SEVEN - SEGMENT LEDS AND DECODER DRIVERS

Student Information

Part 1. Objectives

• To verify experimentally the characteristics of a seven - segment LED array.
• To properly connect a BCD - to - seven segment decoder driver to a seven - segment LED array.

Part 2. Discussion

The light - emitting diode is an important optical device commonly used as status indicator and display to numerous circuits. It operates on the principle of radiant emission from a semiconductor pn junction. In the process of recombination of the electrons with the majority - carrier holes (forward - biased pn junction), photons are emitted. The wavelength (and color) of the emitted light depends on the material used. The table below shows the thypical LEDs available in the market with the typical values current and voltage ratings.

GaAsP = gallium arsenide phosphide

Gap = gallium

GaAs = gallium arsenide

If, = forward current, Vf, = forward - biased voltage

The light intensity from an LED is a function of the forward current through it. It is important to drive an LED from an approximate current source of from a voltage source in series with a resistance.

Seven - segment Display

A seven - segment LED display consist of seven LEDs designed in such a way to display all 10 decimal digits and 9 distinct letters. Two types of seven - segment indicators are available: common cathode and common anode.

Seven - Segment Decoder Driver

The seven - segment decoder driver which can be used to drive a seven - segment LED. There are two types of decoder driver corresponding to the two different seven - segment LED configurations shown in Figure.

It is necessary to add the seven current limiting resistors externally. But there are decoder driver such as the 7448IC that has built - in resistors, thus is no need to add them externally.

Seven-segment decoder driver connection

Part 3. Equipment and parts required:

• 1 common cathode seven - segment LED display
• 1 Fairchild FND500, or equivalent
• 1 7448 decoder driver
• 1 variable power supply, 0 to 10 dc
• 1 resistor, 680 ½ W

Part 4. Procedure

1. Evaluate the data sheet for the FND500 seven - segment LED display. Record the typical value of forward voltage for , == 5mA.
Vf:
2. For Vo = +5V dc, a series resistant of 680 is required to supply an illumination current of 5m AIR = (5-1.7) (5Ma)]. Use a supply voltage of Vo = +5 V do and a standard resistance of 680 ohms to illuminate the decimal point (D.P) on the FND500.
3. Double the illumination current by increasing the supply voltage to +10 V dc. Have the illuminate current by reducing the supply voltage to +2.5 V dc. In each case, observe the light intensity. Although observed light intensity is a variable quantity subject to individual interpretation, try to establish a lower current limit which provides acceptance light intensity. Lowest acceptable
If:
4. Examined the 7448 seven - segment decoder driver data sheet. Note the power supply voltage required. Vcc and the maximum power supply current,Icc Vcc: Icc:
5. The logic diagram for the 7448 driving a FND 500 is equivalent to that in the figure. The seven current - limiting resistors are an integral part of the 7448. Use the data sheets to supply the proper piin connection in the fig. Now connect the two ICs accordingly.
6. The lamp test (LT) and BI/RBO inputs serve as an operational test for unit. Ground LT and apply +5 V dc to BI/RBO and note that all segment illuminate. If thet do not, either the units are incorrectly wired or faulty. Determine which is the case before you proceed.
7. Grounding BI/RBO also serves as a mode and will extinguish all segments regardless of the other inputs. Verify this operation.
8. Now, connect LT and BI/RBO to = Vcc and apply 0000 to the 4 BCD inputs on the 7448 (0=0 v dc, 1 =+5 v dc). The numerical 8 should be displayed.
9. Verify the other nine numericals by applying the proper BCD
10. Use the 7448 truth table to apply the 6 nonnumeric BCD inputs, and verify the resulting displays.
11. Connect a 680 - ½ - W resistor from V cc to segment A of the FND 500, and note the increase in brightness. Seven such resistors can be used to increase the brightness of the display if desired.

Part 5. Questions

1. Wanted a ritance would be required to supply 10ma illuminationg current to one segment of an FND500 from a 5v dc power supply?

2. 1. Wanted a ritance would be required to supply 10ma illuminationg current to one segment of an FND500 from a 5v dc power supply?

3. Are there any distinct letters which can be displayed using the 7448 decoder driver?

4. What form is displayed with a BCD input 1010 to the 7448?

5. Demonstrate why the capital letter B, D, G, I, and S are not distinct.
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