Constructing a NAND Gate as Inverter: Troubleshooting Needed

[dwn]

Homework Statement

Constructing a NAND gate as an inverter.

Homework Equations

The Attempt at a Solution

I've attached the image at my attempt to construct the circuit. The output is always on, which it should be except when both inputs are zero. Could someone explain what I am doing wrong?
Thank you.

 

[Korisnik]

Is U1A's upper wire connected properly?

 

[dwn]

I'm afraid I don't understand what you mean. Shouldn't both ports be connected to the switch? In doing that, the gate will output whatever is opposite the input, or does it require an input from the voltage source? I'm really confused about all of this.

 

[gneill]

I don't see a pullup resistor at the switch. How will you get a logic 1 for your inputs?

 

[dwn]

Based off what you and Korisnik are saying, I need to run a lead from U1 to the voltage source in order to have a HIGH input value.

Tried that to see if it would work...no go.

 

[Korisnik]

Based off what you and Korisnik are saying, I need to run a lead from U1 to the voltage source in order to have a HIGH input value.

Tried that to see if it would work...no go.

No, I just asked if it was properly connected, as I can't see a junction there. I don't know about the pull up thingy, I've never worked with anything but logic (I don't know how "input", as in Vcc can be connected like this and pull-up to still work). Maybe if this is TTL logic you can't put Vcc on the output.. you need it on the input, also connect to ground and put some pull up resistors (~10k Ohms) before the gates (after switches) so it doesn't float. http://s22.postimg.org/ynlu13ech/DLr5_Bna.png

 

[dwn]

Thank Korisnik. I implemented the changes you suggested in your image, however, the circuit still doesn't function correctly. I found a couple example circuits online, to use NAND as an inverted logic gate, but I must not be making the connections correctly.

 

[Korisnik]

Thank Korisnik. I implemented the changes you suggested in your image, however, the circuit still doesn't function correctly. I found a couple example circuits online, to use NAND as an inverted logic gate, but I must not be making the connections correctly.

Check all junctions, for example where U1A's upper wire connects to the wire coming from the switch. "Bigger" red dots should be junctions. Does your circuit work properly with any configuration of logic gates?

 

[CWatters]

Can I suggest you take a look at U4A. For example if you understand what U4A is doing you might see a way to simplify your circuit considerably.

PS: Inverters only have one input so why does your circuit have two switches?

 

[NascentOxygen]

You could use a switch like this at the input, to provide the logic HIGH or logic LOW for your inverter.

You won't need a complicated arrangement of gates if you use this to set logic 1 or 0 as input.

The indicator at the output isn't right. All you need from the gate's output is a resistor + LED to ground. The light will glow if the gate's output is HIGH, and it won't glow if output is LOW.

Start over afresh. You've got way off-course by using the wrong arrangement at the input.

 

[CWatters]

If you don't have that type of switch use a pull-up resistor on the input and a single pole switch to pull it down.