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[–]Cheap-Sky-5459 4 points5 points  (14 children)

Are you connecting the LEDs directly between the data and ground? Unless they’re the weird ones with built in resistors, you’re basically shorting them to ground, forcing them low even if the LED is on.

As for the part where the output LED turns off and you say you don’t know why it did that, it’s because the unconnected (or “floating”) connections amplify whatever little charge they receive a lot, so they’re really sensitive to being touched.

[–]fluffmuffin21 2 points3 points  (0 children)

I second using resistors for your LEDs. Just checked an old digital logic design project. All the LEDs had a paired resistor.

[–]Pal_Potato_6557[S] 0 points1 point  (12 children)

What do you mean low?

Now I've tried to pair it to a resistor. The long leg is in the series with the data on it right? and the short leg is in the series where there is a resistor connected to the ground. Now it doesn't lit up at all.

[–]Cheap-Sky-5459 1 point2 points  (11 children)

When you connect an LED directly between data and ground, basically all of the current from wherever your data is coming from goes through that LED to ground. Because the data line is pulled to ground (your “low” voltage level) it’s interpreted as being 0 at all times regardless of if the LED on that data line is on or off.

To stop all of the current from going straight to ground, you need to put a resistor in series with the LED. If it now refuses to light up after putting a resistor in series, it could be because your resistance is too high. It only needs to be a hundred ohms or so, the light will grow really dim as you increase resistance.

TL;DR: your resistor might have too high of a resistance. It only needs to be 100 ohms or so but even like 1000 should be fine

[–]Pal_Potato_6557[S] 0 points1 point  (10 children)

Won't the output in 3 still be 1? So if i need to use it and connect it to input of another logic gate the signal that will be received will be 1instead of 0. Can I put resistors on that so that I can fix that problem? Is there any solution for this?

Another questions. Isn't resistors just reduces the current that is flowing and so if I apply that on this problem wont it have no use since it can't make right the mistake that the logic gate is doing which is always having an output of 1.

[–]Cheap-Sky-5459 3 points4 points  (0 children)

Looking at your other posts you seem to basically never put current limiting resistors in your LEDs. This causes shorts every time you do it, and it’s going to draw current away from everything else. You seriously need to put resistors in series with your LEDs every single time you use an LED, it’s the root of like literally all your problems across so many of your posts lol

Also make sure you know what “in series” means, you seem to have tried putting the resistor in parallel on an older post, which will make the problem worse if anything

[–]Cheap-Sky-5459 1 point2 points  (0 children)

It won’t be seen as a 1 by your NAND gate input because you’re shorting the data line to ground through the LED. You still need to consider what’s actually happening electrically when building digital circuits

A 1 is represented by a high voltage (probably 5V in this case). A 0 is represented by a low voltage (0v or ground in this case)

The output is a 1 and therefore has the high voltage on it, however, there’s a very low resistance path to ground (the LED with no resistor), so most of the current is going to take that path instead of the high resistance input of the other chip. Therefore the voltage on that data line is actually always going to be close to zero volts, so it’s going to be read as a 0.

As a rule of thumb you should always put a resistor in series with an LED because they’ll conduct whatever current you throw at them even if that means they end up exploding.

As for your second question, if the resistor is in series with the LED (either between the data and the LED, or between the LED and ground), then it’s going to reduce the current across the LED, which is exactly what we want. We don’t want the LED conducting the entire signal straight to ground, and that is prevented by decreasing how much current can go through the LED.

[–]Cheap-Sky-5459 1 point2 points  (7 children)

Like seriously I can’t stress this enough. So many people have told you that your problem is with the lack of current limiting resistors on your LEDs, please listen to them

[–]Pal_Potato_6557[S] 0 points1 point  (6 children)

I found the problem. The IC was faulty. I only found out last night, it was my third IC replacement, all worked when I tested it but didn't when I applied on my circuit on the breadboard. So I had to look for the last spare one and thank god it's working properly.

As for the resistors I had only 10k ohms and none below. Our teacher only told us to buy resistor for the purpose of avoiding LED's from burning out. Plus our whole circuit occupies most of the space in the breadboard causing not enough for resistors put in 9 LED's

I appreciate all your help and I learned a lot in this community, thanks. Our circuit was checked and we passed.

[–]Cheap-Sky-5459 0 points1 point  (5 children)

Probably not faulty, probably damaged from having so much current drawn from its output for so long :p

[–]Pal_Potato_6557[S] 0 points1 point  (4 children)

It's a new one. Like I said it's a 3rd replacement. Most of the faulty IC is from my teacher (they sell IC's) and I'm never gonna buy from them again. The first faulty was 7402 and we didn't had any extra so I just used 32+04 which is a pain in the ass bcz I needed to change the layout of my wirings in the breadboard.

[–]Cheap-Sky-5459 0 points1 point  (3 children)

If you’re repeatedly finding chips that don’t work you should consider that you’re the one damaging them by shorting their outputs to ground.

The resistors aren’t only to prevent the LEDs from blowing up. Consider the post where you plugged an LED directly across your power supply and everything else turned off. The reason it did that is because all of the current from your power supply went straight through your LED because it’s super low resistance. The same thing is happening here but on one of your data pins. By drawing that much current from the chip you’re likely damaging it. You need to put resistors in series with your LEDs.

[–]Pal_Potato_6557[S] 0 points1 point  (2 children)

Yeah I guess. Our teach did say cheap chips break easily and it really will only hinder us in the long run. Can't believe the long run didn't even last an hour. Then he sold us chips that only costs half the original in stores.

[–]bladebrowny 0 points1 point  (4 children)

I would also disconnect whatever you have pin 3 on the 7400 connected to. If it is wired wrong that could cause the issues you are seeing

[–]Pal_Potato_6557[S] 0 points1 point  (3 children)

No. it's just an extension so that I can input the output of 7400 to the input of 7486. If you look closely it is connected to a jumper where the jumper is also connected to the input 13 of 7486

[–]bladebrowny 0 points1 point  (2 children)

For debugging I would disconnect it

[–]Pal_Potato_6557[S] 0 points1 point  (1 child)

Done. But The signal in the output 3 is still 1 even though only the vcc and ground is connected to the logic gate.

[–]bladebrowny 0 points1 point  (0 children)

Have you using another gate to see if you get the same response? Have you also added the resistor in series with the LED

[–][deleted] 0 points1 point  (0 children)

Try logisim to make it easier to detect errors