circuit-debugger

---
name: "circuit-debugger"
description: "Systematic approach to diagnosing hardware issues in maker projects."
license: "MIT"
---

# Circuit Debugger

Systematic approach to diagnosing hardware issues in maker projects.

## Resources

This skill includes bundled tools and references:

- **scripts/generate_debug_sketch.py** - Arduino sketch generator for I2C scanner, GPIO tester, ADC checker, PWM tester
- **references/measurement-procedures.md** - Comprehensive multimeter and oscilloscope guide

## Quick Start

**Generate I2C scanner:**
```bash
uv run --no-project scripts/generate_debug_sketch.py --i2c --output i2c_scanner.ino
```

**Generate GPIO tester:**
```bash
uv run --no-project scripts/generate_debug_sketch.py --gpio --pins 2,3,4,5 --output gpio_test.ino
```

**Generate all debug sketches:**
```bash
uv run --no-project scripts/generate_debug_sketch.py --all
```

**Interactive mode:**
```bash
uv run --no-project scripts/generate_debug_sketch.py --interactive
```

## Trigger Phrases
- "My circuit doesn't work"
- "Component is getting hot"
- "No power to the board"
- "Sensor not responding"
- "Intermittent/random failures"
- "Works sometimes, not others"

## Debugging Protocol

### Phase 1: Power System Check (Do First!)

**Visual Inspection (30 seconds)**
```
□ Check for smoke, burn marks, or melted plastic
□ Verify power LED on microcontroller is lit
□ Look for loose wires or cold solder joints
□ Confirm correct polarity on polarized components (LEDs, caps, diodes)
```

**Multimeter Tests (Set to DC Voltage)**
```
Test Point              | Expected Value  | If Wrong
------------------------|-----------------|------------------
VCC to GND on MCU       | 3.3V or 5V      | Check regulator, power source
Sensor VCC pin          | Match datasheet | Check wiring, broken trace
Motor driver VCC        | Logic + Motor V | Separate supplies needed?
Battery terminals       | Rated voltage   | Dead/discharged battery
```

**Common Power Issues:**
| Symptom | Likely Cause | Fix |
|---------|--------------|-----|
| No voltage anywhere | Disconnected power, blown fuse | Check continuity from source |
| Low voltage (< 4V when expecting 5V) | Overloaded supply, bad regulator | Reduce load, check current draw |
| Voltage drops under load | Undersized power supply | Calculate total current, upgrade PSU |
| Reverse polarity | Swapped wires | Check all connections, may have damaged components |

### Phase 2: Ground Continuity

**Critical Rule:** All grounds must be connected together.

```
Multimeter: Set to CONTINUITY (beep mode)

Test these pairs - ALL should beep:
□ Arduino GND ↔ Sensor GND
□ Arduino GND ↔ Motor driver GND  
□ Arduino GND ↔ Display GND
□ Arduino GND ↔ Power supply negative
□ All breadboard GND rails connected
```

**Ground Problems Cause:**
- Erratic sensor readings
- I2C/SPI communication failures
- Motors behaving randomly
- Displays showing garbage

### Phase 3: Signal Verification

**Digital Signals (Set multimeter to DC Voltage)**
```cpp
// Add this debug code to verify pin states
void debugPins() {
    Serial.println("=== Pin States ===");
    Serial.print("D2: "); Serial.println(digitalRead(2) ? "HIGH" : "LOW");
    Serial.print("D3: "); Serial.println(digitalRead(3) ? "HIGH" : "LOW");
    // Add more pins as needed
}
```

**Expected Readings:**
| Signal Type | HIGH | LOW | Floating (Bad!) |
|-------------|------|-----|-----------------|
| 5V Logic | 4.5-5.5V | 0-0.5V | 1-3V unstable |
| 3.3V Logic | 2.8-3.6V | 0-0.3V | 0.8-2V unstable |

**I2C Troubleshooting:**
```cpp
// Run this I2C scanner first
#include <Wire.h>

void setup() {
    Serial.begin(115200);
    Wire.begin();
    
    Serial.println("I2C Scanner");
    for (uint8_t addr = 1; addr < 127; addr++) {
        Wire.beginTransmission(addr);
        if (Wire.endTransmission() == 0) {
            Serial.print("Found device at 0x");
            Serial.println(addr, HEX);
        }
    }
}
void loop() {}
```

| I2C Problem | Check |
|-------------|-------|
| No devices found | SDA/SCL swapped? Pull-ups present? Correct address? |
| Address conflict | Two devices same address? Check AD0/AD1 pins |
| Intermittent | Weak pull-ups (try 4.7kΩ), long wires, noise |

### Phase 4: Component Isolation

**The Divide-and-Conquer Method:**
```
1. Disconnect ALL external components
2. Verify MCU works alone (blink LED)
3. Add ONE component at a time
4. Test after EACH addition
5. When failure occurs, problem is last added component
```

**Component-Specific Tests:**

**LEDs:**
```
□ Correct polarity? (long leg = anode = positive)
□ Current limiting resistor present? (330Ω-1kΩ typical)
□ Test LED alone with battery + resistor
□ PWM pin? Try digitalWrite first
```

**Motors/Servos:**
```
□ Never connect directly to MCU pin (use driver!)
□ Separate power supply for motors
□ Flyback diode on DC motors
□ Check stall current vs driver rating
```

**Sensors:**
```
□ Correct operating voltage (3.3V vs 5V!)
□ Level shifter needed for mixed voltage?
□ Decoupling capacitor (100nF) near VCC pin
□ Pull-up resistors for I2C (4.7kΩ typical)
```

### Phase 5: Software vs Hardware

**Quick Software Test:**
```cpp
// Minimal test - does the MCU even run?
void setup() {
    pinMode(LED_BUILTIN, OUTPUT);
    Serial.begin(115200);
    Serial.println("MCU is alive!");
}

void loop() {
    digitalWrite(LED_BUILTIN, HIGH);
    delay(500);
    digitalWrite(LED_BUILTIN, LOW);
    delay(500);
    Serial.println("heartbeat");
}
```

| If This Works | If This Fails |
|---------------|---------------|
| Hardware likely OK, check your code | Check USB cable, bootloader, board selection |

## Quick Reference: Common Failures

| Symptom | First Check | Second Check | Third Check |
|---------|-------------|--------------|-------------|
| Nothing works | Power supply | USB cable | Board selection in IDE |
| Gets hot | Short circuit | Reversed polarity | Overcurrent |
| Works then stops | Power brownout | Overheating | Memory leak |
| Erratic behavior | Floating inputs | Missing grounds | Noise/interference |
| I2C fails | Pull-ups | Address | Wire length |
| Motor jerky | Power supply | PWM frequency | Driver current |

## Multimeter Quick Guide

```
Measurement    | Setting      | Probes
---------------|--------------|------------------
DC Voltage     | V⎓ (20V)     | Red=signal, Black=GND
Continuity     | ))) or Ω     | Either direction
Resistance     | Ω            | Component out of circuit!
Current        | A or mA      | IN SERIES (break circuit)
```

## When to Ask for Help

If after this protocol you still can't find the issue:
1. Take clear photos of your wiring
2. Draw a schematic (even hand-drawn)
3. List exact components with part numbers
4. Share your complete code
5. Describe what you expected vs what happened

## References
- See [references/multimeter-guide.md](references/multimeter-guide.md) for detailed measurement techniques
- See [references/common-mistakes.md](references/common-mistakes.md) for beginner pitfall gallery

## Shared Output Contract

Use [the shared Arduino skill contract](../../docs/arduino-skill-contract.md):
state assumptions, required tools and versions, implementation steps,
tests/evidence by proof stage, known limitations, and recovery/security notes.