troubleshooting
Why won't my LED light up?
Short answer: 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.
See it happen
This is the broken circuit, simulated live. The readout under the board tells you what the parts are doing.
simulating…
What's going on
An LED is a diode — current passes in one direction only. The longer leg is the anode (+ side); the shorter leg, next to the flat edge on the rim, is the cathode (− side). Put it in backwards and it blocks like an open switch. Nothing is damaged; a 4.5 V battery is far below the few volts in reverse that would hurt it.
The simulation above shows exactly that: a complete loop, a good resistor, and 0 mA everywhere, because the diode is holding the door shut.
The fix
Pull the LED out, rotate it 180°, push it back in. Long leg toward the battery's + side (the red wire). If you've trimmed the legs, look for the flat spot on the rim — that side is −.
If it still won't light, work through the list below in order. Each one takes ten seconds.
simulating…
Also check
- The loop isn't closed. A breadboard row is five holes joined together; a part in the wrong row is a gap you can't see. Check that every part shares a row with its neighbour.
- The switch is off — or it's a push button that only connects while pressed.
- The resistor is huge. 10 kΩ on 4.5 V gives 0.25 mA: technically lit, practically invisible in daylight. Swap for 220 Ω.
- The battery is flat. Three AAs should read about 4.5 V; below 3.5 V a red LED barely glows and a blue one won't at all.
- The LED is already dead from an earlier no-resistor experiment. Try a fresh one.