Activity 1.1.6 Component Identification: Digital
Introduction
In this activity you will investigate both combinational and sequential logic gates. You will be
asked to simulate simple circuits using basic logic gates. You will then complete a truth table
for each logic gate based on the outputs generated from your simulation. The name of many of
the fundamental logic gates in digital electronics are based on the logic output from the gate.
From analysis of a truth table, could you determine the name and understand function of the
gate?
You will examine the basic building block of sequential logic: the flip-flop. The investigation will
conclude with a look at the 555 IC and how it is used to trigger events in a circuit.
Equipment

Circuit Design Software (CDS)
Procedure
Using the Circuit Design Software (CDS), enter the circuit shown for each gate.
Combinational Logic
1) Use a switch for the input X and a probe for the output Z.
Toggle the switch to complete the truth table shown.
X
Z
0
1
1
0
From analysis of the truth table, why do you think this is called an “INVERTER” gate?
Getting Started Note:
When the switch is at GND, the input is a (0). When it is at +5V, the input is a (1).
Likewise, when the probe is on, the output is a (1). When the probe is off, the output is a (0).
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Digital Electronics Activity 1.1.6 Component Identification: Digital – Page 1
2) Use switches for the inputs X & Y and a probe for the output Z.
Toggle the switches to complete the truth table shown.
X Y Z
0 0 0
0 1 0
1 0 0
1 1 1
From analysis of the truth table, why do you think this is called an “AND” gate?
It is only active when both inputs are on.
A:
3) Use switches for the inputs X & Y and a probe for the output Z.
Toggle the switches to complete the truth table shown.
X Y Z
0 0 1
0 1 1
1 0 1
1 1 0
From analysis of the truth table, why do you think this is called a “NAND” gate? (NOT AND)
The only time the circuit is inactive is when both inputs are on.
In this course we will use 2-input gates predominantly. There are gates available with more
than 2 inputs.
3-Input AND
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Digital Electronics Activity 1.1.6 Component Identification: Digital – Page 2
4) Use switches for the inputs X & Y and a probe for the output Z.
Toggle the switches to complete the truth table shown.
X Y Z
0 0 0
0 1 1
1 0 1
1 1 1
From analysis of the truth table, why do you think this is called an “OR” gate?
It is active when either input is on.
5) Use switches for the inputs X & Y and a probe for the output Z.
Toggle the switches to complete the truth table shown.
X Y Z
0 0 1
0 1 0
1 0 0
1 1 0
From analysis of the truth table, why do you think this is called a “NOR” gate? (NOT OR)
It is active when OR gates aren’t, and is inactive when OR gates would be active
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Digital Electronics Activity 1.1.6 Component Identification: Digital – Page 3
6) Use switches for the inputs X & Y and a probe for the output Z.
Toggle the switches to complete the truth table shown.
X Y Z
0 0 0
0 1 1
1 0 1
1 1 0
From analysis of the truth table, why do you think this is called an “XOR” gate?
(EXCLUSIVE OR)
It is only active when either gate is active, and won’t activate when both gates are on.
Sequential Logic
In the previous combinational logic circuits, YOU made all the outputs change based on the
inputs YOU entered with switches. What if you wanted the changes to happen without YOU
needing to flip a switch? There are two ICs that allow us to do this.
1. Using the Circuit Design Software (CDS), enter the 74LS74N test circuit shown below.
Use a switch for the input T and probes for the outputs Que and NOT_Que.


The 74LS74N has two inputs. (Data in and a clock signal CLK). In this circuit, the
clock signal input will be a switch T that you flip. The Data in is tied to the NOT_Q.
The 74LS74 also has preset and clear inputs. In this circuit the PR (preset) and CLR
(clear) are connected to 5v (high), which makes them both inactive. These PR and
CLR inputs on the 74LS74 said to be “active low” inputs. (It takes a low signal to
activate them)
74LS74N Test Circuit
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Digital Electronics Activity 1.1.6 Component Identification: Digital – Page 4
a) To help with the class discussion (and make sure everyone is seeing the same thing),
toggle the input T several times until T is low and NOT_Que is low. (Toggle means to
change the state of the switch. One “toggle” of the switch would change the state of the
switch from low to high. Another toggle of the switch would change the state of the
switch from high to low.)
b) Starting with the switch on GROUND, what happens to output Que when the switch is
moved from GROUND to 5v? One toggle of the switch.
c) What happens to output Que when the switch is moved from 5v to GROUND? One
toggle of the switch.
d) Toggle the switch one more time. What happened to output Que? What does this tell
you about when Que changes (toggles) in relation to when the input at the CLK
Changes. (Helpful hint: It might help to cycle though a number of changes to spot this
relationship of when Que changes.)
e) What is the relationship of D Data in to Que always?
f) What is the relationship of Que to NOT_Que always?
g) The 74LS74N is called a “flip-flop” Based on your observations can you explain the
relationship between the D Data in, Que, NOT_Q, and the CLK signal? What does a
flip-flop do?
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Digital Electronics Activity 1.1.6 Component Identification: Digital – Page 5
Later in this course we will learn how to combine flip-flops to make transitions to desired
outputs. The flip-flop is holding bit of information, waiting for a signal to change (memory). But
in this example YOU are still making the transition by flipping a switch. How can we make the
flip-flop change without us providing the input directly?
Clock Signals (The 555 Timer)
2. Using the Circuit Design Software (CDS), create the three LM555CN test circuits shown
below on the same sheet. The 555 Timer Oscillator is one of the most common circuits
used in introductory electronics. We will use this design to generate a clock signal in the
Random Number Generator (our first full circuit design). We will look more closely at this
circuit in the next lesson. In each of the three circuits, the only value that is changed is
capacitor C2.
Circuit A
Circuit B
Circuit C
When C2 is changed from 12μF to 6μF. What happens?
When C2 is changed from 12μF to 24μF. What happens?
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Digital Electronics Activity 1.1.6 Component Identification: Digital – Page 6
Conclusion
1. Can you combine the 555 Timer circuit with the flip-flop circuit so the changes on the
flip-flop happen without your input at a rate you desire? Share this simulation with your
instructor.
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Digital Electronics Activity 1.1.6 Component Identification: Digital – Page 7