Tricky Circuit

LED + resistor

Make light obey.

One part glows; another keeps the current under control. Power up and discover how they work together.

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LED · Light-emitting diode
Light-emitting diode. A semiconductor component that emits light when forward current flows. In this circuit, a resistor limits that current.
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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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One circuit. Two different roles.

Red LED model: fixed 2 V drop, 20 mA limit. Brightness is illustrative. Real parts vary; reverse breakdown and heating are not simulated.

Light module = resistor + LED in series. The resistor works on either side: both carry the same current. I ≈ (Vsupply − 2 V) / R; resistor power P = I²R. Check real component ratings, including maximum reverse voltage.

Difficulty

  1. Build · Your first light

    Power on. Reach 5–20 mA with the LED facing the correct way.

    Need a hint?

    Start with 5 V and 330 Ω. The power button is below the circuit.

  2. Explore · Light without overload

    At 5 V, reach 8–12 mA. Then reverse the LED and explain what changes.

    Need a hint?

    About 3 V remains across the resistor. For 10 mA: R = 3 / 0.010 = 300 Ω.

  3. Solve it · Choose protection

    At 9 V, reach 10–15 mA. Choose the resistance yourself.

    Need a hint?

    The resistor drops 7 V. A resistance between about 467 and 700 Ω works.

Extra reading and circuit recipes

The tiny light that changed the world

An LED is a semiconductor junction. When electrons and holes recombine, some energy can leave as light. The material helps determine the colour; the plastic cover alone does not. A resistor turns a fragile light source into a controllable circuit.

I = (Vs − Vf) / R → (5 − 2) / 330 ≈ 9.1 mA

The LED takes about 2 V in our model, leaving 3 V across the resistor. Double that resistance and the current halves. Real forward voltage depends on the LED, current and temperature.

A Nobel-worthy colour

Efficient blue LEDs earned the 2014 Nobel Prize in Physics. They helped make bright, energy-saving white light sources possible.

Current is not used up

In this series loop, the current before the LED equals the current after it. Components transfer energy; they do not consume electrons.

The resistor can move

Put the resistor before or after the LED in the same series loop: the ideal current is unchanged. It still limits the whole loop.

A useful trap to avoid

Two parallel LEDs should normally have a separate resistor in each branch. Small differences in forward voltage can make one LED take more current than the other.

You move the resistor to the other side of the LED. What happens?

  1. The LED becomes brighter
  2. The ideal current stays the same
  3. The current stops
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There is still one series path with the same supply, LED and resistance. The order does not change the loop equation.

Pocket indicator

A gentle 3 V indicator. Predict the current before opening it.

About 3.03 mA. The light works, but this is below the first mission’s 5 mA target.

9 V beacon

Use more resistance when the supply gets bigger.

About 10.29 mA. The resistor drops 7 V and dissipates about 0.072 W in this model.

Nobel Prize: blue LEDs