# help with a 4 bit full adder using four 3×8 decoders

Discussion in 'The Projects Forum' started by RadwaU, Sep 4, 2011.

Sep 4, 2011
4
0
Hi everyone,,

two months ago we were assigned with this beautiful project ^^
anyway we spent plenty of time and money on it but it didn't work,,
we have tried almost everything,,

the project was a 4 bit full adder using four 3×8 decoders and displaying the output on 2 seven segments display,,

the input is in binary (we used switches for it),, so the max number that should appear on the 7-seg.s' is 30

we made the truth table of course,,
here it is
Here

now the problem was to convert the five binary output (S1-S4, C1) from the gates into BCD so that we can use it with the two BCD to 7 segment decoders,,

we bought this chip for it,,
TDA4092
it's suppose to convert the 5 binary outputs (S1, S2, S3, S4, C1) from the decoders into BCD,,

TDA4092 Datasheet,,
http://www.alldatasheet.com/datasheet-pdf/pdf/25060/STMICROELECTRONICS/TDA4092.html

but still it didn't work,,

we tried something else,,

that is to convert binary to BCD using gates,, we did it with the truth table of course,,

here is the Truth Table

the plan was as follows:
we want the output (M0 - M7) from the input (S1-S4 , C1)
so we got each (M) using (AND & OR gates),,
for example,,

M2 is active high form 20 to 30,,
so M2 = (S3*C1) + (S1*S3*C1) +(S2*S3*C1) +.... etc

here is the schematic,,

Here

it's too large,, and is very complicated,, 5 people including the our Teacher Assistant have checked this over 20 times,, so it is 99.9% correct,,

so?? where the hell is the problem

Apr 5, 2008
15,805
2,389
Hello,

The post was made invisible by the forum software.
Perhaps there are some issues with the site you use to show us the pictures.
We prefer to have the pictures uploaded to our server, so they will be preserved better, as external sites may "disappear" at once.

Sorry for the inconvenience.

Bertus

3. ### vrainom Member

Sep 8, 2011
109
19
Hey look again at that TDA4092 datasheet, at the very beggining it says: "5 bit binary to 7 segment decoder driver" so its output is not BCD but direct 7 segments display data

Sep 4, 2011
4
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ok here are the pictures,,

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