LED circuit simulator
Get the LED resistor right — then watch it light.
The most common first circuit, done properly. Size the current-limiting resistor, solve the operating point with a real diode model, and see the LED brighten by the current that actually flows through it.
Sizing the current-limiting resistor
The resistor drops the difference between your supply and the LED forward voltage, and that drop divided by your target current is the resistance you need:
R = (Vsupply − Vf) / If
For a 5 V supply, a red LED at about 2 V, and 15 mA, that is roughly 200 Ω — so a standard 220 Ω part sits it safely just under target.
Typical forward voltages
- Red
- 1.8 – 2.2 V
- Green / yellow
- 2.0 – 2.4 V
- Blue / white
- 3.0 – 3.4 V
Check the datasheet for your exact part.
Questions
- Why does an LED need a series resistor?
- An LED is a diode: once it passes its forward voltage, its current rises almost without limit and it destroys itself. A series resistor sets the current by dropping the difference between the supply and the forward voltage.
- How do I choose the resistor value?
- Use R = (Vsupply − Vf) / If. Pick your target current (often 5–20 mA), read the LED forward voltage, and the resistor value follows. The LED resistor calculator does it and rounds to the nearest standard part.
- Does the LED actually light in the simulator?
- Yes — by real forward current. The LED is solved with a diode model, so it brightens when the operating point pushes enough current through it, not on a fixed timer.
- What forward voltage should I use?
- Roughly 1.8–2.2 V for red, 2.0–2.4 V for green and yellow, and 3.0–3.4 V for blue and white. Use the datasheet value when you have it; it matters most on low supplies.