Electronics

Resistors & Components

Passive components used in cockpit builds — reading colour codes, calculating LED resistors, and choosing capacitors.

Resistors

Resistors are the most common passive component in any cockpit build. They protect LEDs from overcurrent, form voltage dividers for analog sensors, and provide pull-up or pull-down biasing for digital inputs.

Reading 4-Band Colour Codes

Colour	Digit	Multiplier	Tolerance
Black	0	×1	—
Brown	1	×10	±1%
Red	2	×100	±2%
Orange	3	×1 k	—
Yellow	4	×10 k	—
Green	5	×100 k	±0.5%
Blue	6	×1 M	±0.25%
Violet	7	×10 M	±0.1%
Grey	8	—	±0.05%
White	9	—	—
Gold	—	×0.1	±5%
Silver	—	×0.01	±10%

Common values for cockpit builds

Value	Use case
68 Ω – 100 Ω	LED current limiting from 3.3 V supply (standard LEDs ~20 mA)
220 Ω – 330 Ω	LED current limiting from 5 V supply; indicator LEDs at reduced brightness
1 kΩ – 10 kΩ	Base resistor for NPN transistors driving LEDs or relays
10 kΩ	External pull-up or pull-down for digital inputs
4.7 kΩ	I²C pull-up (SDA / SCL lines)

LED Series Resistor

Every LED needs a series resistor to limit current. Without one the LED will draw as much current as the supply can provide and burn out instantly.

Formula: R = (V_supply − V_forward) ÷ I_forward

LED colour	Typical V_f	Typical I_f	R from 3.3 V	R from 5 V
Red	1.8–2.2 V	20 mA	56–75 Ω → 68 Ω	140–160 Ω → 150 Ω
Yellow / Orange	2.0–2.2 V	20 mA	55–65 Ω → 68 Ω	140–150 Ω → 150 Ω
Green	2.0–3.0 V	20 mA	15–65 Ω → 33–68 Ω	100–150 Ω → 100 Ω
Blue / White	3.0–3.4 V	20 mA	0–15 Ω → use 33 Ω min	80–100 Ω → 100 Ω

Annunciator LEDs in cockpit panels are typically run at 5–10 mA (not 20 mA) for comfortable panel brightness. Use a higher resistance (220–470 Ω from 3.3 V) to keep the panel from being blinding in a dark room.

Pull-Up and Pull-Down Resistors

A floating GPIO pin (nothing connected) will give random readings. A resistor anchors the pin to a known voltage when the switch is open.

Configuration	Resistor connects	Idle (switch open)	Active (switch closed)
Pull-up	GPIO → 3.3 V	HIGH	LOW (switch to GND)
Pull-down	GPIO → GND	LOW	HIGH (switch to 3.3 V)

The ESP32 has built-in ~45 kΩ pull-up resistors on every GPIO — enable with pinMode(pin, INPUT_PULLUP). External 10 kΩ pull-ups provide a stronger reference and are preferred when the wire run is long (>30 cm) or the signal line is near power wiring that could induce noise.

NPN Transistors — Driving Higher Loads

An ESP32 GPIO can source/sink a maximum of 40 mA (12 mA recommended). For higher loads — multiple LEDs, a buzzer, a relay coil — use an NPN transistor as a low-side switch. The GPIO drives the base through a resistor; the transistor switches the load.

Transistor	I_C max	Use case
2N2222	600 mA	Single LED strings, small buzzers
BC547 / BC337	100–800 mA	Annunciator LED banks, relay coils
TIP120 (Darlington)	5 A	Motor control, large LED arrays, solenoids

Base resistor calculation: R_B = (V_GPIO − V_BE) ÷ I_B, where I_B = I_C ÷ h_FE. A 1 kΩ base resistor is a safe starting point for most NPN transistors at GPIO logic levels.

Capacitors — Decoupling & Filtering

Capacitors appear in two roles in cockpit builds: decoupling (suppressing voltage spikes near ICs) and filtering (smoothing power rails). They are not optional near motor drivers and stepper controllers.

Value	Type	Use case
100 nF (0.1 µF)	Ceramic	Decoupling cap across VCC/GND on every IC; place as close as possible
10 µF	Electrolytic or ceramic	Bulk decoupling near ESP32 VIN; helps with Wi-Fi transmit spikes
100 µF	Electrolytic	Required across VMOT/GND on A4988 / DRV8825 stepper drivers
1 000 µF +	Electrolytic	Main power rail filter when multiple servos share a supply

Electrolytic capacitors are polarised — the longer leg (or stripe on the can) is positive. Installing them backwards will cause them to fail, sometimes explosively. Ceramic capacitors are not polarised.

Diodes — Protection

Two diode types appear regularly in cockpit wiring:

Diode	Purpose	Where to use
1N4007 (rectifier)	Flyback / freewheeling — absorbs voltage spike when inductive load switches off	Across relay coils, motor driver outputs, solenoid valves
1N4148 (signal)	Reverse-polarity protection; signal isolation in diode matrix keyboards	Button matrices, input protection on sensitive GPIO lines
Schottky (e.g. 1N5819)	Low forward voltage drop (0.3 V) — power path selection, OR-ing supplies	Dual-power schemes (USB + bench supply), OR-ing two ESP32 power inputs