Sepideh Mohammadi — PCB Portfolio

LAB 02 Solderless Breadboard

555 Timer Lab Report — NE555 vs TLC555

This lab compares the NE555 bipolar timer and the TLC555 CMOS timer in astable mode. I built the circuit on a solderless breadboard, measured frequency, rise time, fall time, and LED load current, and then selected the better timer for the PCB design.

≈463 Hz NE555 measured frequency

89 ns NE555 rise time

14.8 ns TLC555 rise time

NE555 selected for PCB design

Step 01

Understand the 555 Timer Circuit

A 555 timer can generate a repeating square wave without an external trigger. The frequency and duty cycle are controlled by two resistors, RA and RB, and one timing capacitor. This makes the 555 timer useful for blinking LEDs, clock pulses, tone generation, and PWM-style timing.

Step 02

Build the Circuit on a Solderless Breadboard

I placed the 555 timer circuit on a solderless breadboard and powered it with 5 VDC. The output was connected to the oscilloscope so I could measure frequency, duty cycle, rise time, fall time, and LED load current using different resistors.

Step 03

Measure the NE555 Timer

For the NE555 timer, the measured frequency was about 463 Hz, close to the theoretical value of about 481 Hz for RA = 1 kΩ, RB = 1 kΩ, and C = 1 µF. The measured rise time was 89 ns, and the fall time was 33.8 ns. The NE555 also provided higher output current when driving the LED load.

Step 04

Measure the TLC555 Timer

I repeated the same test with the TLC555 CMOS timer. The measured frequency was close to the NE555 result, but the TLC555 switched much faster. The measured rise time was about 14.8 ns, and the fall time was about 11.1 ns.

Step 05

Compare the Results

Both timers produced nearly the same oscillation frequency, but their output behavior was different. The TLC555 had faster switching edges, while the NE555 provided stronger current drive for the LED load.

Measurement	NE555	TLC555
Rise time	89 ns	14.8 ns
Fall time	33.8 ns	11.1 ns
Frequency	≈463 Hz	≈485 Hz
LED current with 1 kΩ	2.5 mA	2.7 mA
LED current with 47 Ω	76 mA	61.7 mA

Step 06

Final Decision for PCB Design

Why I selected the NE555

The TLC555 is faster and produces sharper switching edges.
The NE555 provides stronger current drive, which is useful for driving LED loads.
The NE555 rise and fall times are still acceptable because the target frequency is only around 500 Hz.
For the PCB design, the NE555 was selected because it is more robust for this load-driving application.

NE555TLC555Astable Mode OscilloscopeBreadboardPCB Decision

LAB 05 SBB Experiment

PDN and Slammer Circuit — Full Lab Report

This lab studies how sudden current switching affects a 9 V power rail using a slammer circuit. I built the circuit on a solderless breadboard, measured voltage droop during slow and fast switching edges, and compared how 1 µF and 1000 µF decoupling capacitors reduce power rail noise when placed near or far from the switching transistor.

1.811 V fast-edge drop without decoupling

640 mV fast-edge drop with 1 µF near

246 mV fast-edge drop with 1000 µF near

2.06 Ω estimated Thevenin resistance

Main Results

Fast switching caused a much larger voltage drop than slow switching.
Without decoupling, the fast-edge voltage drop reached about 1.811 V.
A 1 µF capacitor reduced the drop but was not enough to fully stabilize the rail.
A 1000 µF capacitor placed near the transistor gave the strongest improvement.
Capacitor placement close to the switching device reduced loop inductance and improved performance.

Open Lab 5 Full PDF →

PDNSlammer CircuitTIP41C MCP601DecouplingLoop InductancePower Integrity

LAB 15 Analysis

Hex Inverter High-Speed Digital Circuit — Switching Noise Report

This lab compares switching noise, edge speed, and rail stability for three layout cases: good layout, good layout with the decoupling capacitor far away, and bad layout. The goal is to show how return path quality and capacitor placement affect high-speed digital behavior.

2.25 ns good layout rise time

4.86 ns bad layout rise time

274 mV good rail collapse

1031 mV bad rail collapse

Main Results

Good layout produced the fastest and cleanest transitions.
Bad layout showed slower edges and larger rail noise due to poor return paths.
Moving the capacitor far away also degraded performance, even with a solid return plane.
The experiment confirms that close decoupling and continuous ground planes are critical for SI and PI.

Layout	Rise Time	Fall Time	Rail Collapse
Good Layout	2.25 ns	1.89 ns	274 mV / 181 mV
Bad Layout	4.86 ns	3.92 ns	1031 mV / 2763 mV
Good, Cap Far	3.78 ns	2.47 ns	709 mV / 829 mV

Open Lab 15 Full PDF →

High-Speed DigitalHex InverterReturn Path DecouplingRail CollapseSignal Integrity

BOARD 02 PCB Design

Hex Inverter Board — Good vs Bad Layout Report

This report demonstrates how PCB layout decisions affect switching noise and power delivery network behavior. The board compares two identical hex inverter circuits: one with good layout practices and one with poor layout practices. The good layout uses close decoupling and a continuous return plane, while the bad layout uses longer return paths and far decoupling.

1.76 ns good layout rise time

5.58 ns bad layout rise time

0.917 V good rail compression

1.85 V bad rail compression

Main Results

The good layout showed faster switching edges and lower power rail disturbance.
The bad layout had slower transitions because of poor return paths and far decoupling.
Good layout rise time was about 1.76 ns, while bad layout rise time was about 5.58 ns.
Good layout fall time was about 2.64 ns, while bad layout fall time was about 9.86 ns.
The experiment confirmed that close decoupling and a continuous ground return plane improve SI and PI.

Measurement	Good Layout	Bad Layout
Rise time	1.76 ns	5.58 ns
Fall time	2.64 ns	9.86 ns
Quiet High Vp-p	690 mV	1900 mV
Rail compression	0.917 V	1.85 V
Output Thevenin resistance	≈66 Ω	—

Open Board 2 Full PDF →

Hex InverterSN74AHC14DRNE555 Good LayoutBad LayoutGround Plane DecouplingSwitching Noise

LAB 16 Signal Integrity

Differential vs Single-Ended Signals — Ground Noise Study

This lab studies how ground noise affects analog measurements. I used a TMP36 temperature sensor, ADS1115 16-bit ADC, Arduino Uno, and I²C communication to compare single-ended and differential measurements under normal and noisy ground conditions.

ADS1115 16-bit ADC

±1.024 V ADC range

31.25 µV LSB size

93 mV single-ended noise

Main Results

With no injected ground noise, both single-ended and differential measurements read about 0.734 V.
With ground-current injection, the single-ended reading shifted because it used the local ground reference.
The differential measurement rejected the common-mode ground shift and stayed stable.
This shows why differential measurement is more robust for noisy analog sensing.

Open Lab 16 Full PDF →

Differential SignalSingle-EndedTMP36 ADS1115I²CGround Noise

LAB 18 Power Analysis

Measuring Inrush Current and Steady-State Current

This lab measures steady-state and inrush current in a 555 timer circuit. I used a 1 Ω sense resistor and oscilloscope math mode to measure the voltage across the resistor and convert it to current.

1 Ω sense resistor

≈360 Hz 555 output frequency

≈3.5 mA steady-state current

≈1 A inrush peak

Main Results

The 555 timer output was measured as a square wave at approximately 360 Hz.
The steady-state voltage across the 1 Ω sense resistor was about 35 mV, corresponding to about 3.5 mA.
During power-up, the capacitor charging transient produced a short inrush current spike.
The experiment demonstrates how current can be measured safely using a sense resistor and oscilloscope math.

Open Lab 18 Full PDF →

Inrush CurrentSteady-State CurrentSense Resistor Oscilloscope Math555 Timer

BOARD 03 PCB Design

Golden Arduino — Custom Arduino Uno Board Report

This board is a custom Arduino Uno-style design built with ATMEGA328, CH340G USB-to-UART, a 3.3 V LDO, USB mini-B, ICSP bootloading, ferrite filtering, crystals, status LEDs, and test points for validating power, USB, I²C, UART, and SPI signals.

ATMEGA328 main MCU

CH340G USB-UART

16 MHz MCU crystal

ICSP bootloaded successfully

Main Results

The ATMEGA328 was successfully bootloaded using the ICSP header.
The board includes USB, power, reset, crystal, and test-point features similar to an Arduino Uno.
The design supports bring-up and signal validation through labeled test points.
The project demonstrates a full PCB design workflow from schematic to functioning embedded board.

Open Board 3 Full PDF →

Golden ArduinoATMEGA328CH340G ICSPUSB-UARTEmbedded PCB

LAB 21 & 22 VRM Characterizer

VRM Characterizer — Breadboard Instrument Report

This lab builds a breadboard version of the VRM Characterizer. The circuit applies a controlled load current, measures open-circuit and loaded voltage, and computes the Thevenin resistance of voltage sources using Arduino, DAC, ADC, op-amp, MOSFET, and a sense resistor.

MCP4725 DAC output control

ADS1115 ADC measurement

48–52 Ω function generator Rth

1.34–1.39 Ω Arduino VRM Rth

Main Results

The circuit measured Thevenin voltage and resistance by sweeping load current.
The function generator stabilized around 48–52 Ω at higher current values.
The Arduino VRM stayed nearly flat around 1.34–1.39 Ω over the measured current range.
The lab validated the core measurement concept before building the final Instrument Droid PCB.

Open VRM Characterizer Full PDF →

VRMThevenin ResistanceMCP4725 ADS1115MOSFET LoadArduino

BOARD 04 PCB Design · Capstone

Instrument Droid — Automatic Rth Measurement PCB Report

Instrument Droid is a custom PCB designed to automatically measure the Thevenin resistance of a voltage supply up to 12 V. The board integrates ATMEGA328 control, DAC/ADC measurement, op-amp buffering, MOSFET loading, OLED display, smart LEDs, buzzer, USB interface, and test points for validation.

4-layer instrument PCB

12 V max supply under test

OLED Rth display

ATMEGA328 control MCU

Main Results

The board was designed to measure Rthevenin automatically for voltage supplies up to 12 V.
The design combines DAC current control, ADC voltage sensing, op-amp buffering, and MOSFET loading.
The ATMEGA328 was bootloaded and the board was validated through test points and scope measurements.
The circular PCB served as the capstone design for the PCB design and manufacture course.

Open Instrument Droid Full PDF →

Instrument DroidRth MeasurementATMEGA328 ADC/DACOLED4-Layer PCBCapstone PCB

Skill	Before	After	Evidence
KiCad PCB Design	2/5	4/5	Schematic → Layout workflow
Altium Designer	1/5	4/5	Board 2–4 full designs
DFM / DRC / Gerbers	1/5	5/5	All boards ordered from JLC
Signal Integrity	1/5	4/5	Good/bad layout Lab 15
Power Integrity	1/5	4/5	Lab 5 decoupling study
Oscilloscope / Probing	2/5	5/5	Inrush, NFE, quiet-node
Embedded / Arduino C	2/5	4/5	Board 4 firmware, I²C driver
Thevenin / PDN Analysis	1/5	5/5	Lab 5, Board 3 & 4

Sepideh
Mohammadi

Engineer who builds
things that work.

Current Proficiency

All Projects

555 Timer Lab Report — NE555 vs TLC555

Understand the 555 Timer Circuit

Build the Circuit on a Solderless Breadboard

Measure the NE555 Timer

Measure the TLC555 Timer

Compare the Results

Final Decision for PCB Design

PDN and Slammer Circuit — Full Lab Report

Hex Inverter High-Speed Digital Circuit — Switching Noise Report

Hex Inverter Board — Good vs Bad Layout Report

Differential vs Single-Ended Signals — Ground Noise Study

Measuring Inrush Current and Steady-State Current

Golden Arduino — Custom Arduino Uno Board Report

VRM Characterizer — Breadboard Instrument Report

Instrument Droid — Automatic Rth Measurement PCB Report

Skills Growth

Core Competencies

Learning Outcomes

Let's connect.

SepidehMohammadi

Engineer who buildsthings that work.

Current Proficiency

All Projects

555 Timer Lab Report — NE555 vs TLC555

Understand the 555 Timer Circuit

Build the Circuit on a Solderless Breadboard

Measure the NE555 Timer

Measure the TLC555 Timer

Compare the Results

Final Decision for PCB Design

PDN and Slammer Circuit — Full Lab Report

Hex Inverter High-Speed Digital Circuit — Switching Noise Report

Hex Inverter Board — Good vs Bad Layout Report

Differential vs Single-Ended Signals — Ground Noise Study

Measuring Inrush Current and Steady-State Current

Golden Arduino — Custom Arduino Uno Board Report

VRM Characterizer — Breadboard Instrument Report

Instrument Droid — Automatic Rth Measurement PCB Report

Skills Growth

Core Competencies

Learning Outcomes

Let's connect.

Sepideh
Mohammadi

Engineer who builds
things that work.