Last week you drew RC filters in simulators. Today you'll build them with real components on a breadboard, drive them from a headphone output, and listen through a piezo speaker amplified by an LM386.
The headphone plug has three wires: Left, Right, and Ground. A 3.5 mm TRS (Tip-Ring-Sleeve) jack assigns: Tip = Left, Ring = Right, Sleeve = Ground. Identify which wire on your plug is which.
Answer
2.
We need a mono signal, but the headphone output is stereo. We can't just short L and R together — the two output amplifiers in your phone would fight each other. Instead, we sum them through resistors: put a 10 kΩ in series with each channel, then join the far ends. Why do the resistors prevent the channels from fighting?
Think of it as two people pushing on the same door. The resistors are like springs between each person and the door — they absorb the difference.
Answer
3.
The LM386 module has three inputs: VCC, IN, and GND. In one sentence each, what does each connection do?
Answer
4.
Why do we need the LM386 at all? A headphone output is already an amplified signal. Why can't we connect the filter directly to the piezo speaker?
Answer
Part 2
Build: Direct Connection (No Filter)
Breadboard
First, verify your signal chain works without any filter. This is your baseline.
Breadboard reminder: The two long rails at the top and bottom are connected horizontally (power buses). The short columns in the middle are connected vertically within each half, split by the center channel.
Connect the 9V battery + to the red power rail, battery − to the blue ground rail.
Connect the headphone plug ground wire to the blue ground rail.
Mono summing: Insert a 10 kΩ resistor from the L wire to an empty column. Insert another 10 kΩ resistor from the R wire to the same column. This column is now your signal column — a safe mono sum of both channels.
Wire the LM386 module: VCC → red rail, GND → blue rail, IN → signal column.
Connect the piezo speaker to the speaker lugs on the LM386 module.
Play music from your phone. You should hear it through the piezo.
Safety note: The LM386 module already has its own output coupling capacitor built in. Always connect or disconnect the speaker wires before connecting the 9V battery — hot-swapping while powered can cause loud pops that are unpleasant and could damage the piezo.
Q1.
Describe what the piezo sounds like compared to your phone speaker. Is anything missing? What frequencies seem emphasized?
Observation
Q2.
Open the sweep tone generator (onlinetonegenerator.com/frequency-sweep-generator.html) or play the AudioCheck sine sweep. Start with your laptop volume very low, then bring it up until the piezo is just audible. Play a 20 Hz–5 kHz sweep. At roughly what frequency does the piezo start producing sound? Where is it loudest?
Observation
CSCM 373-01 — Fuzzbox Physics
Breadboard Filters (continued)
Part 3
Build: Low-Pass Filter (Treble Cut)
Breadboard
Now insert an RC low-pass filter between the headphone signal and the LM386 input. Use R = 22 kΩ and C = 10 nF.
A.
Before wiring, sketch the schematic. The signal enters on the left, the output goes to the LM386 on the right. Draw R in series and C to ground.
Low-pass schematic
Wiring steps: Disconnect the headphone signal wire from the LM386 IN. Insert the 22 kΩ resistor between the signal column and a new column (call it the output column). Insert the 10 nF capacitor between the output column and the ground rail. Reconnect the LM386 IN to the output column.
B.
Play the same music. Describe what changed compared to the unfiltered sound.
Observation
C.
Now play the sine sweep through the filter. Compare to your unfiltered sweep from Part 2. At roughly what frequency does the sound start getting quieter? This is your measured cutoff.
Measured cutoff
D.
Calculate the cutoff frequency using fc = 1 / (2πRC). Show your work. Does it match what you heard?
Calculation
E.
Now swap the capacitor for the 47 nF (473). What happens to the sound? Calculate the new cutoff frequency.
Observation + calculation
F.
Swap the resistor to 10 kΩ (keep 47 nF). What is the new cutoff? Does the sound change in the direction you predicted?
Prediction, calculation, observation
Part 4
Build: High-Pass Filter (Bass Cut)
Breadboard
Swap the positions of R and C. Use C = 100 nF and R = 22 kΩ.
A.
Sketch the schematic. The capacitor is now in series and the resistor shunts to ground.
High-pass schematic
Wiring steps: Remove the old R and C. Insert the 100 nF capacitor between the signal column and the output column. Insert the 22 kΩ resistor between the output column and the ground rail. LM386 IN stays on the output column.
B.
Play music again. Describe what you hear. What is missing compared to the unfiltered sound?
Observation
C.
Calculate the cutoff frequency for this high-pass filter (100 nF, 22 kΩ).
Calculation
D.
Replace the 100 nF with a 10 nF (103) capacitor. What happens? Calculate the new cutoff and explain why.
Observation + calculation
CSCM 373-01 — Fuzzbox Physics
Breadboard Filters (continued)
Part 5
Component Swap Table
Breadboard + Pencil
Fill in the table as you swap components. For each combination, calculate fc, predict the sound, then build and listen.
Filter type
R
C
Calculated fc
Prediction
What you heard
Low-pass
22 kΩ
10 nF
Low-pass
22 kΩ
47 nF
Low-pass
10 kΩ
47 nF
High-pass
22 kΩ
100 nF
High-pass
22 kΩ
10 nF
Part 6
Thinking It Through
Pencil
1.
In the low-pass filter, explain what the capacitor is physically doing to the high-frequency parts of the signal. Where does that energy go?
Answer
2.
You used the same formula fc = 1/(2πRC) for both low-pass and high-pass. How can the same R and C values give different behavior just by swapping positions?
Answer
3.
A guitar's tone knob is a low-pass filter: a potentiometer (variable R) in series with a capacitor to ground. When you roll the tone knob down, R decreases. What happens to fc? Does that match what you hear (darker sound)?
Answer
4.
The piezo speaker itself is not flat — it naturally emphasizes upper-mids and treble, acting as a kind of built-in bandpass filter. Did this make it harder or easier to hear the filter's effect? Which filter type (low-pass or high-pass) had the more obvious result on the piezo, and why?
Answer
Challenge: Bandpass Filter
If you finish early, try cascading both filters: a high-pass followed by a low-pass. Use C = 100 nF / R = 10 kΩ for the high-pass, then R = 22 kΩ / C = 10 nF for the low-pass. Calculate both cutoff frequencies, sketch the combined circuit, build it, and describe the resulting sound. What range of frequencies survives both filters? Does it sound like a telephone?