Build your first electronic circuit
The classic first circuit is a battery, a resistor and an LED. It is small, but it teaches the three ideas behind almost everything else: a complete loop, polarity, and limiting current with Ohm’s law. This guide builds it on a breadboard, measures it with a multimeter, lists the usual mistakes, and shows the same circuit in a simulator.
What you need
- A 9 V battery and a battery clip with leads
- A red 5 mm LED
- A 470 Ω resistor (yellow, violet, brown, then the tolerance band)
- A breadboard
- A digital multimeter
Why the resistor, and how to choose it
Once an LED conducts, the voltage across it stays close to its forward voltage, about 2.0 V for red. Connected straight to 9 V it would try to drop the other 7 V across almost nothing, and a large current would destroy it within moments. A resistor in series takes up that 7 V and sets the current.
Choose a current of about 15 mA, inside the 20 mA continuous rating typical of a 5 mm LED. Ohm’s law gives R = V ÷ I = (9 − 2.0) ÷ 0.015 = 467 Ω. The nearest standard value is 470 Ω, which gives (9 − 2.0) ÷ 470 = 14.9 mA. The resistor dissipates 7.0 V × 14.9 mA = 104 mW, comfortably under a quarter-watt (250 mW) rating.
A fresh 9 V battery measures nearer 9.5 V and a tired one 7 V or less, so the current will drift between about 16 and 11 mA over its life. That is why the calculation uses a margin rather than the LED’s maximum.
| Supply | Voltage across resistor | Exact value | Standard value | Current |
|---|---|---|---|---|
| 3 V (two AA cells) | 1.0 V | 67 Ω | 68 Ω | 14.7 mA |
| 5 V (USB) | 3.0 V | 200 Ω | 220 Ω | 13.6 mA |
| 9 V battery | 7.0 V | 467 Ω | 470 Ω | 14.9 mA |
Polarity: which way round the LED goes
An LED conducts in one direction only. The anode (+) is the longer leg; the cathode (−) is the shorter leg, next to the flat edge on the rim of the plastic. Current flows from anode to cathode, so the anode must face the battery’s positive terminal. A resistor has no polarity and can go either side of the LED: what matters is that it is in the same loop.
Build it
- Leave the battery disconnected while you build.
- Push the LED into two different rows of the breadboard: anode in row 10, cathode in row 11.
- Put one leg of the resistor in row 11 with the cathode and the other in row 15.
- Connect the battery clip’s red lead to row 10 and its black lead to row 15.
- Clip on the battery. The LED lights.
Measure it with a multimeter
Never put the meter across the battery while it is set to current: on that range it is nearly a short circuit, and it will blow the meter’s fuse or worse.
- Set the meter to DC volts (V with a straight line) and measure across the battery: about 9 V.
- Measure across the LED, probes on its two legs: about 1.9 to 2.1 V.
- Measure across the resistor: the rest, about 7 V. The two add up to the battery voltage, which is Kirchhoff’s voltage law in action.
- Work out the current without breaking the circuit: divide the resistor voltage by its resistance. 7.0 V ÷ 470 Ω = 14.9 mA.
- To measure current directly, move the red probe to the mA socket, set the dial to mA, and put the meter in series: unclip the black lead from row 15 and hold one probe on it and the other in row 15. Then move the probe back to the V socket.
Common mistakes
- LED in backwards: it stays dark and nothing is harmed. Turn it round.
- Both LED legs in the same row: they are shorted together by the breadboard, so the LED cannot light.
- No resistor: the LED flashes brightly and dies, sometimes with a pop.
- A missing connection: the circuit must be a complete loop from + back to −.
- Wrong resistor: 47 Ω (yellow, violet, black) looks like 470 Ω (yellow, violet, brown) and lets ten times the current through. Check with the meter’s ohms range before fitting it.
- A flat battery: measure it if nothing works.
The same circuit in a simulator
Building it in Simscuit first is a good way to check your sums. Start the Battery Torch template, which is a 9 V battery, a push button, a 1 kΩ resistor and a blue LED with no Arduino and no code, or place the parts yourself: battery, breadboard, a 470 Ω resistor and a red LED. Press Run Simulation and hover over the resistor to read its voltage and current, or place the simulated multimeter from the Systems category and measure exactly as you would on the bench.
Then try the mistakes safely. Turn the LED round and it stays dark; replace the resistor with a wire and the LED burns out on screen, with the current it reached and the rating it broke. The damage limits come from the LED’s own record, so it is the same arithmetic as above.
Questions
What resistor do I need for an LED on a 9 V battery?
For a red LED (about 2.0 V) at 15 mA, (9 − 2.0) ÷ 0.015 = 467 Ω, so use 470 Ω. A 1 kΩ resistor also works and gives about 7 mA, dimmer but kinder to the battery.
Why does my LED not light?
Most often it is in backwards, both legs are in the same breadboard row, or the loop is not complete. Measure the battery, then the voltage across the LED.