Debugging Your first builds will not work. That is normal and not a verdict on you — it is the actual activity. What separates a frustrating evening from a productive one is having a method instead of swapping parts hopefully.

1. No current-limiting resistor on an LED. Dead LED, possibly a stressed pin.
2. Electrolytic fitted backwards. Heat, bulge, bang.
3. No common ground between two boards or supplies. Nothing works, everything measures fine individually.
4. Floating input pin. Random readings; needs a pull-up or pull-down.
5. Driving a motor or relay straight from a pin. A pin gives ~20mA; a motor wants hundreds. Transistor plus flyback diode.
6. Missing flyback diode on a coil. The transistor dies after some number of switching cycles, which makes the cause hard to spot.
7. Missing decoupling capacitors. Intermittent faults that feel like software bugs or bad luck.
8. Brownout under load. The board resets whenever the motor starts. Bigger supply, bulk capacitance, separate ground returns.
9. Non-logic-level MOSFET on a 3.3V gate. Half-on, hot, and misleadingly almost-working.
10. Breadboard rail split in the middle, or a component straddling the wrong column.
11. Reversed supply polarity. Add a series Schottky on anything you care about.
12. 5V signal into a 3.3V pin. Sometimes survives, sometimes silently degrades the pin, sometimes kills the board.

Most faults are wiring, power, or a wrong assumption — not a dead part. Beginners invert this order, replace a transistor that was fine, and end up with two problems.

Techniques that narrow things quickly

• Halve the problem. Disconnect the circuit into two halves and test each. Two or three halvings localises almost anything.
• Simplify to the minimum. Strip back to the smallest thing that should work — one LED, no sensors, no radio. Add one element at a time. This is tedious and it is also the fastest route.
• Change one thing at a time. Two simultaneous changes mean you learn nothing from the result.
• Compare against a known-good. If you have two identical modules, swapping tells you instantly whether the fault travels with the part or stays with the circuit.
• Check the current draw against what you expect. Far too high means a short; near zero means it is not running at all.
• Re-read your own schematic aloud. Absurdly effective. The error is very often in the plan rather than the build.

1. Power off immediately at the first smell, smoke or heat.
2. Identify the casualty — discoloured, cracked, bulging or too hot to touch.
3. Ask why it failed. A burnt resistor is a symptom: something upstream drew more current than intended. Replacing it without answering this burns the replacement too.
4. Check its neighbours. Failures cascade; a shorted transistor often takes a driver pin with it.
5. Fix the cause, then the casualty, then power up through a current-limited supply.

Damaged components are tuition, not disaster. A resistor costs pennies; understanding why it burnt is worth considerably more than that, and you will not forget the lesson.