Switching low power with transistor
If you're switching voltage is say 5V from the Arduino, a ubitquitous PN2222 BJT will probably work just fine if you switch it in the saturation zone:

Vbias=5.0-0.7 = 4.3
V=IR
R=V/I
looking for I &lt; 10mA,
R = 4.3/0.010


so R the biasing resistor somewhere around 4300 ohms should easily put the transistor into saturation and only require 10 mA from the pin, well within range.  A more standard value will be 4700 ohms, it little lower drain on the Arduino pin and a bit less delivered current with the PN2222's beta.

There are many pages describing the use of transistors as switches; here's just one of countless. . Keys to look for are:


Limiting bias current from the Arduino to the base of the transistor.  Keep it less than 20mA for sure on the Arduino, yet,
Provide enough voltage to the base of the transistor to put it solidly in the "saturation zone," where it "cannot be on harder" ;) .  The calculation I gave above slams the small-signal 2222 into the saturation zone, where the transistor acts like a switch and not like an amplifier.