The OR Gate - Truth Table, Circuit Symbol, Diode Circuit & Transistor Circuit

 The OR Gate:

The OR gate performs logical OR function. The output is high when any one input is high. 

Learning Objectives:

• Introducing OR gate implementation using

• Switches

• Diodes

• BJT

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:

2 Input OR Gate
Input A	Input B	Output
0	0	0
1	0	1
0	1	1
1	1	1

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

Before starting the circuit model of OR gate, it is better to understand the concept of OR logic with the help of switches. The figure shows the switch model of OR gate. Two switch (switch A and switch B) are connected in parallel in between input and output. Input doesn't reach when both switches are off (open). If anyone switches on (closed) input reaches to output. 

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.