LED Circuit Designer & Resistor Calculator
Build LED circuits with a battery, resistor, LED, switch and meters, watch real current and brightness, break them on purpose, and use 11 resistor calculators.
About the LED Circuit Designer & Resistor Calculator
Free led circuit designer & resistor calculator. Build LED circuits with a battery, resistor, LED, switch and meters, watch real current and brightness, break them on purpose, and use 11 resistor calculators. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for engineering, the led circuit designer & resistor calculator runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Build LED circuits with a battery, resistor, LED, switch and meters, watch real current and brightness, break them on purpose, and use 11 resistor calculators. Use it to explore engineering ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the LED Circuit Designer & Resistor Calculator
- Use the controls to change Guide me: tell me what to do next, LED colour, LEDs in series, Battery voltage, Resistor, and more. The simulation reacts instantly.
- Pick an option such as Red (1.8 V), Green (2.2 V), Blue (3.0 V), White (3.2 V) to switch modes or load an example.
- Press "Best resistor for 20 mA", "Replace burnt LED", "Reset to quick circuit" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Leave the 5 V, 150 ohm circuit and press Best resistor for 20 mA.
- Drag the battery voltage up until the LED burns, then press Replace burnt LED.
- Load Lesson 2 and close the switch to see why an LED needs a resistor.
- Rotate the LED backwards and read the error message.
- Use the LED resistor calculator for 9 V, 2 V and 20 mA, then place the 360 ohm resistor in the circuit.
- Open the fault-finding cases and diagnose each one with the meters.
Key ideas you can learn
- An LED needs a series resistor to limit its current: R = (supply voltage − LED voltage) ÷ current.
- Ohm's law, V = I × R, links the voltage, current and resistance of every resistor in the circuit.
- Series resistors add; parallel resistors share the voltage and the total is smaller than the smallest.
- A resistor turns power into heat, P = I² × R, so it needs a wattage rating above what it dissipates.
- An LED only conducts one way and only above its forward voltage; too much current burns it out.
- Real parts vary: resistor tolerance and LED voltage change the current, so good designs leave margin.
Where this is used in the real world
Every torch, phone indicator, traffic light, TV backlight and Arduino project uses a current-limited LED, and the same nodal analysis, resistor networks, dividers and tolerance checks are used to design real circuit boards.
Who is this simulation for?
Engineering and technology students, makers, robotics clubs and teachers of design and technology. It gives a hands-on feel for how machines behave before you build a real one.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
How do I choose a resistor for an LED?
Subtract the LED's forward voltage from the supply, then divide by the current you want, usually 10 to 20 mA. For 9 V, a 2 V LED and 20 mA that is 350 ohms, so use the nearest standard value, 360 ohms, which gives 19.4 mA.
Why does an LED burn out without a resistor?
Once an LED conducts, its resistance is tiny, so the current is limited only by the battery. The current soars past the LED's maximum and it overheats. Try it in the simulator with the switch.
How does the simulator work out the currents?
It builds the circuit you draw into a network, then solves it with Modified Nodal Analysis, the method used in circuit simulators such as SPICE. It finds every node voltage, then each current and power. The results were checked against an independent NumPy solver.
How do I read a resistor colour code?
Read the bands from the end nearest the edge. On a 4-band resistor the first two bands are digits, the third is the multiplier and the fourth is the tolerance, so brown, black, red, gold is 10 × 100 = 1 kΩ ±5%. The colour code calculator works in both directions and also handles 5- and 6-band resistors.
What are E12, E24 and E96 resistor values?
They are the standard values that resistors are made in, with 12, 24 or 96 steps per decade. If you calculate 347 ohms, the nearest E12 value is 330, E24 is 360 and E96 is 348. The E-series calculator shows the nearest lower, nearest and nearest higher value.
Is this accurate enough for real electronics?
The mathematics is standard, but it is an educational model: the LED is a fixed forward voltage with a small resistance, and parts are treated as ideal. Real LEDs and resistors vary with temperature and tolerance, which the tolerance calculator explores.
Is the LED Circuit Designer & Resistor Calculator free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the LED Circuit Designer & Resistor Calculator work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.