MAE 2055: Mechetronics I
Mechanical and Aerospace Engineering
Lab Exercise #7
Name
Partner 1
Partner 2
Partner 3
Objectives – Build a digital logic circuit whose output is the enabling signal for the starter in an
automobile with a pushbutton ignition system.
In the last homework, you designed a digital logic circuit whose output, a starter solenoid enable signal,
is high (S = 1) when all of the following three conditions are true:



The engine is not already running
The clutch is depressed
The emergency brake is on or the footbrake is depressed
The inputs to the circuit are:

E
E = 1 if the engine is running
E = 0 if the engine is not running

C
C = 1 if the clutch is depressed
C = 0 if the clutch is not depressed

EB
EB = 1 if the emergency brake is on
EB = 0 if the emergency brake is off

FB
FB = 1 if the footbrake is depressed
FB = 0 if the footbrake is not depressed
The circuit’s output is a signal that enables the starter solenoid:

S
S = 1 – starter is enabled
S = 0 – starter is disabled
The simplified circuit you designed in your homework should look something like this:
E
C
EB
FB
S
MAE2055 Mechetronics I
Lab Exercise #7
Your circuit may have been slightly different. That’s OK, because, as we’ve seen, there is more than one
way to construct a circuit that will provide the same functionality.
Fill out the following truth table for the circuit above:
E
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
C
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
EB
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
FB
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
S
Using a 74HC04 inverter chip, a 74HC08 AND gate chip, and a 74HC32 OR gate chip, build the circuit on
your bread board. (See the pin-out diagrams on the following page.) Be sure to insert the chips
straddling the centerline of the bread board so that pins on opposite sides of the chip are not shorted
together. Wire a bus above the chips to +5V, and a bus below the chips to ground (0V), so that you can
easily wire the supply pins on each chip to the appropriate supply bus. The positive supply (+5V) pin is
the VCC pin.
Connect the output of the circuit to an LED in series with a current-limiting resistor, so you can easily
observe the state of the output.
E
C
EB
FB
S
330Ω
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MAE2055 Mechetronics I
Lab Exercise #7
Build the following circuit on your bread board to supply the logic signal inputs to your circuit. When a
switch is closed that input is high (5V), when the switch is open the input is pulled down to 0V by the
1KΩ resistor connected to ground. A resistor used this way is called a “pull-down resistor”.
S1
E
S2
C
S3
EB
S4
5V
FB
1KΩ
1KΩ
1KΩ
1KΩ
Once your circuit is connected, step through each line of the truth table, configuring the switches to
provide the appropriate input, and verify that your circuit provides the correct value of the output, S, in
each case.
Pin-outs for the logic chips:
74HC32
74HC08
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74HC04
MAE2055 Mechetronics I
Lab Exercise #7
The following picture illustrates how you may want to wire your circuits. The red wire coming in from
the left connects to the +5V power supply. The black wire connects to ground. Notice how connecting
the +5V supply and ground to the supply buses simplifies wiring power to each chip.
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