Electronics and Circuitry

Key Units

Quantity	Unit	Symbol
Voltage	Volt	V
Current	Ampere	A
Resistance	Ohm	Ω
Power	Watt	W

Ohm’s Law

$V=I\times R$

Derived forms:

• I = V / R (current = voltage ÷ resistance)
• R = V / I (resistance = voltage ÷ current)

Kirchhoff’s Voltage Law (KVL)

The sum of all voltages around any closed loop in a circuit equals zero.

In series: V_source = V_component1 + V_component2 + ...

LED Resistor Calculation

Formula: R = V_R / I_R

Where:

• V_R = voltage drop across resistor = V_source - V_LED
• I_R = desired current through LED

Worked Example

Given:

• Supply voltage: V_s = 12V
• LED forward voltage: V_LED = 2.2V
• Target LED current: I = 20mA = 0.02A

Step 1: V_R = V_s - V_LED = 12 - 2.2 = 9.8V Step 2: R = V_R / I_R = 9.8 / 0.02 = 490Ω Step 3: Choose next standard resistor ≥ 490Ω (e.g., 510Ω or 560Ω)

Minimum resistance protects LED from burning out.

GPIO Electrical Limits (Raspberry Pi / reTerminal)

Limit	Value
GPIO logic level	3.3 V
Max current per GPIO pin	16 mA
Max total current (all GPIO)	50 mA
5V power pin max	2 A (from USB supply)
3.3V power pin max	~50 mA

Never connect 5V directly to a GPIO input — it’s 3.3V-tolerant only.

Multimeter Usage

Mode	Measures	Connection
Voltage (V)	Voltage across component	Parallel to component
Current (A)	Current through circuit	Series in circuit (break circuit)
Resistance (Ω)	Component resistance	Disconnected from circuit

See Also

• Ohm’s Law concept
• Kirchhoff’s Law concept
• GPIO and reTerminal topic
• electronics-basics source