# design a 4-digit-number display system

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#### ashleychong1988

##### New Member
can anybody help me to figure out the circuit?
a multiplexing system to display 4-digital number on the 7-segment LED display. The data input consists of 4BCD digits.
component provided:
-three 74LS157
-one 74LS47 BCD-to-7-segmant decoder
-one 74LS139 decoder
-LED common-cathode 7-segment displays
-resistor
-inverter

thank for the help..

#### duffy

##### Well-Known Member
The outputs of the 74LS47 go to the segments on all 4 displays through resistors. The cathodes of the displays go to the 74LS139. The 74LS157's are used to select which 4-bit BCD drives the 74LS47. The inverter is probably used to decode the 2-bit address signal (which you did not mention) for the 74LS157's.

#### brianelite87

##### New Member
have read your post on the above title..i cant understand the last part of the solution.(The inverter is probably used to decode the 2-bit address signal for the 74LS157's) what is 2-bit address signal? and why we should insert a inverter for 74LS157. and the inserted resistor is it prevent the malfunction of the led?

thank u

1-->>http://img123.imageshack.us/img123/5231/43883077.jpg

2-->>http://img123.imageshack.us/img123/5039/96268344.jpg

so this is the question..
and why we have put a inverter to the decoder?
your help is much appreciated ..

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#### duffy

##### Well-Known Member
Yes, the resistor is to prevent malfunction of the led - it must have the current limited. Without it, you have voltage sources (+5V) and voltage drops (for instance, 1.7V of a red led segment) but nothing to limit the current from the difference (3.3V).

I cannot answer the other question because not enough information has been provided. The 2-bit address signal is what drives the 74LS139, it is what also selects the digits. If you don't have something like that, you are going to need an oscillator and a counter.

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#### hjtorz

##### New Member
I am dealing with this question as well , refering to the figure . how should the circuit be construct ?

There are 3 multiplexer does that mean more 74LS139 is needed ?

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#### duffy

##### Well-Known Member
You will have TWO "data select" inputs - call them G1 and G2. The 74LS157's are connected like a tree - two 74LS157's go into one 74LS157 which goes to the 74LS47 like this -
Code:
#1┐
├#3
#2┘
The input to the first two 74LS157's are your 4-bit BCD numbers A0-3, B0-3, C0-3, and D0-3 inputs.

G1 connects to G1 on #1, and #2. G2 connects to G1 on #3, to EN on #1, and through the inverter to EN on #2. EN on #3 is ground, same as schematic.

#### hjtorz

##### New Member
Is the G1 and G2 data select input is connect to 74LS139 before connect to #1,#2,#3 ?

#### ashleychong1988

##### New Member
thanks for all of u..
finally i solved it..
there is no inverter on it i think..
and it is about the same as what duffy described..
two data select input G1 and G2..
G1 connect to #1 and #2..same times connect to A1 of 74LS139 decoder..
G2 connect to #3 and B1 of 74LS139..
and EN of both MUX is connected to ground..

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