CMOS or TTL for "messing" with logic gates

[rstofer]

One last question:

If, say, I'm using a chip with four NAND gates but I only actually need one, do I have to tie the inputs of the unused gates to ground/Vcc?

It is highly recommended, particularly with certain families of CMOS where the input will float midway and lead to heating of the output stage.
In my view, the answer is "YES", terminate the inputs.

So, pull-up or pull-down.  I almost always use a resistor and pull up.  If I do it this way, instead of connecting the pin to solid ground, I can probably use the gate later by just removing the resistor (or not) but I won't have to cut a trace on a PCB.  Also, check the pin input current for Vcc and Gnd.  I think you will find that pulling up requires less current and potentially generates less heat.

See I_IH and I_IL on page 5 here:  https://www.ti.com/lit/ds/symlink/sn74ls04.pdf

Note that I_IL is much larger than I_IH

[Wimberleytech]

One last question:

If, say, I'm using a chip with four NAND gates but I only actually need one, do I have to tie the inputs of the unused gates to ground/Vcc?

If you are fooling around with a breadboard as your design develops or just learning...then "no."  If you are building PCB to take it to the next level...I would say "yes."

[james_s]

It's not strictly necessary but it's good practice to do so, floating inputs can lead to unpredictable behavior.

[Ian.M]

Breadboards are bad enough for digital signal integrity without encouraging ground bounce by allowing CMOS gates with floating inputs to hammer your Vdd and Gnd nets as capacitively coupled glitches on their inputs take them in and out of their linear regions.

Spend a minute or two adding jumpers to tie unused gates' inputs to the nearest rail, or waste hours chasing glitches and malfunctions when your circuit complexity grows till you can no longer beat the odds.

[PerranOak]

Cheers all.

[TomS_]

If you just want to play with logic, you can look at some simulation software. Titles I have used are logicly and logisim.

Logicly also has a web based version (at logic.ly) so you can try it out before you might decide to buy a copy. You can also build up your own modules to include into future circuits which I found was quite valuable because you can build up familiar 74 series logic chips to make circuits that are more like what you can build on the real world.

Logisim seems more limited in that respect, but has a lot of other things that logicly doesn't, like ROM/RAM simulators.

I've used both for proofing some of my own designs before committing to a bunch of soldering or ordering PCBs.

Personally I've found it easier to whip something up in software like this and make changes with a few mouse clicks than having to deal with the tediousness of a breadboard, and once I have something that works the way I need I look at designing it into my project.

[westfw]

You might learn more by buying 100 quad 2-input nand gates and building everything with them (you can.)


But even in the old days a lot of stuff would be built with bigger chips (flip flops and counters in particular.)


I 2nd the “cmos cookbook” recommendation.

[TomS_]

I've built a project entirely out of NOR logic. NOR and NAND are universal gates, and the right combination can implement any other logic function.

The Apollo Guidance Computer was also built entirely out of NOR logic and was the inspiration for my project.

[tkamiya]

If you are using CMOS, yes.  Just make sure it's input.  An advantage of using TTL is that you don't have to worry about any of that. 

Amazon has IC assortment kits.  Use sockets, because even with TTL, it isn't that difficult to kill it.