Project 2.2.5 Universal Gates and K-Mapping:
Fireplace Control Circuit
Introduction
The Acme Fireplace Company has hired you to redesign the fireplace control circuit for their
latest residential gas fireplace. The fireplace burner is equipped with four thermal sensors
that output a logic (1) whenever a flame is present. These sensors are connected to the
fireplace control circuit which outputs a (1) to the emergency cut-off valve to keep the gas
flowing (i.e., a zero will turn the gas off).
The original design of the fireplace control circuit was quite simple. For the gas valve to
remain on, all four sensors needed to output a logic (1). During field testing it was discovered
that variations in gas pressure and humidity cause the thermal sensors to occasionally output
a logic (0) even when a flame was present. This caused frequent unnecessary shut downs
and constant customer dissatisfaction.
Emergency
Cut-Off Valve
A
B
C
D
Gas Line
Thermal
Sensors (4)
Fireplace
Control
Circuit
For the redesign, it has been determined that the emergency cut-off value should remain
open as long as three of the four sensors indicate that a flame is present.
Additionally, the designers have asked you to add a second output indicator to the control
circuit. This indicator will output a logic (1) when the four sensors do not all agree (i.e., not all
on or not all off). This indicator will be used by the service technician to diagnose whether a
faulty sensor exists.
Equipment




Circuit Design Software (CDS)
Breadboard (DLB or DMS)
#22 Gauge solid wire
Integrated Circuits (74LS00 & 74LS02)
© 2014 Project Lead The Way, Inc.
Digital Electronics Project 2.2.5 Fireplace Control Circuit – Page 1
Procedure
Design
Design a combinational logic circuit that meets the above detailed design specifications.
Additionally:



The Karnaugh mapping technique must be used to obtain the simplified logic
expression for both outputs.
The circuit that controls the emergency cut-off valve must be implemented
using only 74LS00 two-input NAND gates.
The circuit for the possible faulty sensor indicator must be implemented using
only 74LS02 two-input NOR gates.
Simulation
Using the Circuit Design Software (CDS), enter and test your Fireplace Control Circuit
design. Use switches for the inputs A, B, C, and D and a probe or LED circuit for the two
outputs. Verify that the circuit is working as designed. If it is not, review your design work and
circuit implementation to identify your mistake. Make any necessary corrections and retest.
Be sure to document all changes in your engineering notebook/portfolio.
Prototyping
Using a Digital Logic Board (DLB) or Digital MiniSystem (DMS), build and test your Fireplace
Control Circuit design. Verify that the circuit is working as designed. If it is not, YOU SHOULD
NOT CHANGE YOUR DESIGN. You know that your design functions because you simulated
it. If your circuit isn’t working correctly, you must have built something incorrectly. Review
your circuit implementation to identify your mistakes, make the necessary corrections, and
retest. Be sure to document all changes in your engineering notebook/portfolio.
Conclusion
Using your engineering notebook/portfolio as a guide, write a conclusion (minimum 250
words) that describes the process that you used to design, simulate, and build your Fireplace
Control Circuit. This conclusion must include all of your design work (i.e., truth table, K-Maps,
etc.), preliminary and final schematics, parts list, and a digital photograph of your final circuit.
The documentation should be complete enough that a student with a similar knowledge of
digital electronics could reproduce your design without any additional assistance.
© 2014 Project Lead The Way, Inc.
Digital Electronics Project 2.2.5 Fireplace Control Circuit – Page 2
Elaborate Explanation
In the “Activity 2.2.5 Fireplace Control Circuit”, I used many different techniques that
helped me to reach the final product. Amazingly the design worked and it was true to the
truth tables that I made before the design. First to the important things to do is to make a
truth table for the Emergency Cutoff valve and the Faulty circuit Indicator. Like these:
BC’+A’D+AB’+CD’
ABD+ABC+BCD+ACD
Figure. 1
Next using those truth tables create the corresponding Un-simplified logic expressions.
Once you have found the Unsimplified expressions, simplify
them using the K-mapping
technique.
This technique can easily simplify
expressions without using Boolean
algebra or De Morgan’s Theorems.
And Now that you have the
simplified Expressions (as seen in figure 1) you can go to the Circuit Design Software
and prepare the circuit to test out. These are the
Circuits that I got from those truth tables
and k-maps. Once you have this get your
breadboard ready and place your circuit there
according to the schematic you made on cds. And
the working end result should be:
VCC
5V
S1
1
U1A
Key = S2
Space
3
74LS00N
2
GND
U1B
5
74LS00N
7
U1C
U2A
Key = Space
VCC S4
6
U1D
8
74LS00N
74LS00N
4
U2D
X1
15
2.5 V
U4A
17
74LS00N
Key = S3
Space
74LS00N
11
74LS00N
U3C
U2B
U3D
14
12
Key = Space
74LS00N
16
U3A
10
9
74LS00N
74LS00N
74LS00N
GND
VCC
5V
A
U5A
18
Key = Space
22
74LS02N
B
19
Key = Space
C
D
24
74LS02N
U7B
26
74LS02N
33
U6C
74LS02N
34
25
74LS02N
U6D
U7C
2.5 V
U8B
32
30
74LS02N
U5D
74LS02N
U7A
29
74LS02N
U5C
21
U6B
U8A
74LS02N
Key = Space
Key = Space
28
X2
23
20
U6A
74LS02N
U5B
GND
U3B
74LS00N
U2C
VCC
13
74LS00N
35
74LS02N
U7D
27
31
74LS02N
74LS02N
74LS02N
GND
© 2014 Project Lead The Way, Inc.
Digital Electronics Project 2.2.5 Fireplace Control Circuit – Page 3
© 2014 Project Lead The Way, Inc.
Digital Electronics Project 2.2.5 Fireplace Control Circuit – Page 4