# Redraw Equivalent Circuit for 3-input OR gate

1. Aug 29, 2011

### Deathfish

How do you redraw a three-input OR gate circuit using only 2-input NAND gates?

2. Aug 29, 2011

### MisterX

Using De Morgan's laws, you should be able to figure out how to make a 2-input OR gate using NAND gates. You can make a NOT gate by wiring the NAND inputs together.

3. Aug 29, 2011

### LCKurtz

DeMorgan's law says
$$\overline{A+B}= \overline A\cdot \overline B$$

or

$$A+B = \overline{\overline A\cdot \overline B}= NAND(\overline A,\overline B)$$

4. Aug 29, 2011

### Staff: Mentor

You know the rules, Deathfish. Show us your work!

5. Aug 29, 2011

### Deathfish

i've worked on this, but i seem to be going back and forth combining all combinations ie. 3C2 with no luck... difficult to explain this here. I know the equivalent of 2-input OR gates... but i am required to draw up 3-input OR gate when i am given just 2-input NAND gates. The question is to output HIGH when at least 2 out of 3 inputs are LOW.

6. Aug 29, 2011

### Staff: Mentor

A 3-input OR gate does not "output high when at least 2 out of 3 inputs are low"...

7. Aug 29, 2011

### Deathfish

its part of the question... A AND B, B AND C, A AND C then combine them together..

8. Aug 29, 2011

### LCKurtz

Part of what question? Your original post says nothing about AND gates.

9. Aug 29, 2011

### Deathfish

figured the first part out just need to combine that using three-input OR gate..

10. Aug 29, 2011

### LCKurtz

Just guessing, are you trying to implement a majority circuit where two or more of A,B, and C are true? If so, it probably will take as many NAND gates to implement the 3 input OR gate by itself as it would to do the whole problem with NANDs in the first place.

11. Aug 29, 2011

### Staff: Mentor

Thanks for being so clear in your OP. In the future, use the Homework Help Template, which asks for the exact problem statement.

I'm out.