|Digital Teaching Aid (DED Philippinen, 86 p.)|
|Counter - Lesson 8|
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.