C-MOS OR gate


One of the nice things about the Quad 2-lnput NAND IC is the way the 3 NAND gates can be combined to make up other logic circuits. It's possible to make up an OR gate from NANDs - as this project proves.

Our last project (C-MOS buffer) has given some clues about how we can use NANDs to make up other logic circuits. Take a look at the schematic for this project - can you trace what happens from each input to the eventual output?
(Sure you can - just give it a try!)

After you build the project, turn power on, and press S1. What happens to LED?

You saw that this circuit indeed behaves like other OR gates you've played with. Have you traced what happens from input to output yet? The answer is in the next column - but no fair peeking.

Let's say you press S1. This inputs 1 to both inputs of the NAND, causing the NAND's output to go to 0. This 0 output is one of the inputs to the NAND gate controlling the LED. Since a NAND's output is 0 only if all inputs are 1, the 0 input causes the NAND's output to go to 1... and LED 1 lights!

We can make up AND, NOR and XOR gates using the Quad 2-lnput IC. Can you figure out how we would connect the NANDs to do this? Take your best guess and make notes ... because we're soon going to find out.







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