# Transistor logical AND circuit

#### Electronic0Noob

Joined Oct 24, 2021
19
I tried to create an logical and operation using npn transistors. I tried connecting: plus of the 12 V DC power source to first transistor collector, then the first transistor emitter to second transistor collector, then the second transistor emitter to potentiometer, from potentiometer(I used an voltmeter to set the resistence to enough ohms to output 1.3V) to 1.3 V led and from the led to the minus of the power. For the switch part I created a parallel circuit from the power source:
plus from the 12 DC power source to potentiometer(outputting 1.0V), potentiometer to switch 1 and switch 2 and switch1 to base 1 and switch2 to base2. The led is on when the second switch is closed, and the first switch can be open or closed and the led is still on.

#### ericgibbs

Joined Jan 29, 2010
16,383
hi E0N,
Welcome to AAC.
Do you have a circuit sketch you could post.?

E

#### Papabravo

Joined Feb 24, 2006
19,295
I tried to create an logical and operation using npn transistors. I tried connecting: plus of the 12 V DC power source to first transistor collector, then the first transistor emitter to second transistor collector, then the second transistor emitter to potentiometer, from potentiometer(I used an voltmeter to set the resistence to enough ohms to output 1.3V) to 1.3 V led and from the led to the minus of the power. For the switch part I created a parallel circuit from the power source:
plus from the 12 DC power source to potentiometer(outputting 1.0V), potentiometer to switch 1 and switch 2 and switch1 to base 1 and switch2 to base2. The led is on when the second switch is closed, and the first switch can be open or closed and the led is still on.
Even a well written paragraph is incapable of conveying the information in a schematic diagram. A schematic diagram will be most helpful in describing and understanding your problem.

#### Electronic0Noob

Joined Oct 24, 2021
19
Is there a free tool where I can sketch a simple diagram?

#### MrChips

Joined Oct 2, 2009
27,129

#### Electronic0Noob

Joined Oct 24, 2021
19
I sketched the worst diagram ever in an online free drawing tool. It is bad, however you get the idea:

#### Electronic0Noob

Joined Oct 24, 2021
19
This is the schemeit:

#### MrSalts

Joined Apr 2, 2020
2,372
Since you are working with NPN transistors, you should put your LED and it's current-limiting resistor above the two transistors (between 12VDC and top transistor collector m

#### MrSalts

Joined Apr 2, 2020
2,372

here is a better option...

#### Electronic0Noob

Joined Oct 24, 2021
19
That doesn't mean that just 1.3 Volts will pass trough transistors? I think I rode in datasheets that the transistors I have have a minimum value for collector emitter flow. I think I'm missing something essential about how the electricity works.

#### BobTPH

Joined Jun 5, 2013
5,770
Voltage does not pass through things, current does. A voltage is measured between two points. The voltage across a transistor, from the collector to the emitter is at maximum when the transistor is off and at a minimum when it is on. Typically, the voltage when fully on is about 0.2 to 0.3 V.

Bob.

#### Electronic0Noob

Joined Oct 24, 2021
19
This should work?:

#### Electronic0Noob

Joined Oct 24, 2021
19
Can this work for NOT?:

And this for a OR?:

#### dl324

Joined Mar 30, 2015
15,114
Can this work for NOT?:
Not more than once, and probably not for long. If the battery/supply can provide sufficient current, the transistor will be destroyed the first time it's turned on.
And this for a OR?:
If the output is taken from the collectors, as would be typically be done, that would be a NOR. It's not a good idea to put BE junctions in parallel. Each transistor should have it's own base resistor.

Why are you drawing extra connection dots? I take it that you're inexperienced, but no one has pointed out that you're drawing some things side ways. It conforms to the left to right flow, but we usually draw the LEDs vertically.

#### Ian0

Joined Aug 7, 2020
6,310
View attachment 251004
here is a better option...
Unfortunately, it is no longer and AND gate - it's now a NAND gate. The output voltage is high unless both transistors are ON.

#### Electronic0Noob

Joined Oct 24, 2021
19
I take it that you're inexperienced
I am not inexperienced. I am a literary noob.

#### dl324

Joined Mar 30, 2015
15,114
It makes more sense to stack transistors for an AND gate and to have them in parallel for an OR.

NOR:

NAND:

#### Ian0

Joined Aug 7, 2020
6,310