OR gate

Shihab Uddin Ahmed | January 20, 2017

OR gate is a digital circuit which performs OR operation on inputs. In OR operation section I discussed about the Boolean logic, OR operation. If any input is true the output is true. So OR gate is a digital circuit where output goes to high state (1) if any of the inputs are high. The only situation OR gate output goes low when all of the inputs are low. OR gate may have two or more inputs. But its output is never more than one.

Three input OR gate symbol

Three input OR gate symbol

Two input OR gate truth table

Truth table for two input OR operation

Figure below shows an OR gate circuit designed only with diodes and resistors:


Let’s discuss about all four possible input states of the OR gate circuit:

  • If A and B both inputs are connected to ground, both diodes will be reverse biased and will not conduct current. Because of the resistor output will be zero, which is in line with OR gate truth table.
  • If A is connected to the ground the diode will be reverse biased like before. But at the same time if B is in high state diode will be forward biased and conduct current. Because of the conducting current through this diode the output will go high.
  • If A is in high state and B is low, situation will be the same as previous combination.
  • If A and B both are in high state, both diodes will be forward biased and because of conducting current output will be high.

Figure below shows an OR gate circuit using resistors and transistors:

OR gate with transistors photo

Working principle of the transistor OR gate is almost like previous one. From the circuit we see any biased transistor is enough to bring output at high state. Output will go low when both inputs are low.

These OR gate circuit with diodes, transistors & resistors are mainly for demonstration purposes. In actual circuits we use integrated circuits (IC) where several OR gates are packed in a single OR gate IC.


Suppose you are working with a boiler. So your concern is whether pressure and temperature is within accepted limit. You can install a pressure transducer and a temperature transducer which triggers by giving high voltage (5 volt). These can be used as inputs of an OR gate. So if pressure or temperature goes beyond accepted limit your OR gate will give high output and that can trigger an alarm connected to OR gate output.

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