How do I design a 2 input or 3 input NAND gates using a 4 input NAND gate (like 7420)?

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Suman Pandit

Joined Nov 23, 2015
1
I want to design a selectable input NAND gate using IC 7420 which should work as a 2- or 3-input NAND gate even when 2 or 3 inputs are applied
 

djsfantasi

Joined Apr 11, 2010
5,577
But as I read his original post, he doesn't know what the unused inputs are. Sometime there is one unused input. Other times there may be two unused inputs. He wants this to be "selectable"?

I can't imagine why. Maybe it could be done with several additional gates. But I still can't imagine why and the TS has not explained why neither.
 

crutschow

Joined Mar 14, 2008
23,299
If you want to use 2, 3, or 4 inputs of a 4-input NAND gate, just tie 2 of the inputs high with a 10k resistor from each input to +5V.
Thus any unused inputs will be high, while the used inputs can be readily pulled low, as needed.
 

djsfantasi

Joined Apr 11, 2010
5,577
I want to design a selectable input NAND gate using IC 7420 which should work as a 2- or 3-input NAND gate even when 2 or 3 inputs are applied
See how the TS specified a selectable NAND gate, which takes either two or three inputs? Tying some gate inputs high, does not make it selectable. I realize he may have meant use a quad input NAND gate as either 2 or 3 input NAND gate, but we didn't ask him (nor has he returned).

Acknowledging I may have perceived this differently, this is what I see [UPDATE: crutschow, I did miss the 10k pull-up resistor. Oops)
SelectableNAND.PNG
The SEL input changes between a 2 or 3 input gate. If it's low, the NAND gate functions as a 3 input NAND gate; if it's high, the NAND gate functions as a 2 input NAND gate.
 

ericgibbs

Joined Jan 29, 2010
8,718
hi,
@djsfantasi
'dj' may have a point ref selectable NAND gate, hopefully the OP will post a more precise specification so that we can give the correct guidance.

E
 
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