Common Peripheral Circuits

After learning components and protocols, the final step is to build a functional circuit. Below are the peripheral modules most commonly encountered in projects.

Buttons and Debouncing

Problem

Mechanical button presses are not clean make/break events:
  V
  │ ┌─┐  ┌──┐
  │ │ │  │  │      ← Contact bounce
  │ │ └──┘  └──────
  └──────────────── t
      ↑          ↑
    Pressed    Stable

Bounce time: Typically 5~20ms
No debouncing → One button press triggers dozens of events

Hardware Debouncing (RC)

Vcc
 │
 Rpullup (10k)
 │
 ├────── GPIO
 │
 ├── R1 (1k) ──┬── Button ── GND
               │
              C1 (0.1μF)
               │
              GND

RC time constant τ = R1×C1 = 100μs (Too short)
Need τ ≈ Bounce time ≈ 10ms
→ R1=10k, C1=1μF → τ=10ms

Disadvantages: Uses extra components, but reliable
Advantages: Does not consume CPU, suitable for interrupt wake-up scenarios

Software Debouncing (Common)

// Simplest and most effective method
if (digitalRead(BUTTON) == LOW) {
    delay(20);  // Wait 20ms (or use a timer, avoid using delay)
    if (digitalRead(BUTTON) == LOW) {
        // Confirm press
        handle_press();
    }
}

// Better: State machine + timer, non-blocking
// Record the time of the last change; only consider it valid if the interval > 20ms

Using MOSFETs as Load Switches

Why use a MOSFET instead of driving directly with a GPIO

GPIO max current: Typically 8~20mA
Loads to drive: LED strips (1A), Relays (100mA), Motors (>1A)
→ GPIO controls MOSFET, MOSFET drives the load

Low-side switch (N-MOSFET, load between Drain and Vcc):
  Vcc ── Load ──┬── Drain
                 │
  GPIO ── Rg ──── Gate
                 │
                Source ── GND

  Advantages: Simple, N-MOSFET Rds(on) is low
  Disadvantages: Load is not grounded (not suitable for some scenarios)

High-side switch (P-MOSFET, load between Drain and GND):
         Source ── Vcc
           │
  GPIO ──┬─ Rg ── Gate
         │
        NPN/ NMOS (Level shifting, because GPIO 3.3V cannot fully turn off P-MOS)
         │
        GND

         Drain ── Load ── GND

  Advantages: Load is grounded on one end (safer)
  Disadvantages: Requires level shifting, P-MOS Rds(on) is higher

Gate Protection (Mandatory!)

            Rg (100Ω~1k)
  GPIO ───┤├─────── Gate
                   │
                   Rgs (10k~100k)
                   │
                  GND

Rg: Limits gate charging/discharging current, suppresses oscillation
Rgs: Ensures Gate=GND (MOSFET OFF!) during GPIO uninitialized/reset periods
     Without Rgs → Floating Gate at power-up → MOSFET may turn on inadvertently

High voltage / inductive loads: Add a TVS or Zener diode from Gate to Ground
  (Prevents drain spikes from coupling back to the gate via Cgd → breaking down the gate oxide)

Optocoupler Isolation

When is an optocoupler needed

- Between high voltage and low voltage (Mains 220V ↔ MCU 3.3V)
- Long-distance signal transmission (large ground potential difference)
- Need to eliminate ground loop noise
- Industrial environments (surges/lightning)

Scenarios where optocouplers are NOT needed:
  - Different voltage domains on the same board → Use level shifter ICs
  - On-board I2C/SPI → No isolation needed (unless industrial)

Basic Circuit

MCU Side (3.3V)          Controlled Side (12V/24V)
    │                       │
  GPIO                      R (Current Limit)
    │                       │
    R (Limit LED Current)  ┌──┴──┐
    │                    │Opto  │
   ┌┴┐                   │Output│
   │LED (Internal)       │      │
   └┬┘                   └──┬──┘
    │                       │
   GND                     GND (Controlled Side)

Typical: PC817 (Cheap, Low Speed), 6N137 (High Speed, 10Mbps)

LED Side: If = 5~20mA, R = (3.3V-1.2V)/If
Output Side: Calculate current limit based on load

Relay Driving

Flyback Diode — Absolutely Essential!

               Vcc
                │
              ┌─┴─┐   ▸├ (Flyback Diode)
              │Relay│   │
              └─┬─┘   │
                │     │
              ┌─┴─┐   │
              │MOSFET│  │
              └─┬─┘   │
                │     │
               GND────┘

At turn-off: Coil current cannot change instantly → Generates reverse high voltage (L×dI/dt)
             Can exceed 100V! → Without flyback diode → MOSFET breakdown

Diode Direction: Reverse parallel (Cathode to Vcc, Anode to MOSFET Drain)
  Normally reverse-biased and non-conducting
  When MOSFET turns off, provides a current path

Diode Selection: 1N4148 (Small relays), 1N4007 (Large relays)
                 Fast recovery or Schottky (Reduces EMI)

Complete Circuit

              Vcc (5V/12V/24V)
               │
              ┌┴┐
              │▸├ (Flyback)
              └┬┘
               │
              ┌┴┐
              │Relay│
              └┬┘
               │
             Drain
  GPIO ──Rg─── Gate  N-MOSFET
               │     (Select low Rds(on), Vds > Vcc×2)
             Source
               │
              GND

Relay Coil Current: I = Vcc/R_coil
  Example: 5V Relay 70Ω → 71mA

MOSFET Selection:
  Vds > Vcc × 1.5
  Id > I_coil × 1.5
  Vgs(th) < GPIO High Level (For 3.3V systems, select < 2.5V)
  Recommended: AO3400 (30V/5.8A, Vth<1V, Cheap and easy to use)

LED Driving

Constant Current vs. Current Limiting Resistor

Current Limiting Resistor:
  I_LED = (Vcc - Vf) / R
  Vcc fluctuation → Current fluctuation
  Vf temperature drift → Current drift
  Severe resistor heating at high power

Constant Current Source:
  Current remains constant, independent of Vcc/Vf changes
  Can connect multiple LEDs in series (Voltage adds up, current remains constant)
  High efficiency

Watchdog

Why is it needed

Embedded device runs for 3 months → Freezes → User has to unplug power
Causes: Memory leaks, dangling pointers, EMI interference, rare bugs

Watchdog = Independent Timer
  Firmware normal: Periodically "feed the dog" (reset timer)
  Firmware stuck: No feed → Timeout → Hardware reset

Key Usage Points

❌ Feeding the watchdog in the main loop (Still feeding if stuck in a sub-function)
✅ Feed the watchdog in an independent monitoring task
   Or: Multiple tasks must all check in before feeding the watchdog

❌ Forgetting to feed the watchdog when using delay/sleep
✅ Break long delays into multiple short delays + feed watchdog

❌ Using a very short watchdog timeout (Might not have time to feed during normal operation)
✅ Timeout > 2 times the longest task execution time

Independent Watchdog (IWDG): Independent clock source, can reset even if system clock fails
Window Watchdog (WWDG): Can only be fed within a specific time window (Stricter)

Keywords: Debouncing, MOSFET Switch, Optocoupler, Flyback Diode, Relay, LED Driver, Watchdog, GPIO