Soldering: Health and Safety guidance

Safety for you and your kit when hacking

Safety for you:

  1. If you're not sure about something, ask someone. You are responsible for your safety.
  2. Don't work with any voltages above 12V. Beware of working with any component that draws more current than 0.25 amps, or has a power rating of more than 1 watt.
  3. Never hack or make anything that uses mains supply electricity without a transformer stepping down the voltage.
  4. Beware of hacking any device that uses large batteries (eg. A number of D cells, car batteries, wheelchair batteries, any battery bigger than, say, an apple)
  5. Don't touch any live circuit unless you are sure that it is safe to do so.
  6. Keep liquids away from circuits and do not handle circuits with wet hands.

Safety for your kit:

  1. Disconnect your project from its power supply before doing any rewiring.
  2. Do not use an Arduino on a metal surface without an insulating layer in between.
  3. If anything gets hot (that isn't meant to), or you smell burning, disconnect the power and rethink.
  4. You can break an Arduino Uno by doing any of the following: Don't!
  • Shorting I/O Pins to Ground
  • Shorting I/O Pins to Each Other
  • Apply more than 5V to I/O Pins
  • Exceed 20mA on any one pin
  • Exceed 100mA in total over pins 1-4, 5-13 or A0-A5
  • Exceed Total Arduino Uno Current of 200mA

uno safety.png

Example load current calculation:

We want to light 10 LEDs simultaneously using an Arduino Uno. How can we wire these without overloading the Arduino?

To know for sure, we need to know the Forward Voltage and Forward Current of the LEDs we are using. These fairly typical red LEDs [] have a Forward Voltage drop of 2.1V, and a Forward Current of 20mA.

Using an online calculator like this one [, enter '1' for array size] we find that we need a 150 Ohm resistor to go with each LED.
If the resistor was not there, we would be putting 5V across the LED, much more than its limit of 2.1V, and sooner or later it will burn out, or explode.
The calculator presumes that we want to put the maximum 20mA through the LED.
So, roughly speaking, each LED will draw 20mA from the Arduino pin, which also happens to be the maximum for any one pin.

The online calculator that I used chose the nearest biggest resistor value to get close to this 20mA. We can check what the actual current draw will be using high school physics calcs:

Total Voltage = Sum of Voltages in series
ie. Total Voltage drop = Voltage drop over LED + Voltage drop over Resistor
ie. 5V = 2.1V + Voltage drop over Resistor
therefore, Voltage drop over resistor = 5V - 2.1V = 2.9V

Current = Voltage/Resistance
Current in resistor = Voltage drop over Resistor / Resistance
Current in resistor = 2.9 V / 150 Ohms = 0.0193333 Amps

Lastly, a handy conversion:
1Amp (A) = 1000 milliAmps (mA)
therefore, Current in resistor = 0.0193333 x 1000 = 19.3333 mA

The current in the resistor will be the same in the LED and that being drawn from the Arduino Uno: 19.3 mA.
This is fine as it is below the maximum current draw off any one pin of 20mA.

The total current being drawn using 10 of these LEDs will then be:
10 x 19.3mA = 193mA
This is fine as it is just below the Arduino Uno's maximum current of 200mA. We could not power any more of these LEDs using one Arduino Uno.

We know we must not exceed 100mA in total over pins 1-4, 5-13 or A0-A5.
So, we can use this simple calculation to find out how many LEDs we can fit in one group of pins:
Max. current draw from one group = current draw of one LED x max. number of LEDs in group
ie. 100 = 19.3333 x max. number of LEDs
max. number of LEDs = 100/19.3333
max. number of LEDs = 5.172
Therefore the maximum number of LEDs we can light from one pin group is 5.

We must split the 10 LEDs between the pin groups so that none has more than 5 in it:
Eg. We can put 4 in the 1-4 group, 5 in the 5-13 group, and one in the A0-A5 group (yes, you can use the analog pins as digital outputs if you want to – see
For example, wire one LED and resistor each to pins 1, 2, 3 and 4, to pins 5, 6, 7, 8 and 9, and to pin A0.