COS 116 The Computational Universe Laboratory 6: Digital Logic I In this lab you’ll construct simple combinational circuits with a “breadboard” and silicon chips. You’ll use these circuits to experiment with digital logic. If you get stuck at any point, feel free to discuss the problem with another student or a TA. However, you are not allowed to copy another student’s answers. Hand in your lab report at the beginning of lecture on Tuesday, April 4. Include responses to questions printed in bold. (Number them by Part and Step.) When you assemble the circuit for Majority-of-3, you need to show it to your TA. Otherwise you will not get credit for the lab. Also, at the end of the lab you must remove any wires or components that you added to the breadboard and place them back in the plastic box you received. Return this plastic box to the TA before leaving the lab. Breadboards are used to build temporary circuits during prototyping. Their name derives from little holes into which you insert electronic components and connecting wires. You can quickly assemble and disassemble circuits. No soldering required! Here is a picture. + – a b c d e f g h i j + –Some of these holes are wired together on the inside, as shown in the diagram on the left. For example, if you insert two wires into the holes labeled “a” and “e” in the same row, they will be electrically connected. The breadboard has plugs at the top to accept power. Do not connect the power supply until you reach the appropriate time in the experiment. Also, your breadboard will come with some wires inserted into it already; please do not remove them at any time. Our silicon chips use a voltage of 5 volts to denote the binary value 1 and a voltage of zero volts to denote 0. These two voltages are also called “power” and “ground” respectively. Notice that the breadboard has two columns on each side marked with the red and blue stripes (and the symbols + and –). All holes in the + columns are connected to the power (red/+) terminal on top and all holes in the – columns are connected to the ground (blue) terminal of your power supply. You have been given a plastic box containing all supplies for today. The lab TA will walk you through the parts that you will need. • 1 set of power supply connectors • 1 breadboard (with pre-wired switches) • 1 set of breadboard jumper wires, multiple colors • 1 LED • 1 resistor • 1 74LS04 chip (6 “Not” gates) • 1 74LS08 chip (4 “And” gates) • 1 74LS32 chip (4 “Or” gates) • 1 chip extractor tool Connect your breadboard to the power supply using the red and black power supply leads. Connect the red terminal of the power supply to the red post on the breadboard (and the black terminal to the black post). Each power supply can feed multiple breadboards since the jacks can stack on top of each other. Switch the power supply on, and verify that its voltage reading is 5 volts. If not, contact the TA. Caution: For safety, before you insert or remove any components from the breadboard, always disconnect the power by unplug the red power supply connector from the breadboard’s connector post. Part 1: Understanding switches, LEDs, and the breadboard In this part you’ll implement a very simple circuit involving a single switch and a light bulb (LED): the switch is used to turn the LED on and off. COS 116 – Lab 6 2Switch position: Left = “1”, Right = “0” Switch 1 output Switch 3 output Switch 2 output Switch 4 output 1234 Your breadboard is divided into two halves, each like the pictures on page 1. On the left half, we’ve wired some switches for you to use in all the experiments. There are four white switches in one red block. For each switch, flipping the switch to the right connects the output to ground, resulting in a binary “0” value; and flipping the switch to the left connects the output to power, resulting in a binary “1”. You can feed the output from one of the switches as an input to another component by using a jumper wire to connect to one of the switch’s output holes, shown below. Your kit has jumper wires of different lengths. You should always route your wires cleanly (laying them flat on the breadboard) as this will help avoid wiring errors. As your first exercise with the breadboard, you’ll construct the following simple circuit involving a switch, LED, and resistor. 1 2 3 4 ResistorLEDJumper wire 1. Disconnect the power by unplugging the red power supply wire from the breadboard. 2. Insert the LED into the right half of the breadboard. Make sure the longer lead is on the left, as shown above! The LED will not function if you connect it in reverse. Use a red LED if possible, as it tends to be the brightest. 3. Use a short jumper wire to connect one of Switch 1’s output holes to a hole in the row where you inserted the LED’s longer leg. 4. Connect a resistor between the row where you inserted the LED’s shorter leg and one of the ground holes in the breadboard’s rightmost column. (Unlike the LED, the resistor can be connected in either direction.) The voltage from COS 116 – Lab 6 3the power supply will damage the LED if you connect it directly to the switch. The resistor reduces the voltage and protects the LED. 5. Reconnect the power supply to the breadboard. Toggle Switch 1, and verify that the LED is on when the switch is to the left and off when the switch is to the right. 6. Rewire your circuit to use Switch 2 instead of switch 1, and verify that it works. Repeat for Switch 3 and Switch 4. If any of the switches do not function correctly, ask your TA for assistance. Part 2: Understanding Logic Gates and Truth Tables Now we understand the gates contained inside our silicon chips, and build a circuit with a single gate. Each chip used today packages several identical gates: 74LS08 chip AND AND AND AND 4 AND gates Power Ground Power 74LS32 chip 4 OR gates Ground OR OR OR OR 74LS04 chip 6 NOT gates Power Ground NOT NOT NOT NOT NOT NOT Notice the small imprinted dot at the top of each chip—you’ll use this to orient the chip correctly on the breadboard. The upper-right pin on each chip should be connected to power (wire this to the + column on the breadboard), and the lower-left pin should be connected to ground (wire this to the – column). The diagram above shows how the remaining pins are
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