The OR Gate:
The OR gate performs logical OR function. The output is high when any one input is high.
Learning Objectives:
Working:
Case 1:
Input A = 0
Input B = 0
Output = 0
Case 2:
Input A = 0
Input B = 1
Output = 1
Case 3:
Input A = 1
Input B = 0
Output = 1
Case 4:
Input A = 1
Input B = 1
Output = 1
In figure 1 the waveforms or timing diagram of OR gate are also given. The output pulse rises from 0 to 1 when any one of the two inputs are high.
Figure 1: OR Gate circuit symbol, truth table, logical expression and waveforms |
Figure 2: OR gate basic working |
Truth Table Of OR Gate:
Logical Expression Of OR Gate:
Y = A + B
Logical OR Gate (Explain with the help of switches) | Switch Model and OR Logic:
Figure 3: OR gate switch model |
Implementation Of OR Gate Using Diode Logic
Figure 4: OR gate diode circuit |
Practically diode circuit models are not used. But for beginners and better understanding of circuit analysis, implementation and designing, it is better to understand this topic.
Case 1:
See schematic in case 1, both inputs are connected to ground. Both diodes (D1 and D2) are off. Output is zero (LED remains off).
Case 2:
See schematic in case 2. Switch S1 is connected to a 5V source, turns on the D1 diode. Switch S2 is connected to the ground, D2 remains off. The current flows from 5V source to LED and the current grounded.
Case 3:
This case is the same as case 2.
Case 4:
See schematic in case 4. Both switches (S1 & S2) are connected to a 5V voltage source. As a result both the diodes are turned on. Current flows from 5V sources to the LED and grounded.
Implementation Of OR Gate Using Transistor Logic:
Figure 5: OR gate Bipolar Junction Transistor circuit |
Case 1:
Look at the schematic in case 1, both switches are open. There is no voltage at the base. Both transistors are turned off.
Case 2:
Look at the schematic in case 2, S1 is closed and hence Q1 is turned on. The transistor is in common collector configuration. Output is taken from the emitter. Once the transistor is ON, LED starts to glow.
Case 3:
Same as case 2.
Case 4:
Both transistors are ON. Current flows from emitter to collector and hence LED turns ON or output is logic high.