Building a Simple LED Circuit

The first real circuit: one battery, one resistor, one LED — and the Ohm's-law math that keeps the LED alive.

You'll need: a battery or DC supply (a coin cell or a 9V battery both work), one LED, one resistor, and a breadboard.

An LED lets current through in only one direction and has almost no built-in resistance of its own — connect it straight across a battery with nothing else in the loop, and it will draw far more current than it can survive, usually within seconds. The resistor's whole job is to limit that current to a safe level, typically 10–20mA for a standard 5mm LED.

To size the resistor: subtract the LED's forward voltage (about 2V for a standard red LED — check its datasheet) from your supply voltage, then divide by your target current. A 9V supply with a 2V LED and a target of 15mA needs (9−2)/0.015 ≈ 467Ω — a 470Ω resistor is the nearest standard value.

Wire the resistor in series with the LED, watch polarity — the LED's longer leg (anode) goes toward the positive side — and connect the pair across your supply. That's the whole circuit.