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design a 4-digit-number display system

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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

thank for the help..


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.


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



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


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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