I'm relatively new to working with transistors, and I'm having some trouble getting this circuit to work. I want to use an Arduino's I/O pins (5V out) to energize a 12V relay coil. To do this, I'm powering the Arduino with 12 volts via the power port on the Arduino. For my transistor circuit, I'm pulling from the Vin pin on the Arduinio, which is also 12V.

I cannot figure out how to make it so that flipping an i/o pin from 0v to 5v switches the transistor, this allowing 12v to go across the relay coil.

I know the equations for the operating regions and I'm assuming I'm trying to drive the transistor into active by having Vbe > 0.7V, but I don't really know how to set up the circuit to properly regulate this.

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    \$\begingroup\$ I am editing the question to point that the 2N2222 is not a MOSFET. \$\endgroup\$ Apr 26, 2015 at 0:28
  • \$\begingroup\$ Sorry, I sometimes just call transistors MOSFET's kind of like calling tissues Kleenex. I don't work with BJT's much in my electronics class. also, I may be wrong, but isn't it active region we want rather than saturation for BJTs? \$\endgroup\$
    – Jake
    Apr 26, 2015 at 0:30
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    \$\begingroup\$ Well engineering is a more exact science than careless use of words. Better get used to being more exacting. \$\endgroup\$ Apr 26, 2015 at 0:32
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    \$\begingroup\$ If you use a BJT as a switch, which is what you're looking for, you must turn it ON to energize the relay. For BJTs this means to be driven heavily in their saturation region. Do not confuse that with the saturation region of MOSFETs, which is analogous to the active region of BJTs. The terminology may seem incoherent, but the concept of saturation is related to the physics of the device, not to the shape of their characteristic curves, so the meaning of "saturation" is different for BJTs and MOSFETs. \$\endgroup\$ Apr 26, 2015 at 11:14

1 Answer 1


This question has probably already been answered 100's of times. But here is maybe the 201'st time.

Use a circuit that looks like this:

enter image description here

  • \$\begingroup\$ There. Even the transistor is the same. Haven't we got the close→duplicate option? \$\endgroup\$ Apr 26, 2015 at 0:42
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    \$\begingroup\$ If it was closed the OP may never have learned that BJT's are not MOSFET's. \$\endgroup\$ Apr 26, 2015 at 0:47
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    \$\begingroup\$ @user2057387 We are not upset, and no apoloigy is required. But... First you ask a repeat question. Then you confuse BJT with MOSFET. Well... Consider stepping up your game. ;) \$\endgroup\$ Apr 26, 2015 at 0:59
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    \$\begingroup\$ 10K on the base might be a little high, depending on the transistor gain you might need a somewhat smaller resistor. \$\endgroup\$ Mar 4, 2018 at 21:49
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    \$\begingroup\$ Depends on Iload but I tend to agree. NO harm in using a 1k series base resistor. \$\endgroup\$ Oct 9, 2020 at 3:34

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