Page 88 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Case 2: A is low and B is high, The high B input voltage (+5V) forward biases the lower diode, producing an output
voltage that is ideally +5V (actually +4.3V taking the diode voltage drop 0.7V into consideration). That is, the
output is in high level. During this condition, the diode connected to input A is under reverse bias or OFF condition.
Case 3: A is high and B is low, the condition is similar to case 2. Input A diode is ON and Input B diode is OFF
and Y is in high level.
Case 4: A is high, B is high. With both the inputs at +5V, both diodes are forward biased, since the input voltages
are in parallel, the output voltage is +5V ideally [+4.3V to a second approximation]. That is, the output Y-is in high
level.
OR gates are available in the IC form. IC7432 is a T.T.L OR gate IC having 4 OR gates inside it.
Simple application of OR gate
Intrusion detection
Simplified portion of an intrusion detection and alarm system is two windows and a door. The sensors are magnetic
switches that produce a high(1) output when windows and doors are opened and a low(0) output when closed. As
long as the windows and the door are secured, the switches are closed and all three of the OR gate inputs are in
low(0). When one of the windows or the door is opened, a high(1) output is produced on that input of the OR gate
and the gate output goes high. It then activities an alarm circuit to warn of the intrusion.
AND gates
The AND gate has two or more inputs but only one output. All input signals must be held high to get a high output.
Even if one of the inputs is low, the output becomes low.
AND gate symbols for 2 input and 3 input gates are shown in Fig 5a and 5b.

Fig 5

Truth table

Two input AND gate
A B Y=AB
0 0 0
0 1 0
1 0 0
1 1 1

Three input AND GATE
A B C Y=ABC
0 0 0 0
0 0 1 0

0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1

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CITS : Power - Electrician & Wireman - Lesson 60-69