🔀 Transistors/MOSFETs as Switches

Why it matters

GPIO pins can only source/sink ~40mA. Motors, high-power LEDs, and relays need more current. Transistors and MOSFETs act as switches controlled by GPIO.

The idea

The Problem

ESP32 GPIO pins have limits:

• Source: ~40mA (driving HIGH)
• Sink: ~28mA (driving LOW)
• Motors need hundreds of milliamps
• High-power LEDs need 100mA+

The Solution: Transistors

Transistors act as switches:

• Base/Gate: Control pin (connected to GPIO)
• Collector/Drain: High-current path
• Emitter/Source: Ground/reference
• Small current at base → large current flows through collector

MOSFETs vs BJTs

• BJT (Bipolar Junction Transistor): Current-controlled, needs base resistor
• MOSFET: Voltage-controlled, very low gate current, better for GPIO
• For ESP32, use logic-level MOSFETs (turn on at 3.3V)

Common Applications

• Motor control — H-bridge with MOSFETs
• High-power LEDs — MOSFET switches
• Relays — transistor drives relay coil
• Power gating — turn subsystems on/off

Demo

Transistors are switches, not demos. Review before designing high-current circuits.

Key takeaways

• GPIO pins have current limits (~40mA source, ~28mA sink)
• Transistors/MOSFETs act as switches controlled by GPIO
• MOSFETs are voltage-controlled and better for GPIO than BJTs
• Use logic-level MOSFETs for 3.3V ESP32 GPIO

Going deeper

For motor control, use an H-bridge (4 MOSFETs) to control direction and speed. Always include flyback diodes to protect against back-EMF. For PWM motor control, use MOSFETs with low R_ds(on) to minimize heat. Common logic-level MOSFETs: IRLZ44N, IRF540N (but check V_gs threshold — must be < 3.3V).

Math details

MOSFET as switch:
  When V_gs > V_threshold: MOSFET turns ON (low resistance)
  When V_gs < V_threshold: MOSFET turns OFF (high resistance)

Power dissipation in MOSFET:
  P = I² × R_ds(on)

  Example:
  I = 1A (motor current)
  R_ds(on) = 0.1Ω (typical for logic-level MOSFET)
  P = (1A)² × 0.1Ω = 0.1W (may need heatsink if >0.5W)

Gate current (MOSFET):
  I_gate ≈ 0 (voltage-controlled, negligible current)

  vs BJT base current:
  I_base = I_collector / β (β = current gain, typically 100-300)
  I_base = 1A / 100 = 10mA (needs base resistor!)

Implementation

LLM Prompt: MOSFET Switch Driver

Write Rust code to control a logic-level MOSFET from ESP32 GPIO.
Include: GPIO setup (output mode), turn ON/OFF functions, and safety
check to ensure GPIO is not driving more than 40mA (MOSFET gate current
is negligible, so this is fine). Target: esp-hal crate.

Lab Exercise: High-Power LED

1. Get logic-level MOSFET (e.g., IRLZ44N)
2. Connect: GPIO → MOSFET gate, 5V → LED → MOSFET drain, MOSFET source → GND
3. Program ESP32: GPIO HIGH = LED ON, GPIO LOW = LED OFF
4. Measure current through LED (should be limited by power supply, not GPIO)
5. Verify GPIO current is negligible (< 1mA)