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(Keep in mind I am fairly new to electronics, and I have the knowledge of one high school digital electronics course)I am working on making an Arduino controlled door lock. I want to actuate a 12VDC electric strike using one of the digital I/O pins. Which would I use, a transistor or a relay? If it'd be okay to use either, which would be easier?

John Fisher
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    If this electric strike uses electromagnets, don't forget to add some mechanism to prevent back-EMF to be a problem (e.g. flyback diode). – tangrs May 31 '17 at 03:15
  • MOSFET fine. Place reverse polarity diode across strike's coil - 1N400x *x = 1...7) is adequate and cheap. Look for "logic FET" that fully tuirns on with <=3V drive if using 5V Arduino. Gate can be driven by Arduino pin. Good i8dea is say 2 x 1k resistors in series to drive gate and a 4V7 zener diode at the join on the two resistors. That saves the Arduino when 12V gets applied to the gate connection accidentally. – Russell McMahon May 31 '17 at 12:51

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Hmm, can you describe what you mean when you say electric strike? I'd have to further understand what you were creating, but I would also go with cheaper and simpler. Both are switches, but relays are electromechanical, while MOSFETs are solid state, which means that mechanical aspect is gonna cost you so cheaper is almost always the transistor. But for instance, if you wanted to electrically isolate your circuit, or were considering the voltage drop across your switch, a relay might better suit your needs.

Always look at your datasheet to determine the current that can flow through your device. If you're trying to use the relay or transistor as a method to turn on or off an actuator that moves a door lock make sure that the current ratings of both the relay or MOSFET could handle it. Other than that, because this project is not too intense, experiment and break things, see what works for you and is easiest. But I would recommend a cheap MOSFET that can handle a drain current of whatever your actuating servo requires.

Zotto
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    The electric strike I described is a kind of door lock, it uses an electromagnet (solenoid?) to pull back a plate allowing a door to open and close without using the mechanical lock on the door knob. https://www.amazon.com/UHPPOTE-Electric-Strike-Secure-Control/dp/B00V45GWTI/ref=sr_1_2?ie=UTF8&qid=1496193756&sr=8-2&keywords=electric+strike – John Fisher May 31 '17 at 02:11
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    Oh then definitely use a MOSFET, just a 200mA current necessary. A MOSFET switch will do the job easy. But you are actuating a solenoid, so make sure you place a flyback in anti-parallel to the electric strike. This prevents overvoltage of your MOSFET. If you do not know of flyback diodes read this (https://www.westfloridacomponents.com/blog/inductor-need-fly-back-diode). Good luck. Let me know if I need to clarify anything. – Zotto May 31 '17 at 02:18
  • Hmm, just a little more information; are there MOSFETs that can be switched by something as little as 20mA? This is because the recommended current for a single pin on my board is 20mA(40mA absolute max). – John Fisher May 31 '17 at 02:23
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    Yes actually, MOSFETs have "theoretically" 0 current draw into the base. This isn't true in application but can almost be assumed true. This is because the gate of a MOSFET is a capacitor, which acts as an open circuit at DC, i.e. when a digital I/O is turned to 1 meaning there is 5V at the pin. Just to note, adding a pulldown resistor should be done at the gate of the MOSFET to ensure the gate isn't floating. 100kohm should suffice. I also tend to add a series resistance to the gate, approximately 100 ohms. This prevents LC oscillations, also called ringing, due to switching transients. – Zotto May 31 '17 at 02:31
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    MOSFETs are voltage switched, not current, so yes. – Wesley Lee May 31 '17 at 02:31
  • I ran out of characters, but (https://electronics.stackexchange.com/questions/68748/question-about-mosfet-gate-resistor) this should help clear some things up. This tutorial is very similar to your application. http://www.electronics-tutorials.ws/transistor/tran_7.html – Zotto May 31 '17 at 02:35