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# digital electronics help !!!

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#### thegreatleo2003

##### New Member
hi ppl im having trouble figuring out the digital electronics prob

design a combination circuit that gives output 0 when any of the inputs 1,2,3,4,5 are high and gives output 8 when any of inputs 6,7,8,9 are high and show result display on 7 seg

like
when 1 is set high output is 0
when 2 is set high output still remains 0

so my guess is that it uses a decimal to bcd encoder i have done the part when
6 or 7 or 8 or 9 is set high output is 8
and its expression is like

A3=6+7+8+9
A0-A2=remains zero as i only need two outputs i.e 0 and 8
(1 to 9 are the decimal lines)

im building the encoder using OR gates but how do i implement the "outputs zero when inputs are high for 1 2 3 4 5" ???

#### xpi0t0s

##### Member
The 6 segments for 0 are always on, and the - segment (that converts 0 to 8) comes on when 6-9 are high.
So you just need a single 4-input OR gate that links 6-9 directly to the - segment, and the circuit effectively ignores inputs 1-5.

However, you haven't said what should happen when (a) all inputs are low and (b) when some of 1-5 and some of 6-9 are high.

#### yctek

##### New Member
You could use a truth table, with all your inputs and the possible output. The inputs are 4 bits and your output is also 4 bits because the maximum output is 8 (1000). The table is as follows:

---------------------------------
inputs | output |
---------------------------------
Q3 Q2 Q1 Q0 | I1 I2 I3 I4 |
0 0 0 1 | 0 0 0 0 |
0 0 1 0 | 0 0 0 0 |
0 0 1 1 | 0 0 0 0 |
0 1 0 0 | 0 0 0 0 |
0 1 0 1 | 0 0 0 0 |
0 1 1 0 | 1 0 0 0 |
0 1 1 1 | 1 0 0 0 |
1 0 0 0 | 1 0 0 0 |
1 0 0 0 | 1 0 0 0 |
---------------------------------
And then use karnaugh map to get the circuit of each output. You will have 4 outputs which you can use to drive the 7 segment decoder.

#### thegreatleo2003

##### New Member
thx guys !

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