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Ready-to-teach lab

LED and resistor lab: choose the right resistor

This LED and resistor lab is a first circuit: one LED, one resistor and 5 V, and the question every beginner asks, which is what the resistor is for.

Ages 10 and up, a first lesson. About 30 minutes. No code.

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The lesson

Objective and parts

By the end, students can

  • Build a complete circuit and say what makes it complete.
  • Tell the long leg of an LED from the short one, and why it matters.
  • Choose a resistor for an LED from the supply voltage and the current wanted.

Parts, already on the bench

  • An Arduino Uno, used only as a 5 V supply
  • One LED
  • One 220 ohm resistor
  • A breadboard and two jumper wires

Nothing to hand out or collect: the circuit opens built and wired.

Steps

What students do

Each step is short enough to read out. Students work in the simulator beside this page.

Open this lab
  1. Open the lab and trace the path with a finger: 5 V pin, resistor, LED, ground.
  2. Press Run. The LED lights, with about 14 mA through it.
  3. Click the resistor and change it to 1 k. The LED dims: about 3 mA.
  4. Change it to 10 k. The LED is almost dark: about 0.3 mA.
  5. Put 220 ohms back, then turn the LED around so its legs swap rows. Press Run again.
  6. Delete one jumper wire and press Run. Put it back.
Questions

Questions for students

Hand these out as they are. The expected answers are under the list, for you.

  1. What is the resistor for?
  2. The LED was turned around and stayed dark. Why?
  3. One wire was removed and the LED went out. What does that tell you about a circuit?
  4. An LED needs about 2 V and you want 10 mA from a 5 V supply. What resistor do you need?
Answer key for the teacher
  1. It limits the current. An LED on 5 V with nothing in the way would pass far more current than it is made for.
  2. An LED is a diode: current passes one way only, in at the long leg (the anode) and out at the short one.
  3. Current needs a complete loop from the supply, through the parts and back to ground.
  4. (5 V - 2 V) / 0.010 A = 300 ohms. 330 ohms is the nearest common value.
Go further

For students who finish early

  • Add a second LED and resistor from the same 5 V pin.
  • Work out the resistor for the same LED on a 9 V battery.
Your class

Give this lab to every student

Assign to my class puts this circuit on each student’s class page as a starter. They change their own copy and hand it in, and the work comes back to you in one list.

The class tools are free for 30 days for one class of up to 20 students, with no card. Opening and running the lab needs no account and stays free.

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