1. 2019 Investing Now Summer Program
University of Pittsburgh Department of Electrical and Computer Engineering
2019 Investing Now Summer Program

2. Overview Background Square-Wave Oscillator Two-Bit Counter
One-Hot Counter
Traffic Light Controller

3. Background
Digital Systems - Binary Codes
The Oscilloscope

4. Digital Systems
In digital circuits such as computers, all data (numbers, text, music, images) are represented by strings of ones and zeros.
Binary numbers: 1110 = 10112
ASCII code: Q =
In many cases, ones are represented by a HIGH voltage (+5V) and zeros are represented by a LOW voltage (0V).

5. Oscilloscope
The oscilloscope is an instrument that can display signals in which the voltage varies as a function of time.
Turn your oscilloscope on.
Connect a scope probe to channel 1.
Connect the red clip to the calibration signal.
Push the Auto-Scale button.

6. Square Wave
This is a square wave that oscillates between 0V and +5V. Such signals are often used as “clocks” in digital systems.
How often does this signal oscillate?

7. Traffic Light Controller
The goal of this project is to build a circuit to simulate the controller for a traffic light.
The outputs of the controller are signals which turn the lights on and off.

8. Traffic Light Controller
The counter has four states. Each corresponds to a different combination of colors for the lights.
The controller must produce outputs that cycle through this sequence.
State
N-S
E-W 1 2 3

9. Square-Wave Oscillator
The first circuit we will build will produce an output signal that varies between 0V and +5V (0 and 1). It will be used as a clock for other circuits to be built later.

10. Breadboard – Power Connections
The first step is to wire the power and ground buses as shown.
Also connect the red and black banana jacks to the power supply using the cables at your workstation.

11. Breadboard – Power Connections
Once this is done, the holes above the red stripe are connected to +5V.
The holes below the blue stripe are connected to ground.

12. Light Emitting Diodes We will use LEDs to display digital outputs.
When an LED is connected between an output and ground, it will light when the output is 1.
You can see the anode and cathode if you hold an LED up to the light.

13. Testing the Power Connections
Connect an LED across the lower power bus, with the anode connected to +5V.
Turn the power on for a few seconds, then off.
The LED should light when the power is on.
LED here

14. 555 Timer
The 555 timer is the smallest chip you have. It will be used to build the oscillator.
There are eight pins, numbered counter-clockwise around the chip.
There is a shiny dot next to pin 1.

15. 555 Timer
Plug your 555 timer into the breadboard, with the two rows of pins on opposite sides of the “gutter.”
Pin 1 should be on the lower left, as shown.
Pin 1

16. Power Connections
Make the power connections as shown: pins 4 and 8 to +5V, and pin 1 to ground.
Also connect pin 2 to pin 6.
Try to keep your wires as short as possible.

17. Capacitors Connect a 0.01 mF capacitor between pin 5 and ground.
Connect a 0.1 uF capacitor between pins 1 and 8.

18. Capacitors Connect a 0.01 mF capacitor between pin 5 and ground.
Connect a 0.1 uF capacitor between pins 1 and 8.
103K
104K
105K

19. Resistors Connect a 1 MW resistor between pin 7 and +5V.
The stripes on a 1 MW resistor are brown, black, green, and gold.
Connect another 1 MW resistor between pins 6 and 7.

20. Completed Oscillator Connect an LED between pin 3 and ground.
Make sure that the anode is connected to pin 3.
Turn the power on. If your circuit is correct, the LED will flash off and on.

21. Two-Bit Counter
Next we will build a circuit that takes a square wave as input, and produces two outputs that count from 0 to 3 in binary (base two).
Decimal:
Binary:

22. 7474 Dual D Flip-Flop
The 7474 has 14 pins, numbered counter-clockwise around the chip.
Instead of a dot, there is a notch to indicate the location of pin 1.

23. 7474 Dual D Flip-Flop Pin 1 Turn the power off.
Install the 7474 across the gutter, leaving a little space between it and the 555.
Place pin 1 on the lower left.

24. Power Connections Make sure the power is off.
Connect pins 1, 4, 10, 13, and 14 to +5V.
Connect pin 7 to ground.

25. Least Significant Bit Connect pin 2 to pin 6.
Connect pin 3 of the 7474 to pin 3 of the Now the 555 will serve as a clock signal for the counter.

26. Least Significant Bit Connect an LED from pin 5 of the 7474 to ground.
Turn the power on. If your circuit is correct, the LED on the right will flash at half the rate of the LED on the left.

27. Most Significant Bit Turn the power off.
On the 7474, connect pin 8 to pin 12 and pin6 to pin 11.
Connect an LED from pin 9 to ground.

28. Completed Two-Bit Counter
Turn the power on.
If your circuit is correct, the LEDs on the 7474 will count: , 01, 10, 11, 00, 01, …

29. One-Hot Counter
Now we will use a binary decoder to convert our two-bit counter to a “one-hot” counter. It has four outputs, which are asserted one at a time as shown above.

30. 74139 Binary Decoder
The 74LS139 has 16 pins, numbered counter-clockwise around the chip.
There is a notch to indicate pin 1.

31. 74139 Binary Decoder Turn the power off.
Install the across the gutter, leaving a little space between it and the 7474.
Place pin 1 on the lower left.

32. Power Connections Make sure the power is off. Connect pin 16 to +5V.
Connect pins 1 and 8 to ground.

33. Connections to the 7474
Connect pin 5 on the 7474 to pin 2 on the
Connect pin 9 on the 7474 to pin 3 on the

34. Completed One-Hot Counter
Connect LEDs from pins 4, 5, 6, and 7 to +5V.
Make sure the anode of each LED is on +5V.
This is different from the previous LEDs.

35. Completed One-Hot Counter
Turn the power on.
If your circuit is correct, the LEDs will flash in sequence from left to right.

36. Traffic Light Controller
The last step is to make use of the counter circuit to simulate the controller for a traffic light.
We will do this by connecting red yellow and green LEDs to the output signals.

37. Traffic Light Controller
The counter has four states. Each corresponds to a different combination of colors for the lights.
Note that the red lights depend only on the most significant bit.
Count
N-S
E-W 00 01 10 11

38. Output Connections Turn the power off. Remove the LEDs from the 74139.
Using a 100W resistor, connect pin 3 of the to the lower portion of the breadboard.

39. Output Connections
Using wires, connect pins 4, 5, 6, and 7 to the lower portion of the breadboard.
Also connect a wire from +5V to the lower portion of the board as shown.

40. Output Connections
Using a 100W resistor, connect pin 8 of the 7474 to the lower portion of the breadboard.
Connect a wire from +5V to the lower portion of the board as shown.

41. Output Connections
Make connections from the wires connected to pins 4 and 5 to spaces just to the left of the left resistor. Now we are ready to connect the red, yellow, and green LEDs.

42. Red LEDs
Connect red LEDs between the resistors and +5V. Make sure that the anodes are connected to +5V.

43. Yellow LEDs
Connect yellow LEDs between pin 7 of the and +5V, and between pin 5 and +5V.

44. Green LEDs
Connect green LEDs between pin 6 and +5V, and between pin 4 and +5V.

45. Completed Circuit Turn the power on and try it out!
If some of the LEDs don’t flash, they may be in backwards.