Troubleshooting a Circuit That Doesn't Work

A calm, repeatable checklist for the moment your circuit stays dark — check these, in this order, before assuming anything is actually broken.

You'll need: your multimeter, a magnifying look at your breadboard, and patience — the majority of "broken" circuits turn out to be one of these ordinary causes, not a failed component.

1. Check the power source first. Measure the supply voltage directly at the battery or supply terminals with your multimeter, before checking anything else. A dead or low battery makes every circuit downstream look broken.
2. Check polarity. LEDs, electrolytic capacitors, diodes, and some ICs only work one way around. A component installed backwards is the single most common beginner mistake in this entire section.
3. Check for loose or wrong breadboard connections. Confirm each leg is in the row you intended — breadboard rows are easy to miscount, and a leg one row off from where you meant it makes a connection that looks right but isn't.
4. Trace the loop with continuity mode. Starting from the supply, test continuity section by section around the loop (Lesson 16) until you find the exact point where the beep stops — that's where your fault is.
5. Re-check your resistor values. A resistor with the wrong color-code reading, or a value ten times too high, can make a circuit seem completely dead when it's actually just working at a barely-visible level.
6. Isolate, don't guess. If the fault still isn't obvious, remove components one at a time and retest the simplest possible version of the circuit, rebuilding it back up piece by piece until the fault reappears.

Working through this list in order, rather than jumping straight to "maybe the part is broken," finds the actual cause far more often than it finds a genuinely dead component.