The simplest useful circuit: one loop with a battery, a switch, a current-limiting resistor and an LED. It is the first circuit most people build, and it shows the rule every LED circuit follows: something has to limit the current.
There is only one path, so the same current flows through the battery, the switch, the resistor and the LED. Close S1 and the loop is complete; open it and the current stops everywhere at once. That single shared current is what makes it a series circuit.
An LED is a diode. Once the voltage across it reaches its forward voltage, about 1.8 to 2.2 V for a red LED, its current rises very steeply for any further increase in voltage. Connected straight across 9 V it would try to draw far more current than it can survive. R1 takes up the difference: the 9 V splits into roughly 2 V across the LED and 7 V across the resistor.
With 7 V across 330 Ω, Ohm's law gives about 21 mA, close to the continuous rating of a common 5 mm red LED. If you want the LED and the battery to last longer, 470 Ω gives about 15 mA and the LED looks only slightly dimmer.
The order of the parts in the loop does not matter: the resistor can sit before or after the LED and the current is the same. The LED's direction does matter. Current flows from anode to cathode, so the anode (the longer lead) must face the battery's positive terminal, or nothing lights.
I = (Vbattery − VLED) / R = (9 V − 2 V) / 330 Ω ≈ 21 mAUse the forward voltage from the LED's datasheet; blue and white LEDs need about 3 V.
R = (Vbattery − VLED) / I = (9 V − 2 V) / 15 mA ≈ 470 ΩRound up to the next standard value so the current lands at or below the target.
P = I² × R = (21 mA)² × 330 Ω ≈ 0.15 WA 0.25 W resistor is enough. Keeping the power under about half the rating keeps it cool.
Working with your own values? The resistor color code calculator does the sums.
Resistor color code calculator| Ref | Part | Value | What it does |
|---|---|---|---|
| B1 | Battery | 9 V | 9 V battery. Any supply works if you recalculate R1 for it. |
| D1 | LED | Red | Red LED with a forward voltage of about 2 V. Anode toward the battery's + side. |
| R1 | Resistor | 330 Ω | Current-limiting resistor. Sets the LED current to about 21 mA. |
| S1 | Switch | — | On/off switch. It can go anywhere in the loop. |
Recovered from the drawing itself: every pin on each net below is joined by a wire, a junction or a shared supply or ground symbol.
| Net | Connects |
|---|---|
| N1 | B1.-, D1.K |
| N2 | B1.+, S1.1 |
| N3 | S1.2, R1.1 |
| N4 | R1.2, D1.A |
For a red LED at about 20 mA, (9 V − 2 V) / 0.02 A = 350 Ω, so use 330 Ω or 390 Ω. For a blue or white LED, (9 V − 3 V) / 0.02 A = 300 Ω, so 330 Ω. For an indicator, 1 kΩ is usually bright enough and saves the battery.
Either. In a series loop the current is the same everywhere, so the resistor limits it from either side of the LED.
Every part sits on the same single path, so the current through each one is identical and the voltages across them add up to the battery voltage.
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