want to know the number of transistor the most suitable

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http://forum.allaboutcircuits.com – I want to control a 6 solenoid valves(Operates at 24DCV and 0.5A) with the use of 6 relays connected to an Arduino's 6 pins via 6 BJT(NPN) Transistor. The Arduino output pin is 5V and can source 40mA maximum. I would like to use a relay to switch the valve on and off. So, suggest me that which transistor is better to use here and value of resistor that I have to put between arduino and transistor. and DC voltage for relay to operate. (Embedded)