ELEC 101, Spring 2005
Prof. Rich Kozick

Laboratory 9
Design and Construction of a Digital Stopwatch

Our main focus during the weeks of March 28 and April 4 will be the design and construction of a simple digital stopwatch. This should be a fun project that will illustrate the basic ideas of digital systems. We will learn how some integrated circuit (IC) "chips" operate, and also how the chips are connected together as subsystems in a larger system.

Little or no written homework will be assigned or collected during this project. We will discuss the design of the stopwatch during class. You will need to understand how all of the subsystems work in order to put together your stopwatch. You will perform the wiring of the stopwatch during lab on March 31, and I would like you to complete the wiring before you come to lab on April 7 (this is your "homework"). Each lab group should demonstrate their working stopwatch during lab on April 7.

The final exam will contain questions about the stopwatch operation, so please make sure you understand the design process that we use.

Stopwatch operation: The stopwatch should work as follows.

  1. If Swich 1 is set to START, then the display will show the count 0, 1, 2, ..., 8, 9, 0, 1, 2, ..., with one second increments.
  2. If Switch 1 = STOP, then the current number will be held on the display.
  3. If Switch 2 is set to CLEAR, then the display will be set to zero.

Reading: Chapter 11 in the Bobrow text, Sections 11.1-11.4, discusses number systems, binary arithmetic, digital logic gates, and Boolean algebra. I think you have seen this material before, but we will review it quickly in class during the weeks of March 28 and April 4.
See the class handouts for specific information.

Lab notebooks: Your lab notebooks should contain material such as notes that you take while wiring and debugging the subsystems, your design that resets the count to zero after "9", and the complete wiring diagram from the handout.

Suggestions for wiring and testing: Your wiring and testing will be most efficient (and enjoyable) if you test the subsystems individually before connecting them together. You should think about the best way to place the subsystems on your breadboard to simplify the connections. A possible sequence is as follows.

  1. Wire the input switches, and verify their operation with the voltmeter.
  2. Set up the clock circuit using the 555 timer IC. Capacitor values of 1 microfarad and 0.1 microfarad (for pin 5 of the 555 IC) will be available. You should choose resistors to obtain a frequency of about 1 second. Verify the operation of your clock with the oscilloscope.
  3. Wire the 7-segment display, beginning with connections of 150 ohm resistors (or a value close to this) to each segment. Verify that each segment lights up when you apply a logic 0 to the appropriate wire.
  4. Wire the 7447 display driver IC. Test it by applying fixed 0 volt and 5 volt values to the A, B, C, D inputs, and verify that the proper segments light up on the display.
  5. Wire the counter. The simplest way to test the counter may be to connect it to the 7447 and 7-segment display.
  6. If you are sure that each subsystem works individually, then connect the subsystems together.

We will design additional logic during the week of April 4 to make the counter reset to zero after it reaches 9. Without this logic, the counter will progress through 16 levels, 0000, 0001, 0010, ..., 1111, in binary.