Arduino controlled Relay

The ATmega328p is rated at 20ma not 40ma. 40ma is the absolute max. The data sheet says operation over 20ma will reduce it's life.

@flowDAQ Thank you for that note! Fortunately this circuit operates at currents far below that, I'm glad for your tip! :)

I know beginners somehow like more the common collector circuit. But its this way only for Arduino Uno 5V family. Better drive it as 12V relay with OC transistor. Logic negation can be made easy in SW. I like to use the Arduino Due which has a 3.3V interface.

i made a switch indicator from this.

its link is this: http://everycircuit.com/circuit/4640475634466816

Triple-Max cloned this circuit and added 40Watt resistor, awesome bullshit extension. Keep on troll.

@hurz that Due is made by Arduino.org, so I'm wondering what you think about the little war going on.

@TimoM consider if the NPN transistor has its emitter connected to the gound. It then needs a resistor (something like 330 Ohm) to limit the current into the transistor base. The transistor could sink current through the coil from the 12V supply (e.g. as a common emitter, see @Hurz suggestion). This would allow most of the 12V to drop across the coil. Unfortunately, as it is shown now only 4.2V drops on the coil, and 7.8V on the transistor. A 12V relay typically needs more than 4.2V to turn on so I am guessing you have a 5V relay. The Due can not source as much current (probably 5mA?) so a 1kOhm may be needed at the base, but the same circuit may work from a 3.3V pin.

I haven't heard about a war? Why and who about what ;-)

@flowDAQ Oh alright that's true! I was using a 12V relay, it would draw 42mA which are 22mA more than the Uno's recommended limit

An old link but... http://hackaday.com/2015/02/25/arduino-v-arduino/