# Using a 74S138 Demultiplexer and a 74SL10 Nand Gate To implement boolean fx

Discussion in 'Homework Help' started by hitmen, Feb 24, 2009.

1. ### hitmen Thread Starter Active Member

Sep 21, 2008
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I am supposed to use them to implement the function F(a,b,c) = ab' + a'c'.
I am aware of how to use a NAND gate. However, I have only used a demultiplexer to convert binary into decimal numbers.

Can some give me a hint on what I am supposed to be doing? 2. ### promage New Member

Aug 3, 2008
3
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The way I did it is to expand the function into:

F = c'b'a'+cb'a+cb'a'+cb'a

F = 000+010+100+101

After that I connect the 3 input, A (LSB), B, C(MSB), into the 74LS138.

So when F = 1, one of the four output Y0,Y2,Y4 or Y5 will be 0.

From this you can get:

F = Y0'+Y2'+Y4'+Y5'

So using the output of the 74LS138, design a logic circuit using the NAND gates.

3. ### randolph New Member

Feb 25, 2009
1
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The way I do it is to expand the function into:

F = c'b'a'+c'b'a+c'ba'+cb'a

which is also the same as:

F = 000+001+010+101

So just put a, b and c into the select input of the 74LS138, where a is the LSB and c is the MSB. Then use the NAND gates to implement this function:

F = Y0'+Y1'+Y2'+Y5'

I did not check my work though, so there may be mistakes.

4. ### tuborggg Active Member

Jan 3, 2009
37
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Well, you can implement this function (as any function) with the NAND only.
You can use the demultiplexer just as a selector between '1' and '0' inputs, and the nand gates output at the control...
You can of course implement it only with the demultiplexer - when a, b and c are at his control...(write a truth table)