Download Activity 3.1.2 Flip Flop Applications

Activity 3.1.2 Flip-Flop Applications
Purpose
In this activity you will simulate and build two introductory applications of flip-flops.
The first circuit is an event detector circuit using a D flip-flop. This design will sound
an alarm if a beam of light is disrupted on its photosensitive detector input.
The second design is a 4-bit shift regisiter design with J/K flip-flops. This shift
register will be used to detect a specific input sequence.
Equipment
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Paper and pencil
Circuit Design Software (CDS)
Digital Logic Board (DLB)
Integrated circuits (74LS74, 74LS76, 74LS04, 74LS08)
fischertechnik® components (12 volt incandescent bulb; phototransistor)
Discrete components (5.6 k, 10 k)
Procedure
1. The circuit shown below is a photosensitive event detector. This circuit sounds
an alarm if the beam of light between the 12-volt incandescent bulb and
phototransistor is disrupted. This circuit would be used as part of a larger burglar
alarm system. As the name implies, a phototransistor is a transistor that is
sensitive to light. In the event detection circuit, the phototransistor acts as a
switch. As long as the phototransistor is in the proximity of the incandescent bulb,
the transistor will be on. This, in turn, will hold the CLK input of the flip-flop at a
logic zero. If the beam of light is disrupted, the phototransistor will be off. When
the transistor is off, the CLK input of the flip-flop will be pulled high through the
5.6 k resistor. This zero-to-one transition will toggle the Q output to a one,
which will turn the buzzer on. The buzzer will remain on until the reset button is
pressed.
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DE – Unit 3 – Lesson 3.1 – Activity 3.1.2 Flip-Flop Applications – Page 1
Shown below is a simplified version of the photosensitive event detector circuit.
In this simplified version, the phototransistor and incandescent bulb have been
replaced with a simple pushbutton switch. This substitution was made because
the CDS does not have the capability to simulate breaking the light beam.
a. Using the CDS, enter the simplified version of the event detector circuit. Be
sure to set the frequency and voltage of the buzzer (SONALERT) to 2 kHz
and 4.5 volts respectively.
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b. Start the simulation and verify that the circuit is working as expected.
Pressing the first push-button switch (Key=Space) will sound the alarm;
pressing the second push-button switch (Key=A) will disable it. If the circuit is
not working as expected, review your circuit and make any necessary
corrections.
c. Using the DLB, build and test the photosensitive event detector circuit (the
real circuit, not the simplified version). Because this circuit uses two supply
voltages (5-volts and 12-volts), take caution when constructing this circuit.
Connecting the 12-volts to the logic gate WILL destroy it. Verify that the circuit
is working as expected; if not, review your circuit and make any necessary
corrections.
2. The circuit shown below is a 4-bit shift register that is used to detect the input
sequence 1,1,0,1. When this input sequence is detected, the output Z will be on.
At all other times, the output Z will be off.
a. Using the CDS, enter the 4-bit shift register circuit.
b. Start the simulation and verify that the circuit is working as expected by trying
various input sequences and confirming that the sequence 1,1,0,1 is
detected, and the others are not. If the circuit is not working as expected,
review your circuit and make any necessary corrections.
c. Make the necessary modification to the circuit so that it will detect the input
sequence 0,1,1,0. Simulate this new circuit and verify that it is working as
designed. If not, make any necessary corrections.
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Copyright 2009
DE – Unit 3 – Lesson 3.1 – Activity 3.1.2 Flip-Flop Applications – Page 3
Conclusion
1. In this activity you studied two of the many applications of the flip-flop. List three
other applications of the flip-flop.
2. Modify the shift register designed to detect 1,1,0,1 to use only 74LS76 J/K flipflops. In terms of the number of gates used, what is the advantage of using the
J/K flip-flop over the D flip-flop?
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Copyright 2009
DE – Unit 3 – Lesson 3.1 – Activity 3.1.2 Flip-Flop Applications – Page 4