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LED Current Limiting

A resistor sets how much current an LED carries. Put a multimeter in series and see it.

Open this circuit and run it

Opens in your browser with the parts, wiring and code already in place. Press Run to watch it work, then change anything you like.

How it works

An LED is not like a resistor: once the voltage across it passes its forward voltage, around 2 volts for green, it conducts almost anything the supply will give it, and burns out. The 470 ohm resistor is what stops that. With a 9 volt battery, about 2 volts drop across the LED and the remaining 7 volts across the resistor, so Ohm's law says the current is 7 divided by 470, close to 15 milliamps, a comfortable amount for an LED.

The multimeter is set to measure current, which means it has to be part of the loop, in series, rather than across a part. The current path runs from the battery through the meter, the resistor, the LED and back to the battery, and the meter reads it directly. The LED shows its brightness from that same real current, so if you change the resistor you will see both the number and the glow change. No sketch is needed.

Parts

Wiring

Arduino code

// LED Current Limiting
// Press Run: the multimeter, set to measure current, sits in series with the LED.
// There is no sketch to write: this is a circuit, so there is nothing for code to do.
void setup() {
}

void loop() {
}

Try changing it

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