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So far, we have seen a lot about LED matrices and seven segment displays in previous blogs. We had controlled common cathode seven segment displays using 4511 driver ICs. One of the major drawback of 4511 is the absence of a control pin to control the dot point (dp) of seven segment displays. One 4511 can control only one common cathode seven segment display. For controlling eight common cathode seven segment displays, eight 4511 ICs are needed which definitely increase the power consumption. Our Next aim is to control maximum number of seven segment displays using minimum number of Integrated Circuits. Pin out diagram of a single common cathode seven segment display is given below.
A common cathode seven segment display has ten control pins, Eight of them are anode pins (a, b, c, d, e, f, g and dp) and two of them are cathode pins. These two cathode pins are shorted inside. So no need to use both. Any one is needed in the circuit. Then, each common cathode seven segment display will have nine control pins (eight anode pins and one cathode pin). That is, if you want to control eight seven segment display, 8*9 control pins will be there.
If we are using 4511 ICs, eight 4511 ICs will be needed to control eight common cathode seven segment displays. But, we cannot control the dot points of seven segment displays using 4511 ICs. To control 8*9 control pins of eight common cathode seven segment displays, a wide controlling system is needed. Our next aim is to reduce the number of control pins from 8*9 to minimum. Circuit for reducing the number of control pins to minimum is given below. If the circuit is done as shown below, 8*9 control pins get reduced to 16 control pins.
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If we are using 4511 ICs, eight 4511 ICs will be needed to control eight common cathode seven segment displays. But, we cannot control the dot points of seven segment displays using 4511 ICs. To control 8*9 control pins of eight common cathode seven segment displays, a wide controlling system is needed. Our next aim is to reduce the number of control pins from 8*9 to minimum. Circuit for reducing the number of control pins to minimum is given below. If the circuit is done as shown below, 8*9 control pins get reduced to 16 control pins.
Here "a" pin of all the common cathode seven segment displays is interconnected to form a common "a" control pin. Similarly, "b" pin of all the common cathode seven segment displays is interconnected to form a common "b" control pin. Same procedure is repeated for c, d, e, f, g and dp control pins. Cathode pins of eight common cathode seven segment displays form another eight control pins. Totally, there will be sixteen control pins (eight anode pins and eight cathode pins). Circuit drawn above can be redrawn in the form of an 8*8 LED matrix as given in next page.
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