LABORATORY ACTIVITY 1
Logic gate circuits (Diode-Resistor Logic)
Objective: To understand basic gate operation of following logic families:
I. Diode-Resistor Logic (DRL)
Overview:
Digital Logic States
All digital electronic circuits and microprocessor based systems contain
hardware elements called Digital Logic Gates that perform the logical operations of
AND, OR and NOT on binary numbers. In digital logic only two voltage levels or states
are allowed and these states are generally referred to as Logic "1" or Logic "0", High
or Low, True or False and which are represented in Boolean Algebra and Truth Tables
by the numbers "1" and "0" respectively. A good example of a digital logic level is a
simple light as it is "ON" or "OFF".
Logic operations can be performed using any non-linear device that has at least
two distinct regions of operation. Obvious choices for the electrical engineer are the
semiconductor diode and the bipolar junction transistor. Particular voltage levels are
assigned to logic levels 0 and 1.
While many voltage level assignments are possible, one common assignment is:
logic 1 (HIGH)---- ~ 5 V
logic 0 (LOW) ---- ~ 0 V.
This is known as "Positive logic" system. There is also a complementary "Negative
Logic" system in which the values and the rules of a logic "0" and a logic "1" are
reversed. But, unless stated otherwise, we shall only refer to the Positive Logic
convention for all the experiments. It is important to note that noise, power source
fluctuations, loading by other circuits, and other factors will cause the logic level
voltages to vary over some range.
Simple Basic Digital Logic Gates
Simple digital logic gates can be made by combining transistors, diodes and resistors
as discrete components. Let us investigate some of such circuits using Diode-Resistor
Logic (DRL), Diode-Transistor Logic (DTL) and Transistor-Transistor Logic (TTL) as
described below.
I. Diode-Resistor Logic (DRL)
Diode logic gates use diodes to perform OR and AND logic functions as shown in the
circuit diagram. Connection of the LED at the output is optional which simply displays
the logical state of the output, i.e. the logic state of output is 0 or 1, if LED is off or
on, respectively. Diodes have the property of easily passing an electrical current in one
direction, but not the other. Thus, diodes can act as a logical switch. Diode logic gates
are very simple and inexpensive, and can be used effectively in limited space. However,
they cannot be used extensively due to the obvious logic level shift when gates are
connected in series. In addition, they cannot perform a NOT function, so their
usefulness is quite limited. This type of logic circuit is rarely found in integrated form.
Circuit Components/Equipments:
1. Resistors (1KΩ, 3 Nos; 10KΩ, 1 No.)
2. 1N914 diodes or equivalent (2 Nos.)
3. A Surface mount dip switch
4. D.C. Power supply (5V)
5. A Red/Green LED
6. Connecting wires
7. Breadboard
Circuit Diagram:
DRL OR gate
DRL AND Gate
Procedure:
1. Assemble the circuit on your breadboard for OR/AND operation.
2. Turn on power to your experimental circuit.
3. Apply all four possible combinations of inputs at A and B from the power
supply using dip switch.
4. For each input combination, note the logic state of the output, Q, as
indicated by the LED (ON = 1; OFF = 0), and record that result in the
table.
5. Compare your results with the truth table of a logic “OR”/ “AND” operation.
6. When you have completed your observations, turn off the power to
your experimental circuit
Truth Tables:
Logic “OR” operation Logic “AND” operation
A B Q=A+B A B Q=A.B
0 0 0 0 0 0
0 1 1 0 1 0
1 0 1 1 0 0
1 1 1 1 1 1
Observations:
Results of Simulation: (Screenshots of simulations)
Circuit 1:
Circuit 2:
