Power and Wiring
This is the chapter that keeps the magic smoke inside the chips. Read it before wiring anything beyond an LED.
The breadboard
The white plastic board with holes: solderless breadboard. Holes in each half-row of 5 are connected underneath; the long outer rails (usually marked red/blue) run the full length for power. Components push in, no soldering.
The #1 breadboard mistake: the center groove splits the board — rows don't connect across the gap. A wire that "should" connect but crosses the groove connects to nothing.
The resistor: your current limiter
Resistors are read by colored bands. For LEDs with 5V, 220Ω (red-red-brown) is the standard choice:
- 5V ÷ 220Ω ≈ 23mA — hmm, that's the unloaded math; the LED drops ~2V itself, so real current is ~(5−2)/220 ≈ 14mA. Safely under the pin's ~20mA limit either way.
You don't need to memorize the bands — phone apps and web calculators decode them. You need to know that every LED gets one.
Power limits (the numbers that matter)
| Limit | Value | Meaning |
|---|---|---|
| Per digital pin | ~20mA | One LED: fine. A motor: no. |
| All pins combined | ~200mA | The chip's total budget |
| 5V pin output | ~500mA (USB) | Powers small sensors/displays |
| Board input (jack) | 7–12V recommended | The regulator wastes the rest as heat |
Careful — the motor rule: Motors (even tiny ones) draw hundreds of mA and generate voltage spikes when they stop. Never power a motor from an Arduino pin. Use a separate power supply and a transistor or motor-driver board (L293D, L298N); the Arduino only sends the signal. Wiring a motor straight to a pin is the fastest way to kill a board.
Careful — the short rule: 5V straight to GND with no load is a short circuit. The board's polyfuse usually saves it, but "usually" is doing heavy lifting — double-check power wiring before plugging in USB.
Powering projects without USB
- 7–12V into the barrel jack — the onboard regulator makes 5V. Above 12V the regulator overheats.
- 5V into the 5V pin — bypasses the regulator (efficient, but no protection — get the polarity right).
- Batteries — a 9V battery works but dies fast (low capacity); 4×AA or a USB power bank lasts far longer.
Wiring checklist (before power-on)
- 5V and GND go where they should — no swaps.
- Every LED has its resistor.
- No bare wire ends touching each other.
- Pins 0/1 free if you're about to upload.
- Unplug USB while rewiring — live rewiring slips, slips short.
Next: Troubleshooting