Week 7
Digital Circuits
1.
Force sensing resistor gives a resistance value
with respect to the force that is applied on it. Try different loads (Pinching,
squeezing with objects, etc.) and write down the resistance values. (EXPLAIN with
TABLE)
The Force sensing resistor is a variable resistor that's impedance is inversely proportional to the amount of pressure being applied to the sensing pad. The table below shows some sample resistance readings as they correspond to the amount of pressure being applied.
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| Sample Resistance Readings from the Force Sensing Relay. Although 1.7 k W is the greatest resistance on the above table in later experiments we we're able to achieve readings as high as 2 MW |
2.
7 Segment display:
a.
Check the manual of 7 segment display. Pdf
document’s page 5 (or in the document page 4) circuit B is the one we have.
Connect pin 3 or pin 14 to 5 V. Connect a 330 Ω resistor to pin 1. Other end of
the resistor goes to ground. Which line lit up? Using package dimensions and
function for B (page 4 in pdf), explain the operation of the 7 segment display
by lighting up different segments. (EXPLAIN with VIDEO).
The 7-Segment display operates as 9 (although only 7 are used in this lab) separate LEDs that share a common power input. Each LED then seeks ground through it's individual input pin. The table and video below explain this function in more detail and identify which pins correspond to the each LED.
7 Segment Display Operation Explained
7 Segment Display Input Configuration.
(Dec = Decimal U=Upper Leading - L=Lower Trailing L=Left
R=Right)
b.
Using resistors for each segment, make the
display show 0 and 5. (EXPLAIN with PHOTOs)
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| 7-Segment Display, with 330 W resistors connecting pins 1,2,7,8,10 and 13 to ground, Displaying the Number 0 |
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| 7-Segment Display, with 330 W resistors connecting pins 1,2,8,10 and 11 to ground, Displaying the Number 5 |
3.
Display driver (7447). This integrated circuit
(IC) is designed to drive 7 segment display through resistors. Check the data
sheet. A, B, C, and D are binary inputs. Pins 9 through 15 are outputs that go
to the display. Pin 8 is ground and pin 16 is 5 V.
a.
By connecting inputs either 0 V or 5 V, check
the output voltages of the driver. Explain how the inputs and outputs are
related. Provide two different input combinations. (EXPLAIN with PHOTOs and TRUTH TABLE)
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| Display Driver (7447) Configured to Send an "8" Signal to the 7 Segment Display |
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7447 Driver / 7 Segment Display Truth Table
UPDATE! You cannot actually measure the output
voltages directly (I challenge you to figure out why!). You need to connect an
LED and a resistor. LED’s positive terminal will go to 5 V. Negative terminal
will be connected to your outputs via a resistor. The circuit would look like
below:
Output Voltages cannot be measured directly because the 7-Segment Display is a ground seeking circuit, therefore; the driver never sends voltage to the display but instead provides paths to ground.
Output Voltages cannot be measured directly because the 7-Segment Display is a ground seeking circuit, therefore; the driver never sends voltage to the display but instead provides paths to ground.
b.
Connect the display driver to the 7 segment
display. 330 Ω resistors need to be used between the display driver outputs and
the display (a total of 7 resistors). Verify your question 3a outputs with
those input combinations. (EXPLAIN with VIDEO)
A Video Demonstration of the Driver/Display Operation Described in Entry 3A
4. 555 Timer:
a.
Construct the circuit in Fig. 14 of the 555
timer data sheet. VCC = 5V. No RL (no connection to pin
3). RA = 150 kΩ, RB = 300 kΩ, and C = 1 µF (smaller sized
capacitor). 0.01 µF capacitor is somewhat larger in size. Observe your output voltage
at pin 3 by oscilloscope. (Breadboard and Oscilloscope PHOTOs)
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| The 555 Timer finds a home above the 7447 Display on our Breadboard |
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| This picture displays the signal the 555 Timer is generating |
b.
Does your frequency and duty cycle match with
the theoretical value? Explain your work.
The theoretical duty cycle would be 50% since the clock is continuously changing to high and low, which is right about what we measured. The theoretical frequency was about 1.92 Hz according to the equation provided in the 555 timer manual.
The theoretical duty cycle would be 50% since the clock is continuously changing to high and low, which is right about what we measured. The theoretical frequency was about 1.92 Hz according to the equation provided in the 555 timer manual.
Video of our Oscilloscope readings of frequency and Duty Cycle
c.
Connect the force sensing resistor in series
with RA. How can you make the circuit give an output? Can the
frequency of the output be modified with the force sensing resistor? (Explain with VIDEO)
Because of the relatively high resistor values used in this circuit the force sensor's effect on frequency is minimal, however if an effort is made to apply the minimal pressure required to activate the force sensor a resistance large enough to affect the frequency without completely opening the circuit can be achieved.
Because of the relatively high resistor values used in this circuit the force sensor's effect on frequency is minimal, however if an effort is made to apply the minimal pressure required to activate the force sensor a resistance large enough to affect the frequency without completely opening the circuit can be achieved.
Here is a video demonstrating a change in frequency due to the change in resistance through the force sensing resistor
5.
Binary coded decimal (BCD) counter (74192). This
circuit generates a 4-bit counter. With every clock change, output increases;
0000, 0001, 0010, …, 0111, 1000, 1001. But after 1001 (which is decimal 9), it
goes back to 0000. That way, in decimal, it counts from 0 to 9. Outputs of
74192 are labelled as QA (Least significant bit), QB, QC,
and QD (Most significant bit) in the data sheet (decimal counter,
74192). Use the following connections:
5 V: pins 4, 11, 16.
0 V (ground): pins 8, 14.
10 µF capacitor between 5 V and ground.
a.
Connect your 555 timer output to pin 5 of 74192.
Observe the input and each output on the oscilloscope. (EXPLAIN with VIDEO and TRUTH TABLE)
The interaction between the 555 Timer and the 74192 counter can be summed up as follows. The timer sends an input signal to the Counter once each second. The counter then increases its output signal by one binary digit each time it receives a signal. Once the tenth input pulse is received from the time the Counter then resets itself back to zero and the process repeats. The Video Below shows this interaction as monitored by the Oscilloscope.
The interaction between the 555 Timer and the 74192 counter can be summed up as follows. The timer sends an input signal to the Counter once each second. The counter then increases its output signal by one binary digit each time it receives a signal. Once the tenth input pulse is received from the time the Counter then resets itself back to zero and the process repeats. The Video Below shows this interaction as monitored by the Oscilloscope.
Chris Explains the Interactions that take place between the 555 Timer and 74192 Counter
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| Truth Table Showing the all possible output combinations of the 74192 counter and their Binary Translations. (Binary Outputs 10-15 are included on the table as they would occur even though our counter resets after 9) |
6.
7486 (XOR gate). Pin diagram of the circuit is
given in the logic gates pin diagram pdf file. Ground pin is 7. Pin 14 will be
connected to 5 V. There are 4 XOR gates. Pins are numbered. Connect a 330 Ω
resistor at the output of one of the XOR gates.
a.
Put an LED in series to the resistor. Negative
end of the LED (shorter wire) should be connected to the ground. By choosing
different input combinations (DC 0V and DC 5 V), prove XOR operation through
LED. (EXPLAIN
with VIDEO)
The Video below demonstrates the workings of an XOR gate. As the video shows the LED will only illuminate when the XOR gate receives mismatching High and Low inputs, and remains off any time both of the gate's inputs match.
The Video below demonstrates the workings of an XOR gate. As the video shows the LED will only illuminate when the XOR gate receives mismatching High and Low inputs, and remains off any time both of the gate's inputs match.
Demonstration of the workings of an XOR Gate
b.
Connect XOR’s inputs to the BCD counters C and D
outputs. Explain your observation. (EXPLAIN with VIDEO)
With the XOR Gate's inputs connected to the Counters C and D the LED blinked in a pattern that caused it to be illuminated slightly less 50% of the time. This is because the C&D represent the counters two most significant bits, and as the truth table in Step 5 shows these two bits mismatch in 5 of the 9 possible output configurations.
With the XOR Gate's inputs connected to the Counters C and D the LED blinked in a pattern that caused it to be illuminated slightly less 50% of the time. This is because the C&D represent the counters two most significant bits, and as the truth table in Step 5 shows these two bits mismatch in 5 of the 9 possible output configurations.
This video demonstrates how the XOR gate works while having its inputs connected to the C and D outputs from the counter.
c.
For 6b, draw the following signals together: 555
timer (clock), A, B, C, and D outputs of 74192, and the XOR output. (EXPLAIN with VIDEO)
Here Chris explains how the signals A,B,C,D, clock, and XOR are related
Diagram of Each Output's Waveform in comparison to the Clock Signal
7.
Connect the entire circuit: Force sensing resistor
triggers the 555 timer. 555 timer’s output is used as clock for the counter.
Counter is then connected to the driver (Counter’s A, B, C, D to driver’s A, B,
C, D). Driver is connected to the display through resistors. XOR gate is
connected to the counter’s C and D inputs as well and an LED with a resistor is
connected to the XOR output. Draw the circuit schematic. (VIDEO and PHOTO)
We can see in the video below the counter counts up from 0 to 9 when pressure is applied to the force sensor and stop at it's current count when pressure is removed. We also see the LED flash in a pattern determined by the XOR Gate as described in Step 6.
We can see in the video below the counter counts up from 0 to 9 when pressure is applied to the force sensor and stop at it's current count when pressure is removed. We also see the LED flash in a pattern determined by the XOR Gate as described in Step 6.
The Final Circuit in Action
Detailed Schematic Diagram of the Completed Circuit
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| The Completed Circuit Laid Out on the Breadboard |
8.
Using other logic gates provided (AND and OR),
come up with a different LED lighting scheme. (EXPLAIN with VIDEO)
For this step we replaced our XOR Gate with an OR gate. The LED pattern changed such that the light stayed on more frequently, this makes since since like an XOR gate the OR gate will provide a input when either input is exclusively high, and it will also provide a positive output when both it's inputs are high.
For this step we replaced our XOR Gate with an OR gate. The LED pattern changed such that the light stayed on more frequently, this makes since since like an XOR gate the OR gate will provide a input when either input is exclusively high, and it will also provide a positive output when both it's inputs are high.
Alternate LED Configurations using an OR Gate
We wanted to attempt a configuration with an AND gate as well but were unable to find one in our kit and were unable to obtain one because we did not get to this step until well after class hours Friday afternoon. In theory if an AND gate was used the LED would illuminate less frequently as the AND gate would require both inputs to be high to provide an output.
