Reference · The compressed essence

Rules of Thumb

Every actionable rule from the course, on one page. Print me; tape me to the bench.

These are heuristics, not laws — they keep beginners out of trouble without math. When a project pushes a limit, that's the time to dig into the detail (or ask your teacher).

Current & protection

An LED always needs a series resistor. ~220–330 Ω on a 5V pin. The LED can't limit its own current and will burn out. Lesson 1
Keep current under control = the whole game. Most "magic smoke" is an accidental short — a near-zero-resistance loop. Lesson 1

Schematics & breadboards

To read any schematic, trace the loop from + to −. Name each part you pass. Lines are wires; joined points are one node. Lesson 2
Breadboard: columns of 5 connect up-down; rails connect along the edges. The center gap splits each column. Put power/ground on the rails. Lesson 2

Series & parallel

Series = same current, voltage splits. Parallel = same voltage, current splits. Power your modules in parallel so each gets full voltage and works independently. Lesson 3

Power & safety (low-voltage)

Know your board's voltage before wiring its pins. ESP32 = 3.3V, Arduino Uno = 5V. 5V into a 3.3V pin can destroy it — use a level converter. Lesson 4
A pin gives ~20–40 mA — enough for an LED, not a motor. Hungry loads get their own supply + a transistor or relay. Lesson 4
All grounds tie together. Multiple + supplies are fine, but every part shares one common GND. Lesson 4

Inputs

Never leave an input pin floating — use a pull resistor. ~10 kΩ pull-up/pull-down, or the built-in INPUT_PULLUP. A floating pin reads random noise. Lesson 5
If a button "double-fires," it's contact bounce. Debounce in code (ignore changes for ~20–50 ms) or with a small capacitor. Lesson 5

Driving loads

Use a transistor to let a pin switch a big current. Pin → base through ~1 kΩ. Load on its own supply. MOSFET for hungry loads. Lesson 6
Coil or motor? Add a flyback diode. Absorbs the turn-off voltage spike that would kill your transistor. Lesson 6

Mains (treat with respect)

Mains can kill — power off and unplugged before touching it, always. Switch the hot wire only. Everything in an enclosure, never on a breadboard. Lesson 7
Use a relay module; check its contact rating exceeds your load. It bundles the driver, flyback diode, and isolation. Unsure? Use a certified smart plug. Lesson 7

Components

Don't memorize resistor colors — use an app or multimeter. ¼ W is plenty for signal-level work. Standard values: 220, 330, 1k, 4.7k, 10k, 100k. Lesson 8
A voltage divider is for signals, not for powering loads. Two resistors tap a smaller voltage; equal resistors = half. Drawing current breaks the ratio. Lesson 8
Put a 0.1 µF ceramic cap beside every chip's power pin. Decoupling kills random resets/glitches. Ceramic 104 = 0.1 µF, no polarity. Lesson 9
Electrolytic caps are polarized — stripe = −, long leg = +. Backwards can pop. Ceramics are non-polarized and safe either way. Lesson 9
Potentiometer: outer pins = power/ground, middle = analog signal. It's an adjustable divider. ESP32 analogRead gives 0–4095; map to what you need. Lesson 10
Diode band = cathode; current flows toward the band. Flyback across coils; a series diode protects against reversed power (Schottky drops less). Lesson 11
New chip? Find pin 1 (notch/dot), give power+GND+0.1 µF, copy the datasheet app circuit. Check operating voltage first. Pins count counter-clockwise from pin 1. Lesson 12

Microcontroller & signals

Keep a pinout diagram for your exact board. It shows PWM/analog/I²C/SPI pins and which to avoid (input-only, boot pins). ESP32 = 3.3V, not 5V-tolerant. Lesson 13
Match the signal to the task: digital (on/off), analog-in (level), PWM (how much). PWM "fakes" analog by varying duty cycle — for dimming, motor speed, servo pulses. Lesson 14
Default to I²C sensors; wire SDA→SDA, SCL→SCL, power, ground. Many devices on two wires + a library. SPI for speed (displays/SD), UART for the PC link. Lesson 15

Motion & the physical world

Never wire a motor straight to a pin — use a driver + its own supply. 2 direction pins (both same = brake) + 1 PWM pin = direction & speed. Share ground. Lesson 16
Servo = angle (3 wires, Servo library); stepper = precise steps (needs a driver). Power servos (beyond tiny) from a separate supply, common ground; only signal goes to a pin. Lesson 17
Before wiring a sensor, check its interface and voltage. Digital → digitalRead; analog → divider + analogRead; smart → I²C/SPI library. Sense → decide → act. Lesson 18

Connectivity

ESP32 Wi-Fi is 2.4 GHz only; keep credentials in an uncommitted secrets file. Station mode joins your network; the router gives it an IP. Add reconnection logic. Lesson 19
HTTP = request/response: browser asks, ESP32 answers; URLs act as commands. Great for one gadget on demand. You have to poll — doesn't push events well. Lesson 20
MQTT = publish/subscribe via a broker; push-based and many-to-many. Topics like home/room/temp. Run your own broker (Mosquitto) for privacy. Lesson 21
For Home Assistant, start with ESPHome; use raw MQTT for full control. Both make your gadget an entity you can automate. HA runs locally — private, no cloud. Lesson 22

Guitar effects

A guitar signal is small, weak AC — handle it gently; start amp volume low. Rig: guitar → input jack → breadboard → output jack → amp, on 9 V. Common ground throughout. Lesson 23
On 9 V, bias the signal to ~4.5 V and use coupling caps (pass AC, block DC). High input impedance (~1 MΩ) avoids tone suck. This input skeleton starts almost every pedal. Lesson 24
Gain = output ÷ input; an op-amp's two resistors set it (one as a pot = drive knob). Modest gain = clean boost; too much gain → clipping → dirt. Lesson 25
Clipping makes dirt: diodes in feedback = soft (overdrive); to ground = hard (distortion). Diode flavor: germanium (warm) · silicon (tight) · LED (loud). Asymmetric = even harmonics. Socket & experiment. Lesson 26
Tone = RC filter: low-pass cuts highs (the tone knob); bigger cap → lower cutoff. Before clipping = tighter dirt; after = tames fizz. Passive filters lose some volume. Lesson 27
Delay/echo → PT2399 chip; modulation (tremolo/chorus) → an LFO. Time-based effects are more advanced — build dirt first, then layer these on the same fundamentals. Lesson 28
Build a pedal one stage at a time, testing as you go. Confirm a clean boost before adding clipping. No sound? Check ground, polarity, bias ~4.5 V, op-amp pin 1. Lesson 29

Bench skills & debugging

Multimeter: volts across (powered on); continuity/resistance/diode (powered off). Expect rail ≈9 V, bias ≈4.5 V, op-amp output ≈4.5 V at rest. Diode reads ~0.6 V Si forward. Lesson 30
Debug in order: look → power/ground → bias points → trace the signal (half-split). You can only debug as far as you know what each node should do. Change one thing at a time. Audio probe = hear the signal. Lesson 32
Noise: hum = grounding/interference; hiss = high gain. Try a battery first. Common ground, no ground loops, decoupling caps, short signal wiring away from screens/wall-warts. Lesson 33

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