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Resistor Values and LED Brightness on Raspberry Pi 4B

Learn how a resistor's value changes an LED's brightness on a Raspberry Pi 4B. Set ohms, kilo-ohms or mega-ohms and check the value with the Digital Multimeter.

Try this directly in the free Raspberry Pi 4B simulator — no hardware or signup required. New to the GPIO header? Start with the interactive pinout guide.

What you’ll need

  • Raspberry Pi 4B
  • LED
  • Resistor
  • Digital Multimeter (Free, for the value check)

Step by step

A resistor limits how much current flows. An LED needs one, or it draws too much current. A bigger value means less current, and a dimmer LED.

  1. Drag a Raspberry Pi 4B, an LED and a Resistor onto the Canvas. Wire Pi pin 37 → the resistor, the resistor → the LED's anode, and the LED's cathode → a Pi GND pin (for example pin 39).
  2. Click the resistor to select it. Its settings show a Name and a Resistance box with a unit list (Ω, kΩ or MΩ). The default is 220 Ω. You can only change it while the simulation is stopped.
  3. Go to the Code tab and write:
import RPi.GPIO as GPIO
 
GPIO.setmode(GPIO.BOARD)
GPIO.setup(37, GPIO.OUT)
GPIO.output(37, GPIO.HIGH)
  1. Click Start and look at the LED with the resistor at 220 Ω. Click Stop.
  2. Set the resistor to 1 kΩ and click Start again. Then try 100 kΩ.
  3. To check a value: drag a Digital Multimeter onto the Canvas, set its Mode to Continuity/Resistance, and wire its two probes to the resistor's two terminals.

What “working correctly” looks like

  • How to test it: with 220 Ω the LED is on and clearly lit. With 1 kΩ it is noticeably dimmer. With 100 kΩ it is almost off. The simulator scales the brightness from the current, using the pin's 3.3 V and the LED's voltage drop.
  • The Multimeter in Continuity/Resistance mode reads 220 Ω for the default resistor, and 1.00 kΩ after you set 1 kΩ. This works even while the simulation is stopped.
  • The resistor's colored bands change to match the value you set.

If something’s wrong

  • The LED never lights → check the anode and cathode orientation, and that pin 37 goes through the resistor to the LED. The script also needs to run: click Start.
  • You cannot change the value → stop the simulation first. The settings are locked while it runs.
  • The value reverts after you type it → the box must hold a valid number. An invalid or empty entry goes back to the last valid value.
  • The Console warns that the LED is lit through less than 100 Ω → the resistor is too small. A real LED would likely burn out, so use 220 Ω or more.

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