Quinn000

Joined Feb 22, 2013
5
So i have looked for countless hours and found nothing that was of great help.

I decided one day that i wanted to build a computer motherboard. I figured that as the years go by, i can keep adding and upgrading circuits.

The first proof-of-concept circuit i tried was a transistor switch. Worked.
Then i built a NAND Gate... Didn't work. Of the A and B inputs, when i connect B, the light turns off no matter what A is doing.

I tried trudging ahead and built a half adder... nothing. Both outputs S and C stay high no matter what.

The attached image shows the Logic chart i was following to build the circuit that i drew up...

I have no idea how to calculate the resistors needed or anything, so i guessed on the 6v(6.5v with new batteries) circuit with a 10k ohm resistor.

Any help would be greatly appreciated, and go easy i'm new

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SPQR

Joined Nov 4, 2011
379
I did something similar recently and learned how to calculate base current on the 2n3904 - your 10k resistors seem a bit high.

HERE is a page on how to calculate the base current.

Quinn000

Joined Feb 22, 2013
5
Thanks, but does my issue sound like a base resistor problem, shouldnt the circuit still partially work... when i connect input 2 to +6V, the light turns on... which means somehow A is getting switched on as well doesnt it?

Quinn000

Joined Feb 22, 2013
5
i meant to say the light turns off when i connect input 2(B), it should only do that when input A and B are both on

SPQR

Joined Nov 4, 2011
379
The LED current limiting resistor was changed to 330ohms, and the two base resistors I changed to 1k each.

Here's the truth table.

___0___1
0__1___1
1__1___0

We have a working NAND gate!

Edit: But I grabbed parts off my table and didn't do formal calculations - If I were to really "build" this thing, I'd calculate the right resistor for the LED, and spend some time doing the base current calculations. I learned a lot on my previous transistor project.

Ron H

Joined Apr 14, 2005
7,014
Your base resistors are fine. Something is wrong with your circuit. It appears that the top transistor is shorted, either internally, or on the circuit board.

Quinn000

Joined Feb 22, 2013
5
So i tried building the circuit again with the 330 ohm resistor for the LED, and the 1k for the bases, and i get this:
0 0 1
0 1 1
1 1 1(a little Dimmer)

i don't think my circuit is shorted anywhere... the first collector is +6V, the emitter is connected to the second collector and the second emitter is grounded...

I'm using brand new parts, and if i suspect a transistor i replace it instantly.

Besides the resistors, what am i missing?

SPQR

Joined Nov 4, 2011
379
It's a pretty straight-forward circuit.
I even looked-up your LED and it's rated at 2V 20mA - and the calculations for 6.2 volts would require a 310 ohm resistor.

2. Check the pinout on the transistors
3. switch the transistors
5. Change the LED
6. I'm flummoxed

Ron H

Joined Apr 14, 2005
7,014
So i tried building the circuit again with the 330 ohm resistor for the LED, and the 1k for the bases, and i get this:
0 0 1
0 1 1
1 1 1(a little Dimmer)

i don't think my circuit is shorted anywhere... the first collector is +6V, the emitter is connected to the second collector and the second emitter is grounded...

I'm using brand new parts, and if i suspect a transistor i replace it instantly.

Besides the resistors, what am i missing?
"I don't think" is not adequate troubleshooting.
It's obvious that current is getting through or around the top transistor, even when there is no base current.
You have check for shorts. Turn off the power and measure the resistance from collector to emitter of the top transistor. Also measure the resistance from collector to base of the top transistor (red lead on the collector in both cases).

Quinn000

Joined Feb 22, 2013
5
in both cases, there is no continuity between the collector and base, or collector and emitter... (i hope continuity is the right word)

i checked the entire circuit, moved it to another breadboard, replaced every single component, and i still get the exact same problem... i'm sorry if i'm sounding repetitive, but i have not a clue on what is wrong

Ron H

Joined Apr 14, 2005
7,014
Well, all I can tell you is that the circuit will work if you have it wired correctly, and your parts are all good.
If you are new at electronics, you may have wired it incorrectly. Can you post a picture of your breadboard?
I would test it a little differently. I would wire the base resistors directly to +v, with the switches going from each base to ground.

SPQR

Joined Nov 4, 2011
379
I've put the circuit together as I stated above with the resistor changes.
It works.
Here are the voltages I measure in different states:

R = supply voltage above the LED resistor
RLED = voltage at the R LED junction
T1b = upper transistor base
T1e = upper transistor emitter
T2b = lower transistor base
T2e = lower transistor emitter (ground)

____Nothing pressed___Upper pressed____Lower pressed___Both pressed
R______6.3______________6.3_____________6.3____________6.3
RLED___2.07_____________2.12____________2.07___________.11
T1b____0________________2.87___________.86____________.86
T1e____0________________2.12___________.01____________.06
T2b____0________________0_____________.78_____________.8
T2e____0________________0______________0______________0