Digital circuit and logic gate

Shihab Uddin Ahmed | March 28, 2016

Digital Circuit

Those electronic circuits which work on digital data or signal are called digital circuits. Inputs and outputs of these circuits have only two possible states; that is high or low (voltage or current). These high and low states represent 1’s and 0’s which are binary digits, from where the name digital came from. In simple words digital circuits use binary number system.

Logic Gate

Logic gate is a type of digital circuit which can have one or more inputs and only one output. Output depends on logic level of inputs and the logic condition used. Logic gate is basically a switching circuit and it is made with diodes, transistors and resistors.

This simple circuit below with switches and a lamp demonstrates basic of logic gate:


Possible states:

  • S1 and S2 open: lamp is off
  • S1 open and S2 closed: lamp is off
  • S1 closed and S2 open: lamp is off
  • S1 closed and S2 closed: lamp is on



We can assume open state of switch as 0 and closed state of switch as 1. Its truth table is like truth table of AND gate.

These circuits are called logic circuits because they take logical decisions which depend in logic level of inputs. In actual logic circuits there are no mechanical switches, instead of those, components like transistors, resistors and diodes are used.

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