# I dont understand the physics behind NOT logic gates

#### Sethyboy

Joined Dec 23, 2017
1
Any time i try to learn about logic gates online i learn what they do, but I can never find anywhere than explains how they do it. For example, it’s easy enough to understand that a Not gate switches a 1 signal to 0 or a 0 signal to 1, but how this is actually done baffles me because how can you physically input a zero, or no current into something and have a one or a current come out the other side? is the current entering the gate through some other line, then only passes through if there is no input?. I would like to get a birds eye view of the physical structure that makes various logic gates work, if anyone can help, or if someone knows where i could find that information.

Thanks.

#### spinnaker

Joined Oct 29, 2009
7,837
And there is never "no input". The input will always be high or low.

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#### BR-549

Joined Sep 22, 2013
4,938
The physics is the same as a see-saw. Output is inverted input. It can be done many ways electronically.

#### WBahn

Joined Mar 31, 2012
24,973
Any time i try to learn about logic gates online i learn what they do, but I can never find anywhere than explains how they do it. For example, it’s easy enough to understand that a Not gate switches a 1 signal to 0 or a 0 signal to 1, but how this is actually done baffles me because how can you physically input a zero, or no current into something and have a one or a current come out the other side? is the current entering the gate through some other line, then only passes through if there is no input?. I would like to get a birds eye view of the physical structure that makes various logic gates work, if anyone can help, or if someone knows where i could find that information.

Thanks.
There are many kinds of NOT gates -- what are called logic families. Relay logic, diode logic, resistor-transistor logic, transistor-transistor logic, complementary metal-oxide-semiconductor logic. There's even non-electrical forms of logic such as hydraulic logic and many others.

Perhaps the easiest to understand is relay logic, which actually lends itself to understanding CMOS logic fairly well.

Do you understand how a basic electromechanical relay works?

#### ebp

Joined Feb 8, 2018
2,332
Ah, the missing power pins problem!
 The power supply pins are almost never shown on the schematic symbols for logic devices. Sometimes they don't appear anywhere on a schematic. When I draw schematics, I sometimes use symbols where the power pins are shown on one section of a multi-section package (e.g. one gate of a "quad" package), but usually only when I need to clearly distinguish how the pins are connected, and that is usually in mixed analog and digital circuitry.

All logic gates require power supplies. In days gone by, 5 volts was very common for TTL and several CMOS families. Newer families will operate with lower voltages, some less than two volts. The old 4000 series CMOS family would operate at any voltage between (if I recall correctly) 3 V and 18 V. Typically there were six inverters in a 14 pin package - 2 pins for each inverter (in & out), a positive supply pin and a "ground" pin.

A NOT gate, or inverter, in its simplest form, uses one NPN transistor and one resistor. The input drives the base of the transistor. The emitter goes to the ground pin. The collector connects through a resistor to positive supply pin. The output pin connects to the collector. When the input is logic LOW, the transistor is off and the resistor "pulls" the output up to the supply voltage for logic HIGH. When the input is HIGH, the transistor is ON and it connects the output to ground for logic LOW. Actual devices are more complex and most modern devices use CMOS, not bipolar transistors, but the overall effect is the same.

I've actually seen CMOS circuits work without the supply pin connected. Under the right circumstances the whole package can be powered through a single logic HIGH input because of the diodes that exist to protect the input from static discharge. But you'd never design a circuit to work that way, unless it was something oddball or you were being sneaky and devious.

You should be able to find diagrams of equivalent circuits for logic devices quite easily on the web. Get some data sheets for some actual parts
some common old & more recent inverters:
7404, 74LS04, 74HC04, 74ACT04 - and a bunch of others ending in 04, 4009 (CD4009), 4049 (CD4049)
I don't know if you can find anything on the web, but the long-obsolete RTL (resistor-transistor logic) family was fairly easy to understand if you know even a tiny bit about bipolar transistors. I don't remember any part numbers.
Ap notes from TI, National Semi, Fairchild (the original one), RCA & Motorola covered this stuff some decades ago. You can probably find some of them on the web.

#### Parth786

Joined Jun 19, 2017
642
Any time i try to learn about logic gates online i learn what they do, but I can never find anywhere than explains how they do it.
I think you are trying to find out many answers within a one question. first define one problem and then try to get solution of that problem.

you are talking about switch and Not gate.

You can divide this in two way
• Manufacturing of device
• use of device
first decide what you want to understand manufacturing of device (switch or not gate) how they make or the use of device (switch or not gate) how we use device in electronics system. if you want to understand the use of device then get one led, switch, NOT gate IC battery and assemble all component and find out what happen when switch open and closed

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