Keeping advice VERY simple for absolute beginners

[SirClickALot]

Right from the start here I understand perfectly that what I am about to ask about / propose is not standard practice; it is the thoughts of a digital dabbler who is working at passing on some real basics to some REALLY junior students.

As such, what I am trying to do is simplify as much as I can while keeping it possible for them transfer their simulation directly to breadboard to see it working!

I would ask anyone replying to go with the flow of my thinking even though oyu'll say to yourself "I would never do it like that for real".

Here we go...

Clearly, in the real world, we can't leave inputs hanging around and have to tie them up.

Clearly (as far as I can see) in Digital™ I can tie UP inputs using:

1. A Components > Wires > Pull-Up resistor
2. A Components > Wires > Constant value
3. A Components > Wires > Supply voltage

All three seem to work perfectly to me.

In the opinion of you real-world community electronicists, which of the above can I get away with on a breadboard — can I get away with omitting the PULL UP RESISTOR and connecting the otherwise floating input to Vcc or is that a real NO NO.

JUST FOR NOW, and just to keep Circuit No. 1 REALLY SIMPLE - it'll all be corrected in Circuit No. 2 I promise!

Thank you

[mengstr]

Very NO-NO in general. Virtually the only case you can leave inputs floating is if you're using real 74LS ICs - they float quite stable to VCC and would work just fine on any home/lab/school design. In a real product you still would want to pull vcc or gnd just to be save from really heavy EMI.

EDIT See next reply.

[mengstr]

Hm... Maybe I didn't read your question thoroughly enough. You actually ask about pulling to VCC - basically whether to connect directly or via a resistor.

Still for lab/home/echool usage it's fine to pull directly to VCC. It's to protect the IC possibly internally acting as a SCR/Thyristor at a power glitch which then will lock up and possible short gnd and vcc. I've tried a few times to have that happen, but never succeed. But it might be good anyways for a real design.

Also. It's bascially only applicable for CMOS-style ICs. LS and older N-MOS (like old CPUs and such) is not affected in general.

And also only when connected to VCC. When directly connected to GND a lockup doesn't happen.

[SirClickALot]

Brilliant, thank you @mengstr!

So, just to clarify again 100%:

1. If I'm using 74LSxx then I'm good to forego the resistor for PULL UP / PULL DOWN?
   (In other words TTL resilient)
2. I still clearly need a current-limiting resistor for LEDs connected to (74LSxx) TTL chip outputs?

I appreciate No. 2 there is a new question but given you grasp of what I'm getting at, I thought I'd ask it right here!

[mengstr]

Yes, for 74LS you don't "really" need resistors for pulls, you can safely connect directly to vcc/gnd. Also since 74LS have an internal pull up as a part of the input stage you can leave them unconnected and they will stay pretty much stable at a high level.

For the LED question people will tell me "euoeuah!- you are a cowboy amateur engineer", but in practice it's fine to connect a LED to a 74LS without a current limiting resistor in many cases. But there are some caveats:

1. Only with the LED to GND. The Totem Pole output stage of a LS has a resistor (usually 200-500 ohms) to VCC internally which limits the current to a max of 20 ma. Considerng BJT transistor saturation and other effects you can't even reach that.

2. Don't do it on "drivers" like 74LS240 and others. They can push/pull a lot more current than normal gates. Sometimes upwards to 50-60 mA specced, and higher when operation outside of proper voltage levels.

3. Don't connect a LED and something else at the same time to an output. The LED will load down the voltage so the other IC might not recognise it.

But maybe use one anyways. A resistor a day keeps the magic smoke away. ^__^

For junior students you might want to cut one leg of the LED short and solder in a resistor in place on the leg for them to use. Then they are safe even connected across the power rails.

[fredcwbr]

These are wise guidelines.
+1

[SirClickALot]

Thank you @mengstr - those were exactly wise words I was after!

[hneemann]

See here: #36 (comment)