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Troubleshooting

The universal checklist (in order)

  1. Is it powered? Meter the rails — actual volts, not "the LED on the supply is lit."
  2. Is the loop complete? Power → components → ground, no breaks. Continuity mode finds opens.
  3. Polarity? LED/diode/capacitor/electrolytic the right way round? Chip notch matching the board marking?
  4. Shorts? Power off, continuity between 5V and GND — beep = don't power it.
  5. The right values? Meter the resistors (color codes lie to eyes); check the math (Ohm's Law).

Most dead circuits confess by step 3.

Symptom → fix

Nothing happens at all

Power first: supply voltage present? Switch on? Battery dead? Then the loop: a single broken connection kills everything — re-seat breadboard wires (they loosen), reflow suspect joints.

LED doesn't light

Resistor value correct? (Meter it.) LED polarity (long leg to positive)? Pin actually driven (Arduino: pinMode OUTPUT)? Try the LED + resistor straight across the supply — isolates the LED from the rest of the circuit.

Works on breadboard, dies soldered

A solder bridge (continuity-test adjacent pins), a cold joint that was a good breadboard contact, or a component cooked during soldering (ICs and LEDs are heat-sensitive — 3–5 seconds max, use a heatsink clip on delicate leads).

Intermittent / works when touched

The classic cold joint or loose breadboard wire — touching flexes it into contact. Reflow every joint in the suspect area; replace the wire. "Works until I move it" is a mechanical problem, not an electrical one — see strain relief in Breadboard to Permanent.

Chip gets hot

Something's drawing too much current: short downstream, backwards power, or the chip driving a load it can't (a motor straight off a logic pin). Unplug immediately — hot chips are telling you exactly what's wrong. Find the excess current with the meter before powering again.

Right voltage in, wrong voltage out

Voltage divider loaded down (see the caveat in Ohm's Law), regulator overheating into thermal shutdown, or the supply sagging under load (9V batteries are notorious).

Analog readings are noisy/jumpy

Missing decoupling capacitor, long unshielded wires acting as antennas, or a floating input (nothing connected — it reads the room's electromagnetic gossip). Add the 0.1µF cap, shorten wires, tie unused inputs somewhere.

The method

Halve the problem: power section vs. signal section, one half at a time. Change one thing. Write down what you measured — "3.1V here, 0V there" beats "it doesn't work" every time, especially when you ask for help (with photos of both sides of the board).

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