ECE 201L Circuit Analysis Laboratory
Lab 9

This lab introduces the 555 timer chip and explores the transient and AC behavior of capacitors and inductors.

Attachment: lab9.zip.

Do the following exercises. Report your results by editing the attached Word document and submitting it in Isidore. Submit one report per group.

1. Build a circuit that flashes an LED on and off. Use a 555 timer for this circuit. Start with C = 1 µF, R1 = 1kΩ, and R2 = 220 kΩ. You may leave pin 5 disconnected. Connect your circuit to an oscilloscope and measure the frequency. Capture an oscilloscope image, showing the measured frequency, and include it in your report.

   

   Verify the formula for the frequency, given below.

   

2. Modify the above circuit to flash the LED on and off about 8-10 times per second. Report your selected values for C, R₁, R₂ and document the circuit by including an oscilloscope image.

3. Construct the following RC circuit. Use a resistance of 1-10kΩ and a capacitance of 10 µF - 0.2 mF.

   

   Set the signal generator for a square wave. Adjust the oscilloscope to obtain a display similar to that shown below.

   

   Display the rise and fall times on the measurement line. Capture at least one image to include in your report.

   Calculate the time constant from the rise/fall times, t_rise = 2.2 τ. Also calculate the time constant τ = RC. Explain or discuss any differences between these calculations.

4. Vary the resistance and record the rise and fall times. Pick at least five values from 10 kΩ to 100 Ω. Plot rise or fall time vs. resistance and fit a straight line. Find the capacitance and the frequency generator output resistance from this line. Note that the slope should be proportional to the capacitance and the x-intercept the frequency generator output resistance.

In each of the following exercises you will need to record the following measurements from the oscilloscope.

• Frequency
• Peak-to-peak voltage of the input waveform
• Peak-to-peak voltage of the output waveform
• Phase difference between input and output

These measurements can all be made using the electronic tools in the oscilloscope. The image below shows a sample measurement. The input waveform is on the top and The output waveform is on the bottom.

Unfortunately, the oscilloscopes in KL 231 display only three measurements at a time, so one value must be recorded manually.

5. Build the circuit shown below, using the same resistor and capacitor as in exercise 3 above. In this exercise, set the signal generator for a sine wave.

   

   Measure the AC response (as described above) for a number of frequencies from 100 Hz to 10 kHz. Determine whether the response is greater at low frequencies or high frequencies. Does the response lead or lag the input?

6. Measure the inducatance of the two inductors included in your plastic box of parts. Report your results and use the smaller value in the next exercise

7. Replace the capacitor with an inductor, as in the circuit below.

   

   Measure the AC response (as described above) for a number of frequencies from 100 Hz to 10 kHz. Determine whether the response is greater at low frequencies or high frequencies and whether the response phase leads or lags the input.

8. Examine the oscilloscope trace example for AC response I provided above. From your observations, you should be able to tell whether this is a RC circuit or an RL circuit. Report your conclusion.

Maintained by John Loomis, last updated 16 December 2010