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Seven segment indicator and decoder

Seven segment indicators

Seven segment indicators are made by seven LED's. LED's are light emitting diodes. (LED function: Free electrons recombine with holes near the junction. As the free electrons “fall” from a higher energy level to lower one, they give up energy in form of heat and light)

Fig. 8-5: seven LED's labeled a through g

Depending on which LED's are light up it is possible to indicate decimal numbers from 0 to 9.

Fig. 8-6: Seven segment indicator, common anode type

Two types of indicators are available:

- common anode
- common cathode

The diodes are controlled by ground (common anode) or by 5V (common cathode).

Fig. 8-7: Seven segment indicator, common cathode type

Seven segment decoder

Seven segment decocer are devices to drive seven segment indicators.

Ex: The 7448

(see Fig. 8-8 on the next page)

Fig. 8-8: Seven segment indicator and decoder

The 7448 is a decoder to drive common cathode indicators.

When the BCD input is set to:

A B C D = 0 1 0 0

The internal logic of the decoder forces the LED's b, c, f, and g to conduct.