I was trying to build a NAND gate out of transistors but every time I plug in the battery in and press the push buttons the LED still stays on. No matter what I do the LED stays on. Even when i push both buttons the LED still stays on. I've tried different ways to build it but it always ends up where the LED stays on. Can someone explain how to fix this problem and make a NAND gate out of transistors?

 

Take a close look at your wiring. You have wired the LED through a resistor, directly to the power rails. The transistors and push buttons are doing nothing!

Post the schematic that you are trying to wire on the breadboard, and we can see where you're making a mistake.

 

Here is the simple circuit highlighted.

 

no....
when both transistors are on, they are shorting LED (output off when both inputs are on).
possible issues are:
a) 3V supply is too low (certainly for LED resistor)
b) resistor values too high (10k instead of 1k), LED resistor should be even smaller for 3V supply.
c) or one or both switches installed in wrong orientation (try rotating them 90deg, one at a time)

 

@panic mode

Your explanation doesn't match the TS description. The LEDs output is never off. No matter what the input, the LEDs remain lit. It's because they're wired that way.

 

Neither transistor's emitter is connected to anything. No emitter current, no circuit.

ak

 

image is not very good, schematics would be good to have but it appears that most connections are fine, except for:
a) missing power rail jumpers.
b) swapped C and E of both transistors. (jumpers on board appear to be fine)

missing jumpers means pressing buttons does nothing and transistors cannot short LED.

 

or version with fewer jumpers (jumpers look so ugly when not needed... and make for more things that can go wrong, plus harder troubleshooting)

 

Why?



Do you know how a Proto Board works?

Adding jumpers there is superfluous.

 

Transistor NAND gate.

 

Why?

View attachment 117519

Do you know how a Proto Board works?

Adding jumpers there is superfluous.

I respectfully disagree - that may be true for smaller boards but on larger ones - power rails are usually segmented and in such cases they do need those jumpers. see original post and image below:

 

Take a close look at your wiring. You have wired the LED through a resistor, directly to the power rails. The transistors and push buttons are doing nothing!

Post the schematic that you are trying to wire on the breadboard, and we can see where you're making a mistake.

I tried to make a schematic of the connections of the circuit. its not that great but I tried. I know that the LED will stay on because it is directly connected to the power. But for some reason I am having a hard time finding a way around this. I looked online to see what other people made and it looks the same as mine does. I'm not sure where I went wrong as far as copying what they made. I do know that the LED cant be connected this way. I just want to know how to make it without having to connect the LED directly to the power. Please post how to connect it. Thanks.

 

I respectfully disagree - that may be true for smaller boards but on larger ones - power rails are usually segmented and in such cases they do need those jumpers. see original post and image below:

View attachment 117520

Well obliviously it is me that does not know how they work. You have a pretty good eye seeing that spacing difference. My board appears to be larger than the one posted and my rails are connected all the way through,

 

All breadboards are not created equal. Some have side power rails that are discontinuous, others are continuous.
Check your breadboard with a continuity tester.
There is no point in arguing over this.

 

I'm going to go insane if I don't figure this out. I tried many different ways to connect it and every time it is a failure. I tried to copy people making them online and it works for them but not for me. The only other possibility is that the transistors are bad. I guess I have to go buy more transistors. If someone can just please figure out how to make it where the LED doesn't stay on when I push both buttons. And please include how to build one. If you can I would be ecstatic.

 

Hello,

As said, there are several types of breadboards around.
The ones with a large gap in the middle have an open connection over there:



Bertus

 

I tried to make a schematic of the connections of the circuit. its not that great but I tried. I know that the LED will stay on because it is directly connected to the power. But for some reason I am having a hard time finding a way around this. I looked online to see what other people made and it looks the same as mine does. I'm not sure where I went wrong as far as copying what they made. I do know that the LED cant be connected this way. I just want to know how to make it without having to connect the LED directly to the power. Please post how to connect it. Thanks.
View attachment 117531

It is not awfully difficult to create an easy readable schematic with so few compoents.

This works





It is likely one of your issue is the combined values of R3 and R5 are so high there is not enough current for the LED to light.

 

Hello,

As said, there are several types of breadboards around.
The ones with a large gap in the middle have an open connection over there:

View attachment 117537

Bertus

This is what my breadboard looks like. I don't think it has gaps.

 

Hello,

Are you using the correct pinout of the transistors?



Bertus

 

Hello,

Are you using the correct pinout of the transistors?

View attachment 117543

Bertus

I thought I knew. I thought it was, Collector, Base, Emitter.