Page 90 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 9 Fig 10

In Fig 11a AND gate controls the passage of a signal (waveform A) to a digital counter. The purpose of this circuit
is to measure the frequency of waveform ‘A’. The enable pulse has a width of precisely 1 second. When the enable
pulse applied at B is high, waveform A passes through the gate to the counter, and when the enabled pulse is low,
the signal is prevented (inhibited) from passing through. Refer Fig 11b for the waveforms of the above process.

Fig 11

During the 1 second interval of the enabled pulse, a certain number of pulses in waveform A pass through the
AND gate to the counter. The number of pulses counted by the counter is equal to the frequency of the waveform
A. For example, if 1000 pulses pass through the gate in the 1 second interval of the enabled pulse, there are 1000
pulses/sec. That is, frequency is 1000Hz.

Combinational gate circuits - NOR and NAND
NOR Gate
In Fig 12a the output y of the circuit equals to the complement of A OR B, because the circuit is an OR gate
followed by a NOT gate. To obtain high output [Logic-1], both the inputs should be tied to low input [Logic-0]. For
the rest of the other three possibilities, output will be zero, the combination of this OR and NOT gate is called as
NOR gate.
Symbol (Fig 12b) :

Fig 12

We can define a NOR gate as follows:
The output of a NOR gate is 0, even if one of the inputs is in logic-1. Only when both the inputs are in logic-0, the
output is in logic-1.

Truth table
A B A + B

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

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