# Transistor logical AND circuit

#### Ian0

Joined Aug 7, 2020
3,747
I saw this on google. I don't understand where do inputs A and B take electricity from
The inputs come from the previous logic stage.
but. . .
It's nonsense - the transistors are not saturated, they are emitter followers.

#### dl324

Joined Mar 30, 2015
13,490
I saw this on google. I don't understand where do inputs A and B take electricity from
It's assumed that they're driven by other logic gates or tied HIGH or LOW to supplies.

It would be better for you to start referring to current and voltage instead of electricity. With bipolar junction transistors (BJTs), it's current that turns them on. With MOSFETs, it's voltage.

Not having an LED in the output of the gates would also be more typical:

When the output is high, the LED is on. When the output is LOW, the LED is off.

#### Electronic0Noob

Joined Oct 24, 2021
19
Looks like I burned the leds, the potentiometers, the transistors and the power source exploded. At least I have a bunch more leds, potentiometers and transistors. I need to buy another DC power source.

#### dl324

Joined Mar 30, 2015
13,490
Looks like I burned the leds, the potentiometers, the transistors and the power source exploded. At least I have a bunch more leds, potentiometers and transistors. I need to buy another DC power source.
Only your inverter circuit should have damaged anything. Are you certain you're connecting things correctly?

Why and where are you using potentiometers?

A nit. We spell LED with all caps because it's an acronym for Light Emitting Diode. We also spell the acronym when we talk about them, because they're LEDs, not leds.

#### Electronic0Noob

Joined Oct 24, 2021
19
Why and where are you using potentiometers?
I bought potentiometers and not resistors because I didn't want to bother with Ohm's law to calculate the resistence needed to drop an amount of voltage. With potentiometers I can adjust the resistence. I use them to drop from 12V to 1.3V(for the LED) and to drop from 12V to 1.0V(for transistors bases)

#### Papabravo

Joined Feb 24, 2006
17,233
I bought potentiometers and not resistors because I didn't want to bother with Ohm's law to calculate the resistence needed to drop an amount of voltage. With potentiometers I can adjust the resistence. I use them to drop from 12V to 1.3V(for the LED) and to drop from 12V to 1.0V(for transistors bases)
That's fine as long as this is a hobby. Just don't imagine this method as a path to a career.

#### Electronic0Noob

Joined Oct 24, 2021
19
That's fine as long as this is a hobby
I am a software engineer. Someday while programming I thought: "I can program this metal thing, but how does it actually work at physical level?" and I am trying just to understand better how is the electricity from the power outlet of my house used by this ingenious machine we use everyday. So it is a hobby and I don't really need this information in my career, however I thought it might be interesting to see how computers were invented.

#### Electronic0Noob

Joined Oct 24, 2021
19
We spell LED with all caps because it's an acronym for Light Emitting Diode. We also spell the acronym when we talk about them, because they're LEDs, not leds.
And what does AMOLED, OLED, QLED mean?

#### AnalogKid

Joined Aug 1, 2013
9,474
All of the circuits posted by everyone so far have the same problem - there is no resistor from the base to the emitter of any transistor, to guarantee that the transistor turns off when the base goes open-circuit. This explains the error condition in post #1.

To the circuit in post #6 (et al), add a resistor from each transistor's base to its emitter. You don't say what the base resistor's value is, but the added pull-down resistors should be about 10x to 50x that value.

ak

#### Papabravo

Joined Feb 24, 2006
17,233
I am a software engineer. Someday while programming I thought: "I can program this metal thing, but how does it actually work at physical level?" and I am trying just to understand better how is the electricity from the power outlet of my house used by this ingenious machine we use everyday. So it is a hobby and I don't really need this information in my career, however I thought it might be interesting to see how computers were invented.
So that would be like me saying maybe I'll bother to learn the syntax and semantics of the language I am using to do my embedded development project. Would you care to opine on my chances of success in that enterprise?

#### Electronic0Noob

Joined Oct 24, 2021
19
add a resistor from each transistor's base to its emitter
How can I add a resistor from the transistor base to emitter? Does that mean the resistors of the 2 inputs A and B from the following image? Why I need 2 resistors and not just one and split the output of that resistor in 2?

So that would be like me saying maybe I'll bother to learn the syntax and semantics of the language I am using to do my embedded development project. Would you care to opine on my chances of success in that enterprise?
Sorry. I didn't understand...

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#### Papabravo

Joined Feb 24, 2006
17,233
How can I add a resistor from the transistor base to emitter? Does that mean the resistors of the 2 inputs A and B from the following image? Why I need 2 resistors and not just one and split the output of that resistor in 2?
View attachment 251016

Sorry. I didn't understand...
You said:

I bought potentiometers and not resistors because I didn't want to bother with Ohm's law to calculate the resistence needed to drop an amount of voltage. With potentiometers I can adjust the resistence.

I made an equivalent statement about intellectual laziness from a perspective (software development) I thought you might be familiar with. Apparently I was mistaken.

#### Electronic0Noob

Joined Oct 24, 2021
19
The problem was that you quoted the part where I told why it is just a hobby. If you quoted the part where I said that I don't want to bother using Ohms law I would have understood the laziness you are talking about.

#### dl324

Joined Mar 30, 2015
13,490
I am a software engineer.
Don't software engineers have at least one basic EE course required? EE's are required to take a programming class.

I bought potentiometers and not resistors because I didn't want to bother with Ohm's law to calculate the resistence needed to drop an amount of voltage.
The math isn't very difficult.

#### Electronic0Noob

Joined Oct 24, 2021
19
Don't software engineers have at least one basic EE course required? EE's are required to take a programming class.
Maybe in US. I went to the University of Automations in my country and a took Java Master of 2 years, no basic EE.

The math isn't very difficult.
It is kind of hard to find the electronic components I need and if I miss the ohm value of the resistor I need I have to wait a week to get another.

#### Papabravo

Joined Feb 24, 2006
17,233
Don't software engineers have at least one basic EE course required? EE's are required to take a programming class.
...
I think you can get away without taking a basic EE course. I wouldn't swear to it though.

#### dl324

Joined Mar 30, 2015
13,490
It is kind of hard to find the electronic components I need and if I miss the ohm value of the resistor I need I have to wait a week to get another.
You could buy a starter kit.

In a pinch, you can use series and parallel combinations to make values you don't have.

#### Electronic0Noob

Joined Oct 24, 2021
19
You could buy a starter kit.
To deliver something from US to Europe it takes months. I remember I bought something from ebay and it took 2 months and I had to pay vamal taxes