Circuit & Ohm's Law Simulator
Adjust the battery voltage and the resistor's resistance and watch the LED respond instantly. It's the same circuit as the basic LED circuit challenge — just without needing to solder anything.
20 mA flowing
The math behind it
How much current flows through a circuit depends on two things: how hard the voltage is pushing, and how much the resistance is fighting back. That relationship is Ohm's Law , one of the most-used equations in electrical engineering:
current = voltage ÷ resistance (I = V ÷ R)
That's why turning up the voltage brightens the LED, and turning up the resistance dims it — more voltage pushes more current through, and more resistance chokes it back down. It's also why the resistor is doing real work here even though it isn't the part lighting up: an LED conducts current very easily once it turns on, so without something limiting that current, it would draw far more than it's built for and burn out almost instantly. The resistor's job isn't to power the LED — it's to protect it.
This simulator treats the LED as a simple current-driven brightness indicator. Real LEDs have their own small voltage drop before they conduct at all (usually 1.8–3.3V depending on color) and their brightness curve isn't perfectly linear — but the core relationship this simulator shows, more current in means more light out, up to a point where the LED can be damaged, holds.
More on current as a concept →