Week 9: 3/14/2016 - 3/18/2016

1.    Measure the resistance of the speaker.

The speaker had a resistance value of 8.6 Ω

2.Build the following circuit using a function generator setting the amplitude to 5V (0V offset). What happens when you change the frequency? (video)

You get different sounding tones as you change the frequency.

Video 1: This video shows the speaker function as you change the frequency on the function generator

Fill the following table.

Frequency (Hz)	Observation
0	No tone
200	buzzing
400	Low toned buzzing
600	High pitched tone
800-1000	Increasingly high pitched tone

Table 1: This table shows the relationship between the frequency and the tone.

3.    Add one resistor to the circuit in series with the speaker (first 47 Ω, then 820 Ω). Measure the voltage across the speaker. Briefly explain your observations.

When you change the resistor in the circuit you change the sound that the speaker makes. The pictures below show the two waves read by the oscilloscope.

Picture 1: This is the oscilloscope with the 47 ohm resistor on the speaker.

Picture 2: This is the oscilloscope with the 820 ohm resistor on the speaker.

Fill the following table.

Resistor Value Ω	Oscilloscope Output	Observation
47	930mv Pk-Pk	Sinusoidal wave with period = 2.035 ms
820	162mv Pk-Pk	Small wave that looked like noise, period = 2.6 ms

Table 2: This table shows the relationship between the output and the resistor attached to the speaker

4. Build the following circuit. Add a resistor in series to the speaker to have an equivalent resistance of 100 Ω. Note that this circuit is a high pass filter. Set the amplitude of the input signal to 8 V. Change the frequency from low to high to observe the speaker sound. You should not hear anything at the beginning and start hearing the sound after a certain frequency. Use 22 nF for the capacitor.

a.    Explain the operation.  (video)

Video 2: This video shows the operation of high pass filter 

b.    Fill out the following table by adding enough (10-15 data points) frequency measurements. Vout is measured with the DMM, thus it will be rms value.

Frequency kHz	Vout V	Vout/Vin V
0	.46	.082
10	.62	.109
20	1.19	.210
30	1.70	.300
40	2.12	.375
50	2.45	.432
60	2.71	.452
65	2.79	.493
70	2.88	.508
75	2.98	.523
80	3.03	.535
90	3.16	.558
100	3.26	.576
110	3.33	.588
120	3.41	.597
130	3.45	.610
140	3.49	.617

Table 3: This table compares the voltage out values compared to the frequency of a high pass filter.

c.    Draw Vout/Vin with respect to frequency using Excel.

Graph 1: This graph shows the output of a high pass filter. However, the X axis should be read in kHz, not Hz.

d.    What is the cut off frequency by looking at the plot in b?

The cut off frequency looks to be at about 60 kHz from the plot in b.

5.    Design the circuit in 4 to act as a low pass filter and show its operation. Where would you put the speaker? Repeat 4a-d using the new designed circuit

You put the speaker in parallel with the capacitor instead of in series as with a high pass filter.

a.    Explain the operation.  (video)

Video 3: This video shows a high pass filter circuit.

b.    Fill out the following table by adding enough (10-15 data points) frequency measurements. Vout is measured with the DMM, thus it will be rms value.

Frequency kHz	Vout V	Vout/Vin V
10	5.40	.954
20	5.21	.920
30	4.93	.870
40	4.63	.818
50	4.26	.753
60	3.90	.689
65	3.75	.673
70	3.59	.634
75	3.45	.609
80	3.32	.557
90	3.08	.524
100	2.85	.503
110	2.66	.469
120	2.49	.439
130	2.30	.406
140	2.19	.387

Table 4: This table compares the Vout to the frequency of a low pass filter

e.    Draw Vout/Vin with respect to frequency using Excel.

Graph 2: This graph shows the output of a low pass filter.

f.     What is the cut off frequency by looking at the plot in b?

The cut off frequency looks to be at about 90 kHz based off of the plot in b.

6. Construct the following circuit and test the speaker with headsets. Connect the amplifier output directly to the headphone jack (without the potentiometer). Load is the headphone jack in the schematic. Speculate the operation of the circuit with a video.

Video 4: This video shows the circuit with a microphone input and speaker output.