■ Arduino & Electronics Safety + Power Notes
■ General Safety Rules
- Never short +5V and GND together. That creates a direct short circuit, which can instantly overheat
wires, reset your Arduino, or damage the USB port.
- Always use resistors with LEDs. LEDs allow current to flow freely; without a resistor, they’ll burn out or
damage the Arduino pin. Common safe values: 220–330Ω.
- I/O pins can’t power devices. Each digital/analog pin is designed for signals, not power. They can
source/sink ~20 mA safely (40 mA absolute max). Trying to run a motor or big LED strip from a pin will
damage it.
- Check polarity before connecting.
- LEDs: Anode (long leg) = +, Cathode (short leg/flat edge) = –.
- Diodes: Stripe = cathode (–).
- Electrolytic capacitors: marked – side must go to ground.
- Passive components (resistors, ceramic capacitors, switches, inductors) don’t have polarity → can be
placed either way.
■ Static / Handling Safety
- Arduino and beginner components are durable, so static isn’t a huge risk — but sensitive ICs (like
MOSFETs, memory chips, sensors) can be affected.
- Discharge static before touching components: touch grounded metal (like a PC case) with your hand.
- Don’t place Arduino directly on metal surfaces (like your PC case or tools). That can short solder joints
on the underside.
- Safe work surfaces: mousepad, cardboard, wooden desk. Avoid carpets for big projects.
■ Powering Arduino & Circuits
### Inputs (ways to power Arduino itself)
- USB (5V) → easiest, safest. Supplies up to ~500 mA from a PC port, or more if from a wall brick.
- Vin pin or Barrel Jack (7–12V) → passes through Arduino’s onboard regulator, which steps it down to
5V. Works best with 7–9V.
- 5V pin → bypasses the regulator. Only safe if you’re giving it a stable, regulated 5V (like from a
breadboard PSU). If more than 5.5V is applied → risk of frying board.
- 3.3V pin → same idea, but only for devices that run on 3.3V. Current is very limited (50–150 mA).
### Outputs (power you can tap from Arduino)
- 5V pin → gives regulated 5V (from USB or Vin). Use it to power sensors, small ICs, LEDs. Don’t
overload past ~500 mA total.
- 3.3V pin → for low-current sensors or logic that need 3.3V.
- Digital/Analog I/O pins → signals only, not for powering loads.
■ Powering External Devices
- Motors, servos, relays draw more current than Arduino can safely provide.
- Always use a separate power supply (AA batteries, Li-ion, DC adapter) for motors.
- Arduino doesn’t power the motor directly → instead, it controls a transistor, MOSFET, or driver board
that switches the motor on/off.
- Grounds must be connected together. Arduino GND ↔ Motor battery GND ↔ Driver GND. This keeps
everything at the same “zero voltage reference” so signals are understood correctly.
- Don’t power motors from the Arduino 5V pin — even small ones can cause resets, overheating, or
damage.
■ Why Batteries Are Better for Motors
- A 9V rectangular battery can power Arduino itself, but dies quickly when used with motors.
- AA batteries in packs (4–6 cells) can supply much higher current for longer, making them better for
motors.
- Rechargeable options (NiMH, Li-ion) are even stronger for robotics.
- General rule:
- USB / Breadboard PSU → logic, sensors, LEDs.
- Battery pack / DC adapter → motors, servos, high-current loads.
■ Arduino Power “Do’s and Don’ts”
- Do:
- Use USB for testing and learning circuits.
- Use Vin or DC jack for portable power.
- Use 5V pin for sensors/LEDs.
- Always add flyback diodes across motors/relays.
- Don’t:
- Power devices directly from I/O pins.
- Exceed 500 mA from Arduino 5V pin.
- Connect unregulated voltage to the 5V pin.
■ Multimeter Basics
### Voltage
- Set to DC volts (V■).
- Red probe → test point, Black probe → GND.
- Use it to check power rails, sensor outputs, Arduino pins (should be 0–5V).
### Resistance
- Set to Ω.
- Only test when the circuit is powered OFF.
- Good for verifying resistor values or checking if a connection has resistance.
### Current
- Move red probe to current port.
- Set to A or mA, start on the highest range.
- Must wire meter in series with the load (not across).
- Use to check how much current your circuit draws (important when running on USB or batteries).
### Continuity
- Mode with a beep (■).
- Use to test if two points are connected (0Ω).
- Great for debugging breadboard wiring errors or checking if a wire is broken.
■ Extra Key Rules We Covered
- Always tie grounds together when using separate supplies. Otherwise, signals float and devices won’t
work.
- Flyback diode across motors/relays: cathode (stripe) to +, anode to –. Protects transistor/Arduino from
voltage spikes when load turns off.
- Base resistor on transistor (220Ω–1kΩ) is mandatory — otherwise, Arduino pin may source >40 mA,
which is unsafe.
- Check polarity on electrolytic capacitors and diodes — reversed = possible explosion or failure.
- Test your power source with a multimeter before connecting to Arduino (make sure it’s in the safe
range).
■ Summary in Plain Words
- Arduino is good at control and logic, not raw power.
- Use Arduino 5V for small stuff (LEDs, sensors).
- Use external supplies + drivers for big stuff (motors, servos, relays).
- Always connect grounds together.
- Always use resistors and diodes where needed.
- Use your multimeter to double-check voltage, current, and continuity before plugging things in.
- Don’t panic about static — just avoid obvious shorts and metal surfaces.