troubleshooting
Something's not working?
Pick your symptom. Each page runs the broken circuit live so you can see the problem, explains the physics with real numbers, then runs the fix.
Why does my LED burn out? Almost always: no resistor. An LED across a battery draws far more current than it can survive. A 220 Ω resistor on 3×AA holds it to about 11 mA. Why won't my LED light up? Nine times out of ten the LED is in backwards. It only conducts one way: long leg toward +. Otherwise check for an open loop or a resistor that is far too big. Why is my LED so dim? The resistor is too big for the battery. 10 kΩ on 4.5 V lets only 0.25 mA through; 220 Ω gives 11 mA and a proper glow. Why are two LEDs in series so dim? Each LED takes about 2 V before it conducts. Two in series need 4 V, leaving a 4.5 V pack only 0.5 V to push current. Use 9 V, or wire them in parallel. Why doesn't my buzzer make a sound? An active buzzer needs about 20–30 mA and the right polarity. A resistor meant for an LED starves it; wire it straight to 3–5 V with + to the long leg. Why does my battery get hot? A wire from + straight back to − is a short circuit. Nothing limits the current, so the battery heats itself up. Disconnect it and find the stray wire.