Source: Electronics Basics — Connected Objects
GPIO Physical Limits (reTerminal)
- Max current per individual GPIO pin: 16 mA
- GPIO voltage level: 3.3 V (inputs are 3.3 V-tolerant)
- Max current from 3.3 V and 5 V power pins: 2 A
Units
| Quantity | Symbol | Unit | Abbrev |
|---|---|---|---|
| Current | I | Ampere | A |
| Voltage | V or E | Volt | V |
| Resistance | R | Ohm | Ω |
Key Concepts
- Current: movement of electrons through a conductor (like water flow in a pipe)
- Voltage: force pushing electrons (pressure in the pipe); always measured between two points
- Resistance: opposition to current flow; also measured between two points
Ohm’s Law
V = I × R
I = V / R
R = V / I
Example: V1 = 9 V, R1 = 200 Ω → I = 9 / 200 = 0.045 A = 45 mA
Multimeter in ammeter mode: must be placed in series (circuit goes through it). Multimeter in voltmeter mode: placed in parallel (samples two points, does not interrupt circuit).
Kirchhoff’s Voltage Law (KVL)
The total voltage drop across all components in a series circuit must equal the source voltage.
Vsource = Vled + Vr
LED Resistor Calculation (full example)
- Source voltage: 12 V
- LED voltage drop (Vled): 2.2 V
- Target LED current: 20 mA = 0.02 A
Step 1: Find Vr
Vr = Vsource - Vled = 12 - 2.2 = 9.8 V
Step 2: In a series circuit, same current flows through all components:
Ir = 0.02 A (same as LED target current)
Step 3: Calculate R
R = Vr / Ir = 9.8 / 0.02 = 490 Ω
See Also
- Ohm’s Law concept
- Kirchhoff’s Law concept
- Electronics and Circuitry topic
- GPIO and reTerminal topic