# Logic Gate, negative voltage

I am new to electronics... Can a logical gate treat negative voltage as a logical 0 and the ground voltage as logical 1 ? Or should the voltages allways be between 0 and +12V ?

• First you should understand the meaning of the word "voltage". It is a difference of potentials, not an absolute value. Now. Can you answer your question yourself? – Eugene Sh. Jun 21 '16 at 14:45
• Yes. If you connect VDD to GND and VSS to -12V. But the output will be in the same range. – Tom Carpenter Jun 21 '16 at 14:53
• Some logic standards use "ground" as the logic high state and a negative voltage as the logic low state. Consider starting with en.wikipedia.org/wiki/Logic_level#Logic_voltage_levels – user2943160 Jun 21 '16 at 15:11
• I meant voltages with respect to the ground voltage (0) ... I do understand the difference between voltage and potential. – user3633438 Jun 21 '16 at 15:27
• Similarly, I have seen digital logic used to gate high-side drivers where VDD went to the high-voltage and VSS was derived to be VDD - 12V. The inputs were level-shifted using opto-isolators. – Tut Jun 21 '16 at 15:29

## 2 Answers

1) 0 and 1 are just to make things easy for humans.

You could also call a certain voltage range False and another one True.

You could even call them Bert and Ernie if you like but that's confusing for humans, also 0 and 1 is more compact to write down.

In the same way, if you wanted a circuit where + 5 V is 0 (zero) and 0 V is 1 (One) then be my guest. It's just a naming convention. The transistors don't care either.

2) A circuit with a +12 V and a 0 V (ground) has +12 V just because it was chosen to be like that. You could also connect that + 12 V node to ground instead but now, since it's ground, we don't refer to it as +12 V anymore because that is confusing. Now we call it ground and then the node that was previously ground is now called - 12 V

In this circuit the traditional 1 (one) would be 0 Volt and the traditional 0 (zero) would be -12 V.

In the early days of logic chips some used a common positive so all voltages in the circuit were negative ! A bit confusing now since almost no one does that anymore but it is possible nonetheless. Ground is just a reference the designer chooses.

• If I understand this correct, I can arbitrarily set 0 to -5V and 1 to 0V... So if i take a AND gate and connect -5V to one terminal and 0V to the other , I should get -5V ---> 0 at the output ? – user3633438 Jun 21 '16 at 15:34
• The inputs and outputs of logic gates must always be between the power supply rails, whatever voltage you wish to call them. – Peter Bennett Jun 21 '16 at 15:44

How about if you negate your already negated output?

If you put in 0V, you get on output of first negator (called N1) logical 1, and if you negate that, you get out logical 0 on N2.

If you put in 12V, you get on output of N1 logical 0 and if you negate that, you get logical 1 on N2.

By that, I don't see really much of an advantage.

• The question is about an input which is not really negated but rather simply shifted by -12V, as I understood. – Dmitry Grigoryev Jun 21 '16 at 15:21