Project # 5 – Combinational Circuits

Combinational logic circuits are circuits that take two or more inputs, and produce an output that depends only on the value of the inputs at the moment, i.e., they have no memory. For example, if you wanted to build a circuit that would turn on an alarm only if the lights are on AND there is a loud sound you would be building a combinational circuit. In this project you will build a circuit that has three inputs and one output. In part II you will try to use what you learned in class to streamline the circuit.

Procedure:

Part I1. Make sure the logic probe you built is working properly (see page 11, steps 4 – 6, in lab book).2. Follow all the PRECAUTIONS (see page 23, steps 1 through 4) for handling CMOS chips.3. Place the following IC’s on your breadboard: 4011 (NAND), 4049 (Inverter), and 4001 (NOR).4. Connect each IC to +9V and to ground. SEE DATA SHEET given at the beginning of the year.

Note that the +V and GND connections are often omitted from the schematic diagrams.5. NOTE: since you do not have OR gates or AND gates, you need to take the output of each NOR and

each NAND, and pass it through one of the six inverters in the 4049 chip. See the example below:

6. Once you finish the circuit, test it by using your logic probe to determine the logic state of Q for each of the eight combinations of the inputs A, B, C shown in your truth table above. Connect the inputs to ground to produce logic 0 or to +9V to produce logic 1.

7. Once the circuit works correctly and agrees with your truth table, obtain teacher’s initials. If the circuit doesn’t work properly see the troubleshooting techniques given later in this sheet. You will receive most of the points for this project if you get this far and skip part II (which is what you should do if you find you are falling behind).

Part II8. To receive full credit for this project, come up with a circuit that will give you the same output using

just ONE CHIP. Hint: I was able to do this with 4 NOR gates (that is, using only the 4001 chip). See Lesson # 5 and Homework # 5 for further ideas on how to do this.

9. Build and test this streamlined version of the circuit. When it works, obtain teacher’s initials.

A B C Gate 1 Gate 2 Gate 3 Gate 40 0 00 0 10 1 00 1 11 0 01 0 11 1 01 1 1

Troubleshooting:1. Is every chip placed correctly (see figure 3 on page 23), right side up?2. Make sure each chip is receiving power by using the logic probe and touching the pins that are

supposed to be connected to power (pin 14 of the 4001, 4011, and pin 1 of the 4049 should be +9V; pin 7 of the 4001, 4011, and pin 8 of the 4049 should be grounded).

3. Have any of the output pins of any chip been connected to +9V or ground? If so this would cause a short circuit.

4. Choose values for the three inputs, and then using your truth table and the logic probe see if the output of each output agrees with your table – if not this will help you locate the problem.

5. If you have tried all of these steps with no luck, try using three new chips (obtain from teacher) by carefully and slowly popping out the old ones and then sticking in the new one WHILE THE POWER IS OFF. The chips you are using are very easily damaged. If this is the problem, then throw out any bad chips.

6. If you still have no luck, then ask ME, I’ll troubleshoot it for you, just make sure you have all schematics, DATA SHEETS, and logic probe ready.

Conclusion:1. Your report must include the following for Part I

a. A schematic diagram of the original circuit.b. A completed truth table for the original circuit.c. The Boolean algebra expression of the original circuit.d. And, of course, my initials.

2. Your report must include the following for Part IIa. A schematic diagram of the streamlined version of the circuit.b. An explanation of what you did to arrive at the reduced version of the original circuit.c. Again, of course, my initials.