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Discrete OR gates may be realized by using diodes or transistors.
The inputs represented as X and Y may be either 0V or +5V correspondingly.
The output is represented by Z .
In the diode of OR gate, when both the inputs are of same value, X=0V and Y= 0V, then both the diodes are in OFF condition.
As a result, no current flows through the resistor and there will not be any voltage drop across the resistor.
Here the output will be Z=0V. Similarly, when both the inputs or either the inputs such as X and Y are equal to +5V, then the corresponding diodes either D1 or D2 or both the diodes are at ON state and act as short circuits.
Here the output will be Z corresponds to +5V. In practical cases the output Z corresponds to +5V-diode drop = +5V – 0.7V = +4.3V, which is regarded as Logic 1 state.
**Many thanks to @snowfats for his feedback**
@snowfats wrote: “A word of warning, this type of ORing might present a problem with lower logic voltage levels and high forward voltage drops. It’s not unheard of for digital logic inputs to require inputs of >=80% of their supply voltage while the voltage drop across the diode will remain pretty constant. Once you reach ~3.3V and lower, the ~0.6V or 0.7V drop can reduce the voltage too low to properly read as an input. You could just stick to higher voltage levels, use lower Vf diodes, and/or use more robust logic inputs (70% of Vcc is pretty common) to avoid this. For very high speed signals you may also run into problems regarding the switching speed of the diode used. Overall it’d be best to stick to schottky diodes for this sort of application.”
Enjoy...😎
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