Hey thank you for the input.
I should have stated a small transistor (2pn2222) vs. mosfet.
The relays can handle up to 10amps but I am only going to draw <1 amp.
So I will use the small transistor and use a smaller resistor between the middle pin and the arduino pin lets say 300 Ohms?
Thanks again !
IF you are going to use a BJT, you size the resistor to (a) limit the current supplied by the Arduino pin and (2) make the current high enough to saturate the transistor.
So, pick a value like 20 milliamps. Safe for the Arduino, and the relay draws 100 milliamps, you need a current gain (beta) of 100/20 or 5.
The 2N2222 current gain is shown as a minimum of 100 at 150 mA Ic, so now we see that 20 milliamps is way more than enough... let's drop Ib down to 10 mA.
So, 5 volts - 0.7 for the base-emitter drop is 4.3 volts. R = 4.3 / 0.010 = 430 ohms. Either 390 or 470 are close, standard values and either one will work.
This is how you figure out the proper base resistor... "dartboard engineering" - saying "well about xxx ohms" is not the way to do it!
Remember, if you use a DARLINGTON BJT, then you have two base-emitter drops, so you calculate with 5 - 1.4, not 5 - 0.7