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Wednesday, 29 October 2014

Circuit to Control a Seven Segment Display, 7490 + 4511 + Press Button Switch (Part 1)

by realfinetime  |  in Seven Segment Display at  20:43

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          So far we have seen a lot of circuits to control seven segment displays. We have also designed a circuit to control a common cathode seven segment display using 4511 driver IC. There, we need four press button switches to control four inputs (D0, D1, D2, D3) of 4511. Here, we are introducing a new technique to reduce the number of press button switches. By putting an IC named 7490 in front of 4511, we can reduce the number of press button switches from 4 to 1. Circuit is done as shown in the following diagram. A common cathode seven segment display is using for our purpose.

Pin out diagram of 7490

          7490 is clearly explained in it's datasheet. It is better to learn the datasheet before using 7490. It is a monolithic counter and contains four master-slave flip-flops and additional gating to provide a divide-by-two counter for which the count cycle length is divide-by-five. Pin out diagram of 7490 is given below.
          From the datasheet, it is clear that, counting will take place, if any of the following conditions is satisfied (Refer Reset / Count Function table in Page 3 of datasheet).

R0(2) and R9(2) are LOW.
R0(1) and R9(1) are LOW.
R0(1) and R9(2) are LOW.
R0(2) and R9(1) are LOW.

          In our circuit, we are grounding R0(2) (pin 3) and R9(1) (pin 6) of 7490 (condition 4). We can design two types of counters using 7490. They are BCD counter and Bi-quinary counter (Refer Function Tables of 7490 given in page 3 of datasheet). Here, we are designing the circuit for BCD counter. 7490 will work in BCD Count mode if, output Qa (Pin 12) of 7490 is connected to the input B (pin 1) of 7490. Clock input is given to the input A (pin 14) of 7490. In our circuit, clock input is given using a press button switch. If all these conditions are met, we will get a BCD counter at the output pins (Qa, Qb, Qc and Qd). Counter will count from 0-9 in each negative transition of clock input.

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All the circuits, published in this blog is only after testing and getting proper results in my private lab. When you try these circuits, you should check the supply voltage, polarity of components, presence of childrens nearby and shorts in the circuits. This website will not be responsible for any harm happened to you or your components caused by your carelessness.

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