Light an LED
An LED will take whatever current the supply gives it, so it always goes in series with a resistor. This is the first circuit in the box: the power module, a 220 Ω resistor and one LED, four parts and two wires, checked with a meter instead of by looking.
You need one LED of any colour, one 220 Ω resistor, two jumper wires and the power module on the breadboard. Build it with the module unplugged from USB.
An LED has no resistance of its own
A light bulb has a thin filament that limits the current through it. An LED has almost nothing. It takes a fixed voltage across it, about 2 V for a red one and about 3 V for a blue one, and once the voltage is there the current climbs steeply for every extra tenth of a volt. Connected straight across a supply, it takes whatever the supply gives until it fails.
So something else has to set the current, and a resistor does it. The module puts out one voltage. The LED takes its fixed share and the resistor takes the rest, and Ohm's law turns the rest into a current.
The module in the box is the standard breadboard power adapter. On its 5 V side the rail reads about 4.8 V, not 5.0, because a diode and a fuse sit between the USB plug and the rail. Every number here uses 4.8 V; measure yours and use that instead. The reason is in the power module's handbook.
Which leg is which
An LED works one way round only. Current goes in at the anode, the + leg, and out at the cathode, the − leg. Three things tell them apart.
Pick the first resistor
A 10 mm LED is usually rated for about 20 mA, and this book aims well under it. These are the currents on the 5 V side for the three smallest resistors:
| Resistor | Red | Yellow | Green | Blue or white |
|---|---|---|---|---|
| 100 Ω | 28 mA | 27 mA | 26 mA | 18 mA |
| 220 Ω | 12.7 mA | 12.3 mA | 11.8 mA | 8.2 mA |
| 1 kΩ | 2.8 mA | 2.7 mA | 2.6 mA | 1.8 mA |
100 Ω is too small for the red, yellow and green LEDs: it is above 20 mA and wears them out sooner. 1 kΩ is safe and dim. 220 Ω gives every colour a comfortable brightness and stays under the ceiling for all five, so it is the first one to use. If the bands are hard to read, the markings page has the code, and the meter page shows how to check a resistor by measuring it.
Build it
Turn the module the right way round for your breadboard (which way round it goes) and put its switch on 5 V (what the two switches do). Plug the USB-C cable in last, so nothing on the board is live while you push parts in.
Measure it
Set the meter to DC volts, the symbol with a straight line over a dashed one, and touch the red probe to the more positive point and the black to the other. A meter on its volts range draws almost no current, so touching the probes to the legs does not change the circuit and you can do it while the LED is lit. The table is for a red LED and a 4.8 V rail.
| Between | Expect | What it means |
|---|---|---|
| The + and − rails | 4.5 to 4.9 V | The power is there. Use your own number for the rest. |
| The LED's two legs | about 2.0 V | The LED is conducting. Blue and white read about 3.0 V. |
| The resistor's two legs | about 2.8 V | The rest of the rail. |
| The current | about 12.7 mA | The resistor's reading divided by 220: 2.8 ÷ 220. |
The LED and the resistor readings add up to the rail reading, within a few hundredths of a volt. Divide the resistor's reading by its value for the current. If you put the meter on its current range in this position you would be reading the wrong thing and could blow the meter's fuse: stay on volts.
Turn the LED round. It goes dark, the LED reads the whole rail and the resistor reads nothing. That is the signature of a backwards LED, and it is the first thing to look for when a circuit like this does not light.
On the 3.3 V side
Put the module's switch on 3.3 V and the same circuit gives about 5.9 mA with a red LED, which is fine, and about 1.4 mA with a blue or white one, which is dim. A blue LED takes about 3.0 V, and 3.3 V leaves 0.3 V for the resistor. An LED that happens to take 3.2 V would leave under 0.5 mA, and may look dark. Measure your own: this is the case where one LED differs most from another.
When it does not work
Three causes cover almost every case. It is in backwards: pull it out and turn it round, which does no harm at this voltage. The resistor's second leg and the LED's long leg are not in the same row, so nothing joins them. Or the wire to the − rail is missing or in the wrong hole. Put the meter across the rail first: if it reads nothing, the fault is the power, not the circuit.
A resistor of 4.7 kΩ or more was used by mistake, or the module is on its 3.3 V side and the LED is blue or white. A blue LED needs about 3 V of its own, which leaves only a few tenths of a volt on 3.3 V. Check the bands on the resistor, and check the module's switch.
Neither should. A 220 Ω resistor passing about 13 mA gives off about 35 mW, which you cannot feel. If something is warm, unplug the module and look for a wire or a leg that joins the + rail straight to the − rail without the LED and resistor in between, usually two legs in the same row.
Close is right. LEDs of one colour differ by a few tenths of a volt, the rail depends on the charger and the cable, and the resistor is only as accurate as its last band. What must hold is that the LED and the resistor add up to the rail. If they do, the circuit is doing what the article says whatever the exact numbers.
Same board, one resistor swapped, and the current moves with it.
Change the resistor →Edit this page — content/books/estkit/light-an-led.mdx
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Electronics Starter Kit, Breadboard, Power Module and Components
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