Change the resistor

This is the first circuit with one thing changed. You need the same LED and board, and whichever of the resistors you want to try: one at a time, with the USB unplugged while you swap.

Pull one out, push another in

Everything on the board stays where it is. The resistor's two legs go into the same two holes, whatever the value, and nothing tells you the value except its colour bands.

The bands drawn here are the five-band kind: three digits, then a multiplier, then a tolerance band. If yours has four, the first two are the digits. Check them before you push a resistor in, because the board looks the same for every one. The markings page has the code.

One LED, six resistors

Pick a colour and a power side, and move through the six.

Side by side

The drawing is not a photograph, and it is not to scale. What it shows is that the steps are uneven. The first one, from 220 Ω to 1 kΩ, takes the LED from bright to about a fifth of that current, and it still looks lit. From 4.7 kΩ on, there is almost nothing to see.

Why the curve looks like that

Current is the voltage across the resistor divided by the resistor. The LED takes its fixed share of the supply, so the same voltage is left over for any resistor, and halving the resistance doubles the current.

This is why the useful values in the box sit in the middle. 100 Ω is on the steep part of the curve, where a small change in resistance is a big change in current. 10 kΩ and 1 MΩ sit on the floor, where nothing you do shows.

Choosing one yourself

The rule is one line, and it works for any colour and any supply:

resistor = (rail - LED voltage) / the current you want

For a red LED on a 4.8 V rail at 5 mA, that is 2.8 V ÷ 0.005 A, or 560 Ω. The box has no 560 Ω, so use the nearest value that keeps the current inside the range: 220 Ω gives 12.7 mA, 1 kΩ gives 2.8 mA. If you want something in between, two resistors in a row add: two 220 Ω in series make 440 Ω, which gives about 6.4 mA. See series and parallel.

On the 3.3 V side

On the module's 3.3 V side every current is smaller. A 220 Ω resistor gives a red LED about 5.9 mA, a comfortable brightness, and a blue or white one about 1.4 mA, which is dim. 1 kΩ gives the blue LED 0.3 mA, which is barely visible. Nothing in the box is damaged by any combination on 3.3 V: the risk is on the 5 V side with the smallest resistor.

Measure it

Use a red LED on the 5 V side, a meter on DC volts, and take these three resistors one at a time. Measure across the resistor, then divide by its value.

The reading across the resistor hardly changes while the current changes forty times. That is the point of the circuit: the LED fixes the voltage, and the resistor sets the current from it. At very small currents the LED's own voltage sags by a few tenths of a volt, so the resistor reads a little more than the table's first rows say. It is not a fault.

To check a resistor itself, take it out of the board, set the meter to ohms and touch one probe to each leg. Expect the printed value within a few percent. The meter page shows how.