Week 9

Blogsheet week 9

1.       Measure the resistance of the speaker. 

We measured the resistance of the speaker to be 9.2 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)

Figure 1: Test setup for the speaker.

Video 1: Frequency explanation

3. Fill the following table. 

Table 1:Frequency of circuit set at 5V with our observations

Frequency	Observation
300 Hz	Small humming sound, can barely hear it
500 Hz	Can start to hear the sound
1.0 kHz	Pitch is higher, kind of like the severe weather notice tone on tv
3.5 kHz	Pitch is even higher,
5.0 kHz	Pitch is even higher now,
12 kHz	Pitch is so high, it’s kind of like dog whistle but louder than what you’d normally hear from one

According to our table, we concluded that the high pitch sound corresponds to high frequency waves while the low pitch sound corresponds to low frequencies. We didn't hear anything until we reached 300 Hz; the pitch below that frequency was much quieter and it was hard trying to hear anything come out of the speaker while hearing other high frequencies in the room. 

4.       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.

Fill the following table.

Table 2: Oscilloscope output and observation at certain resistor values

Resistor value	Oscilloscope output	Observation
47 Ω	Freq: 1.000 kHz; 128 mV pk-pk 0.26 Vrms	Higher the frequency, the shorter the period
820 Ω	Freq: 1.000 kHz; 50.0 mV pk-pk 0.025 Vrms	Higher the frequency, the shorter the period. The amplitude is slightly different from a different resistor value

According to our table, the frequencies were the same. However, we measured different peak-to-peak and rms values, as shown in the table. The amplitude was also different when we used 820  Ω (see pictures below). 

Image 1: The input of the oscilloscope

Image 2: Oscilloscope reading with 47 Ω

Image: Oscilloscope reading with 820 Ω

5.       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.

Figure 2: Test setup for the high pass filter.

a.       Explain the operation.  (video)

Video 2: 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.


*For our Vin theoretical: 5.6 mV

Data Points	Frequency (kHz)	Vout (rms)	Vout (rms) / Vin (rms)
1	1.0	0.68 V	0.12 V
2	1.5	1.07 V	0.19 V
3	2.0	1.32 V	0.23 V
4	2.5	1.67 V	0.29 V
5	3.0	1.93 V	0.34 V
6	3.5	2.20 V	0.39 V
7	4.0	2.45 V	0.43 V
8	4.5	2.68 V	0.47 V
9	5.0	2,90 V	0.51 V
10	5.5	3.10 V	0.55 V
11	6.0	3.28 V	0.58 V
12	6.5	3.45 V	0.61 V
13	7.0	3.60 V	0.64 V
14	7.5	3.74 V	0.66 V
15	8.0	3.86 V	0.68 V

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

Graph 1: High pass filter. Vout/Vin with respect to the frequency.

d.      What is the cut off frequency by looking at the plot in b?
By looking at our plot, the cut off frequency is just about at 0.7 kHz

6.       Design the circuit in 4 to act as a low pass filter and show its operation. Where would you put the speaker? 

In the circuit, the speaker would be in parallel with the capacitor.

Figure 3: Low pass filter schematic. Vout represents the speaker

a.       Explain the operation.  (video)

Video 3: Operation of low 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.

Vin = 5.6 V

Data Points	Frequency	Vout (rms)	Vout (rms) / Vin (rms)
1	100 kHz	89.3 mV	0.016
2	200 kHz	294 mV	0.052
3	300 kHz	280 mV	0.05
4	400 kHz	120 mV	0.021
5	500 kHz	55.25 mV	0.0098
6	600 kHz	28 mV	0.005
7	700 kHz	15 mV	0.0027
8	800 kHz	0.80 mV	0.00014
9	900 kHz	0.41 mV	0.000072
10	1 MHz	0.34 mV	0.00006
11	1.1 MHz	0.12 mV	0
12	1.2 MHz	0 mV	0
13	1.3 MHz	0 mV	0
14	1.4 MHz	0 mV	0
15	1.5 MHz	0 mV	0

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

Graph 2: Low Pass Filter. Vout/Vin with respect to the frequency

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

The cut off frequency in the plot is at 0.05 kHz.

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: Circuit speculation