Monday, February 20, 2017

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)

Figure 1: Table displaying force sensing resistor values
  • The more pressure present on the sensor, less resistance will be applied.

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). 

Figure 2: Video displaying 7 segment display.

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

Figure 3: Displaying a 0 with the 7 segment display.

Figure 4: Displaying a 5 with the 7 segment display.

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) 

Figure 5: Picture representation of circuit.

  • In the circuit, the display driver was used to light up the LED to show that voltage was being given because it could not be measured with the DMM. 
Figure 6: Truth table for the 7447 display driver.

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)

Figure 7: Video showing how the 7447 display driver works. 

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 8: Picture representation of circuit.
Figure 9: Output voltage of pin 3 of circuit in figure 8.

b. Does your frequency and duty cycle match with the theoretical value? Explain your work. 

  • Based on the calculations we acquired, the frequency and duty cycle are accurate. We found this by using the equations below, which are dependent on Ra, Rb and the capacitor.
Figure 10: Equations used to calculate 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)

Figure 11: Video showing how the force sensing resistor works.

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)

Figure 12: Video explaining how the 555 timer works in combination with the 74192.
Figure 12:Data Table

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)

Figure 13: Video showing the XOR operation with the LED.

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

Figure 14: Video explaining the XOR outputs vs inputs.

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)

Figure 15: Signals for 555 timer, A,B,C and D outputs of 74192 and XOR output.
Figure 16: Video explaining output signals.

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)

Figure 17: Video showing entire circuit functioning.
Figure 18: Picture of entire circuit.

8. Using other logic gates provided (AND and OR), come up with a different LED lighting scheme. (EXPLAIN with VIDEO)
Figure 19: Video showing different lighting scheme.
  • We didn't have access to a different logic gate so instead we just used the XOR gate but with different input values.


  1. I like your truth table #3 that provides which segment is lit up, I will add that to my blog as well. What combination did you use to get the light to turn on for 2 seconds and off for 2 seconds?

    1. We used channels A and B i believe from the 74192 into the XOR gate to recieve the on for 2 off for 2. Thanks for the feedback!

  2. For Question 1, I noticed that you guys also tried placing a phone on your force sensor, however we got very different readings. Ours came out to be 300k, in hindsight I think it may have only been partly on the sensor to give off such a high resistance compared to yours.

    1. A lot of people were receiving a lot different values for the force sensing resistor. I think there are different values for them. Thanks for the feedback !

  3. For number 8, where the pattern of the flashing of the LED was changed, which pins were used to create the 2 second flash duration as opposed to the 4 second one, and what aspect of the circuitry and truth tables causes this difference?

  4. For 3b did you guys run into any resistor problems with the way your resistors were set up. My group did because we had them set up just like that and were getting wrong values or the circuit wouldn't work in general. Also for 7 I like how you guys broke down the signals, and explained each one. It was easy to understand how they all worked together from your video.