Monday, February 20, 2017

Week 7

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)

Table 1.1 Various Force sensor resistance values.

Note in the table above that the resistance value at zero force is actually overlimit for the multimeter and not infinite. This only means that the resistance value is too high for the multimeter to read.

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).

Video 2.1 Explanation of seven segment display operation

b. Using resistors for each segment, make the display show 0 and 5. (EXPLAIN with PHOTOs)

Figure 2.1 Seven segment display showing 0

Figure 2.1 Seven segment display showing 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)
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:

Figure 3.1 Using 7447 display driver combination to display 0 on seven segment display

Figure 3.2 Using 7447 display driver combination to display 2 on seven segment display

Figure 3.3 Truth table 7447 display driver possible combinations and corresponding output for 0 to 9

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)

Video 3.1 Showing operation of each segment of the seven segment display using the 7447 display driver

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)

Figure 4.1 555 timer connections top left of breadboard

Figure 4.2 555 timer clock signal measured on the oscilloscope

b. Does your frequency and duty cycle match with the theoretical value? Explain your work. 
Yes the frequency and duty cycle seem to be close to the theoretical value. Actual Frequency= 1/T = 1/0.55 = 1.8 Hz. 
Actual Duty cycle= 60% on 40% off
Theoretical Frequency=1.44/C(Ra+Rb)=1/(1uF(150K+300K))=1.89 Hz
Theoretical Duty cycle=Rb/(Ra+2*Rb)=300K/(150K+2*300K)=.40 The on/off time was 60% on and 40% off according to what we measured and the signal completed 1.8 cycles every second.

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)

Video 4.1 Showing the force sensing resistor in series with Ra

The force sensing resistor in series with Ra does not change the frequency of the 555 timer's output. The frequency is determined by the capacitors.

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)

Video 5.1 Input and output combinations of 74192 readings on the oscilloscope

Table 5.1 Input and output combinations Truth table 74192 for values 0 to 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)

Video 6.1 Using LED to prove XOR gate operation with different input combinations

b. Connect XOR’s inputs to the BCD counters C and D outputs. Explain your observation. (EXPLAIN with VIDEO)

Video 6.2 XOR inputs coming from BCD counter C and D outputs, connected to an LED to show XOR output of 1 or 0

When XOR output is 1 (BCD counter outputs are the opposite) the LED lights up, and when the XOR output is 0 (BCD counter outputs are the same) the LED turns off.

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)

Video 6.3 Explanation of the clock signal and A,B,C,D, and XOR output in a drawing

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)

Video 7.1 Shows the operation of the 555 timer, pressure sensing resistor,LED, display driver, seven segment display and XOR gate working together to light up an LED and to count from 0 to 9

Note that our seven segment display has a burnt out segment (top left vertical segment) that was later verified by directly providing power and ground to the segment with no operation.

Figure 7.1 Drawing of entire circuit above showing important components of circuit

8. Using other logic gates provided (AND and OR), come up with a different LED lighting scheme. (EXPLAIN with VIDEO)

Video 8.1 Proves operation of AND gate and OR gate 


  1. For Question 1, Im curious to what you used in your table for the different readings of light, moderate, heavy. Like were you guys pinching it or squeezing it? I noticed with our group the more area that the object we placed on the higher the reading.

    1. Yea we did squeeze it and that is it. We also realize the same thing.

  2. How did you determine how to connect the display in order to show 0 and 5 for number 3, and how significant is the difference between the frequency of the timer input and the various output frequencies of the 74192 digital counter?

    1. For number 3, we did look at the manual to know how to determine to connect the display in order to show 0 and 5.

  3. For number 1, are you sure that the value is too high for the DMM? My guess is that when there is no pressure it just isn’t connected so there is no resistance. You may be right though. You may want to explain 2b just because it says to. For 3a you are supposed to measure the output voltages. For 5a it may be beneficial to show outputs alongside the timer reading. Check out our video if you are confused on what I mean. I like your detailed circuit diagram at the end. Very easy to follow.

    1. You are right more explaining might help.

  4. I like how you guys included infinite as the value for no force on your resistor in question 1 no one else has thought to do that. The more I think about this lab the more I'm thinking that that is true. Why do you think that is? By the way nice blog!

    1. I did not really get your question :(. sorry about that.

  5. For your videos, good job on getting your circuit to work and explaining them, try using wires instead of just alligator clips if you can. Also schematics are difficult to read on lined paper. For #4 how did you measure/record the Duty for the cycle? We couldn't figure that out but were still able to calculate the theoretical.

    1. You are right. The reason why we use alligator clips is because we don't have long wires anymore.
      Thank you.