Switching +12VDC by arduino
Thank you for clarification! Then this simplified circuit is reliable:







simulate this circuit &ndash; Schematic created using CircuitLab

M1 is not IRF530, it is any N channel Logic Level MOSFET with max drain to source voltage 20 V or more. R2 must be rated for dissipation of 72 mW, it is two 0402 SMD resistors or one 0603 or larger SMD resistor with rated power 80 mW, or almost any leaded resitor.

Same circuit with BJT:







simulate this circuit

Q1 must be a switching BJT with transition frequency 200-400 MHz, current transfer ratio h21 no smaller than 50 at collector current 10 mA and max collector - emitter voltage higher than 20 V. Plenty of BJTs are suitable.