E11 Lab #4
In lab
2008

Before you start this lab, be sure to read the lab rules.

## Procedure:

Setting up the function generator

Connect a Wavetek signal generator directly to the oscilloscope.  Set the output of the signal generator to give a square wave that goes from 0 to 1 volt (the frequency isn't important for now).  If you don't get this correct your analysis will be much more difficult.  If you are not sure you have it set properly, ask me.  If you don't recall how to do this, refer to the first lab.  After setting the amplitude and DC offset, don't adjust these knobs for the remainder of the lab.  If you do change them, nothing bad happens, but the analysis is more difficult.

### Circuit 1: A 1st order RC circuit.

Remember: Don't change the amplitude or DC offset of the function generator!

Connect the circuit below with Vi coming from the function generator, R=1kΩ, C=1μF, and your function generator set to about 50 Hz (the exact frequency isn't important).  For this circuit it is easier to use the large resistor and capacitor boxes than it is to use the breadboards.

1. Set the triggering so that a falling edge of the input is at the center of the screen (trigger on the input, slope set to falling edge)  and set your scales as large as is convenient so that you can get an accurate measurement.
• Measure and record the initial and final values of the input and output.  (Needed for report)
• Measure and record the time constant by finding where the output voltage has gone through 63% of its total change (use the scope's cursors to measure τ).  (Needed for report)
• Save a screenshot showing the measurement of the time constant.  (Needed for report)
• Download the output data from the scope, you'll use this for a curve fit.  (Needed for report).  Refer to lab 1 for directions on how to save data if you don't remember how to do this.
2. Double the resistance and repeat the measurements of part a (though you needn't do the last step of downloading the data unless you want to).  If you are already at the maximum resistance, you can halve it instead of doubling.
3. Set the resistance to its original value.  Predict and then observe the output if the capacitance is doubled.   No measurements or recording necessary.  (If you are already at the maximum capacitance, you can halve it instead of doubling)
4. Put the resistance and capacitance back to the original values.  Set the triggering so that a rising edge of the input is at the center of the screen, and repeat the measurements of part a (though you needn't do the last step of downloading the data unless you want to).

### Circuit 2: Another 1st order RC circuit.

Remember: Don't change the amplitude or DC offset of the function generator!

Connect the circuit below with Vi coming from the function generator, R=1kΩ, C=1μF, and your function generator set to about 50 Hz (the exact frequency isn't important).  For this circuit it is easier to use the large resistor and capacitor boxes than it is to use the breadboards.

1. Set the triggering so that a rising edge of the input is at the center of the screen (trigger on the input, slope set to rising edge)  and set your scales as large as is convenient so that you can get an accurate measurement.
• Measure and record the initial and final values of the input and output.  (Needed for report)
• Measure and record the time constant by finding where the output voltage has gone through 63% of its total change.  (Needed for report)
• Save a screenshot.  (Needed for report)
• You may also want to save the output data so that you can do a curve fit, though this isn't necessary.
2. Predict and then observe the output if the resistance is doubled.   No measurements or recording necessary.  (If you are already at the maximum resistance, you can halve it instead of doubling)
3. Set the resistance to its original value.  Predict and then observe the output if the capacitance is doubled.   No measurements or recording necessary.  (If you are already at the maximum capacitance, you can halve it instead of doubling)
4. Put the resistance and capacitance back to the original values.  Set the triggering so that a falling edge of the input is at the center of the screen.  Predict and then observe the output.   No measurements or recording necessary.

### Circuit 3:  A 1st order LR circuit

Use the same the amplitude and DC offset that you used for circuit 1!

Connect the circuit below with Vi coming from the function generator, R=1kΩ, L=112mH, and your function generator set to a frequency that allows the output to come to steady-state before the input changes.

1. Set the triggering so that a rising edge of the input is at the center of the screen (trigger on the input, slope set to rising edge)  and set your scales as large as is convenient so that you can get an accurate measurement.
• Measure and record the initial and final values of the input and output.  (Needed for report)
• Measure and record the time constant by finding where the output voltage has gone through 63% of its total change (use the scope's cursors to measure τ).  (Needed for report)
• Save a screenshot showing the measurement of the time constant.  (Needed for report)
• You may also want to save the output data so that you can do a curve fit, though this isn't necessary.
2. Predict and then observe the output if the resistance is doubled.   No measurements or recording necessary.

### Circuit 4:  The 555 Oscillator - another application of RC circuits

In this lab, we'll use an RC circuit to create an oscillator with the help of an LM555 timer. Hook up the oscillator circuit  below with RA=33kΩ, RB=20kΩ and C=0.1μF.  Use Vcc=5 V.  The circuit is repeated below for your convenience. Note that I have put the output of the LM555 through a 1kΩ potentiometer (located on the bottom of your breadboard) so you can control the volume.

 RA=33kΩ RB=20kΩ C=0.1μF Vcc=5 V

What do you hear from the speaker?  (If you don't hear anything try turning the potentiometer)

1. Observe pin 6 and pin 3 on the oscilloscope for at least one full oscillation.
2. Get a screen shot. You will need this for your report.
3. Save the data from pin 6 on your computer (refer to lab 1 for directions on how to save data if you don't remember).  You will need this data for your report.
4. Measure and record the frequency of oscillation.  You will need this for your report.
5. Make sure you understand why the circuit oscillates.
6. Try this - put your fingers across the leads of the capacitor or the resistors.  What happens?

Links to hydraulic 555 simulator from prelab (if you want to play with it): Matlab555.m, Matlab555.fig.  Put both files in the same directory.  Open Matlab555.m with Matlab, and then run it.

### Circuit 5:  The 555 Oscillator from Lab 3 (blinky and squealy)

This part is optional.

Set the potentiometers to their midpoints.  Measure the frequency of the squealy oscillator under three conditions:

1. button not pushed,
2. button pushed and LED of blinky stage is on, and
3. button pushed and LED is off.

## Before You Leave

Make sure the lab is cleaned up, and that you have all the information you will need for your report (see below).  Make sure the resistors are put back in the proper drawers -- either measure them with an Ωmeter, or read the color-code (how to read the color-code).

Things you need for your report.

email me with any comments on how to improve the information on this page (either presentation or content), or to let me know if you had any particular difficulties with this lab.