g" of your LED display.

Once more, click on the image of the resistor you just installed to continue.

0.5" Yellow Jumper

Locate or prepare a 0.5" yellow jumper, and install it as indicated in the assembly diagram. This jumper establishes the connection to segment "d" of your 7-segment LED display.

Click on the image of the jumper you just installed to continue.

0.8" Yellow Jumper

Cut a 1-9/16" length of yellow hookup wire and remove ¼" of insulation from each end. Form the ends of this jumper as shown in the pictorial, with a spacing of 0.8" between the exposed ends. Install this jumper as shown to the right.

Again, click on the image of the jumper you just installed to continue.

1.6" Green Jumper

Cut a 2-9/16" length of green hookup wire and remove ¼" of insulation from each end. Form this jumper as shown in the pictorial, so that this jumper will bridge the 1.6" distance between the indicated 1K resistor and segment "e" of the 7-segment display, while holding itself above the orange jumper you installed earlier.

Install this jumper as indicated, then click on its image to continue.

Orange Jumper

Locate or prepare an orange jumper of a suitable length to reach the logic switches on your test bed. Connect one end of this jumper to the location indicated in the assembly diagram. Connect the other end to S0.

Once more, click on the image of the jumper you just installed to continue.

Orange Jumper

Locate or prepare another orange jumper, and connect one end to the location shown to the right. Connect the other end to S1.

Again, click on the image of the jumper you just installed to continue.

Orange Jumper

Locate or prepare a third orange jumper, and connect one end to the location shown in the assembly diagram. Connect the other end to S2.

As before, click on the image of the jumper you just installed to continue.

Orange Jumper

Locate or prepare one more orange jumper, and connect one end to the location shown to the right. Connect the other end to S3.

Once more, click on the image of the jumper you just installed to continue.

5" Black Jumper

Locate or prepare a 5" black jumper, and connect one end to the ground bus as shown to the right. Leave the other end unconnected for now; you will use this jumper in your experimental procedure.

Once more, click on the image of the jumper you just installed to continue.

Assembly Complete

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, proceed with the experiment on the next part of this page.

Restart assembly procedure
Continue assembly procedure


Performing the Experiment

Step 1. Set switches S0 through S3 to logic 0, and turn on power to your experimental circuit. Note the currently displayed digit on the LED display. Record the displayed digit on the top line of the table to the right.

Step 2. Set S0 (input A) to logic 1 and note the effect on the 7-segment display. Record the displayed digit in the second row of the table to the right.

Step 3. Continue by setting the four input switches, S0 through S3, to each of the 16 possible binary combinations. For each input combination, record the corresponding output display on the appropriate row of the table. If the display is completely turned off, enter the word 'blank' in the table.

Step 4. Set S0 through S3 all to logic 1 (display will be blank). Now, briefly touch the free end of your black grounding jumper to pin 3 of the 4511 IC (the top end of the leftmost 10K resistor you installed for this experiment). Note the effect of this on the 7-segment display. Try it for various settings of the logic switches. What would be the purpose of this input?

Step 5. Set S0 through S3 all to logic 0. Now, briefly touch the free end of your black grounding jumper to pin 4 of the 4511 IC (the top end of the middle 10K resistor you installed for this experiment). Note the effect of this on the 7-segment display. Try it for various settings of the logic switches. What might you use this input for?

Step 6. Use your black grounding jumper to hold pin 5 of the 4511 IC (the top end of the rightmost 10K resistor you installed for this experiment) at ground, thus bypassing this 10K resistor. Now change the four logic switches to some other input state. What happens? Briefly remove the ground connection, then re-establish it. Repeat this step for various input combinations. What is the effect of this input?

When you have completed all steps in this experiment, turn off the power to your experimental circuit and compare your results with the discussion below.

Input
3 2 1 0
D C B A
Output
Display
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


Discussion

In Step 1, you applied a binary input of 0000 to the 4511 IC, and it responded by displaying the digit 0 on the 7-segment LED display. Since that input combination also corresponds to a Binary-Coded Decimal (BCD) input of 0, this display is correct and appropriate.

In Steps 2 and 3, you expanded on this, testing the display for each possible input combination. You should have found that as long as the inputs formed a valid BCD number (0000 through 1001, or 0-9), the display correctly reflected that value as a single digit. However, for input combinations 1010 through 1111, which are not valid BCD numbers, the display simply turned off. There are 7-segment decoder-driver ICs that will display these combinations as the letters A through F, thus producing a hexadecimal digit display. However, the 4511 is intended only for decimal applications.

In the rest of this experiment, you determined the purpose of the three control inputs to the 4511. The first, at pin 3, is the Lamp Test input (LT). Properly, it is LT', because it is activated when this input is grounded. When you grounded this input in Step 4, all segment immediately turned on regardless of the states of the data inputs. Thus, the LT' input, as its name suggests, may be used at any time to verify that all segments of the display device are working correctly, and to identify any display unit that needs replacement due to a defective segment.

The input at pin 4, which you demonstrated in Step 5, had just the opposite effect. This is the BLanking input (BL'), and when it is brought to logic 0, it turns off all segments regardless of the input data. The BL' input is commonly used to suppress the display of leading zeros in an integer, or trailing zeros in a decimal fraction. It is also useful in preventing "bleedover" between digits in a multi-digit multiplexed display, by momentarily blanking the entire display when it switches from one digit to the next. That way we can't accidentally display the same number on two different display digits during the switching interval. This application is transparent to the user.

The final input is at pin 5, which you demonstrated in Step 6. This is the Latch Enable (LE) input. While this input is logic 1, any input data is passed immediately on to the display. However, when you grounded this input, the internal latch took effect and held the displayed digit regardless of changes to the BCD inputs.

This latching action may seem unimportant for a single digit, but consider a multi-digit display for which a count must be gradually accumulated. A digital multimeter is one example of this; a frequency counter is another. Of course, we could just watch the count accumulate and then make it pause before starting a new count. In fact, early frequency counters (known as EPUT counters, for Events Per Unit Time) did just that. But a stable display is much easier to read, so the preferred approach is to accumulate the needed count and then transfer it to a set of latch circuits. This count is displayed while the new count is generated. If each displayed digit has its own 7-segment driver IC, the latch can be part of that IC. If the digits are multiplexed to reduce power requirements and component count, the latches must be separate. But one way or another, such latches are now standard in digital displays.

This completes the experiment on the 7-segment decoder/driver IC. Make sure power to your experimental circuit is turned off, but leave all experimental components in place on your breadboard socket. You'll be using them in your next experiment.


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