Main Ideas For Today

• Digital Voltmeter
• Protoboard
• Logic Circuits
• Oscilloscope
• Wavetec Signal Generator

Activities

Complete the following activities with the assistance of the TA's and Prof. Laws. Be sure to keep a complete record of what you have done in your lab notebook. (The Lab Notebook specifications are now on-line.)

The Digital Voltmeter:

The Digital Multimeter can be used to measure voltages in a circuit, or more specifically the difference in voltage between two points in a circuit. The following activities will familiarize you with the use of the multimeter as a voltmeter and the concept of voltage.


• Configure the multimeter as a voltmeter.
• Measure the voltage between the ground (0V - black post) on the protoboard and the red post labeled +5V. To do this place the black lead from the multimeter on the black post of the protoboard and the red lead from the multimeter on the red post of the multimeter.
• Reverse the leads of the multimeter and measure the voltage.
• Measure the voltage between the ground on the protoboard and the post labeled +15V. Reverse the leads and measure the voltage again. You may need to adjust the range of the multimeter.
• Measure the voltage between the ground and the post labeled -15V. Reverse the leads and measure the voltage again.
• Measure the voltage between the +5V post and the +15V post.
• Measure the voltage between the +5V post and the -15V post.
• Place both leads of the voltmeter on the +15V post.
• Does what you have observed agree with the idea that you are measuring the difference in voltages?

Having measured all of those voltages it is worth noting that most voltage measurements that we will perform this semester will be made with respect to ground.

Wire the circuit shown below. The 7408 chip contains four 2 input AND gates. Measure the output voltage. Verify that the 7408 does in fact compute the AND function. This can be done by varying the inputs (0V = Logical 0, +5V = Logical 1) to one of the AND gates of the 7408 chip while measuring its output. (Hint: Generate a truth table.)






The 7432 chip also contains 4 copies of a common logic gate. The gate inputs and outputs are connected to the same pins as the 7408 chip you were just using. Identify the type of gates contained in the 7432 chip.

When you have identified the gates in the 7432 chip verify your answer by looking the 7432 chip up in the TTL Cookbook. You should familiarize yourself with the TTL cookbook. You will use it to identify the pin-outs of many of the chips we will be using this semester.

Build and test a circuit that implements the following Boolean function:


a XOR (NOT b)

Use the TTL cookbook to identify the chips that you will need to use.

The oscilloscope provides a convenient means for looking at voltages as a function of time.


• Zero the A channel of the scope to the center of the screen.
• Setup the A channel of the scope to display 2V/division at 1ms/division.
• Use the scope to look at the voltage between the +5V post and ground on the protoboard.
• Use the scope to look at the voltage between the +15V post and ground on the protoboard. You will need to adjust the vertical scale.
• Zero the B channel of the scope to the bottom of the screen.
• Use the B channel of the scope to look at the voltage between the -15V post and the +5V post on the protoboard.
• Display both the A and B channels of the scope at the same time.

• Set the Wavetec to output a 100Hz square wave between 0V and 7V.
  Note: A 100Hz square wave looks as follows:
  
  
  
• Connect the Wavetec output to the scope.
• Use the scope display to tune the Wavetec output to be as close to the desired as possible. This may require changing the time and voltage scales on the scope.
• Change the amplitude of the square wave to be between 2V and 6.3V.

In the real world voltages are rarely exactly 0V and +5V to represent logical 0 and 1. This exercise will allow you to determine just how tolerant the logic gates we will be using are to variations in voltages.


• Use the Wavetec, protoboard and scope to connect the circuit shown below:
  
  
  
• Determine the input voltage at which the output of the NOT gate goes from 1 to 0.
• Determine the input voltage at which the output of the NOT gate goes from 0 to 1.