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Spring 2020
/ Circuits and Electronics
Spring 2020

Circuits and Electronics (Spring 2020)

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In Lab 2 we learned how to make a 4-bit DAC with the Teensy. Unfortunately, our DAC could only drive a piezo speaker, which is a high-resistance load, and thus does not "load down" the DAC too much, as we learned in ex03.

In Lecture, we have now introduced op-amps, which can be used to fix this problem.

In this mini-project, you will use the tools you've learned thus far in class to adapt your DAC circuit in order to drive a real speaker. In addition, the circuit should implement volume control.

1) Specifications

Here are the specs:

  • You'll be driving a little coil speaker that we have in lab. The data sheet for the speaker is here. The resistance of the speaker is around 32 \Omega.

  • You want to adapt your DAC circuit by incorporating op-amp(s) in order to be able to drive the speaker to its continuous power rating (0.2 W) or to the max output voltage of the Teensy (a sinusoid of 3.3 Vpp), whichever is lower. Note that the power here is a the RMS power rating, which for a sinusoid of V_m peak-to-peak voltage, is \frac{(V_m)^2}{8R}. Don't go beyond these limits, or the speaker will blow!

  • In addition, you'll want to incorporate volume control in your op-amp circuit. You can use 10 k\Omega potentiometers ("pot") that are found in your lab kit or around the lab. If you are unfamiliar with the circuit diagram of a pot, a staff member can show you (or look it up online).

  • For op-amp, we recommend using an LM358. You'll want to power this with a dual supply. Also note that output voltage will only go to within 1.5 V of the positive rail and to within ~0 V of the negative rail.

  • We strongly recommend that you use a 100 \mu F capacitor in-between the circuit and the speaker. This will block any DC voltage from getting to the speaker, which will help prevent speaker failure. The 100 \mu F capacitors in the lab are electrolytic capacitors. It is essential that you connect these as shown:

  • The speaker and op-amp can be found at the staff table.

2) Grading

When your system is ready, create a short video of it working that goes over the following:

  • show a schematic of your circuit and explain:

    • why you chose this circuit
    • how you chose the values of any resistors in the circuit
  • play music from Lab 2 through the speaker:

    • adjust the volume from no sound to full volume
  • show a scope trace of the input (from the DAC) and output (at the speaker) signals, showing how the output amplitude changes when you adjust the volume, while the input amplitude does not. Point out the volts/div and secs/div on the display.

You must upload your video to Youtube or any other site that has a timestamp associated with it and is accessible by the staff.

Enter the url for the video

Grading is explained on the grading page. Grading is binary, and depends not only succesful demo but also demonstrating that you understand how the circuit works.

This mini-project will add 0.8 percentage points to your grade (for reference, a pset or lab is worth about 1.6% of your final grade).

The staff can provide only very limited help with the mini-project.

We will grade projects that have videos submitted by midnight March 6th.

Remember, this is supposed to be fun. Enjoy yourself!