I am using an Arduino to control an electric lock strike but I don't know how to power the lock with sufficient power. To control the lock from the Arduino, I am using a TIP31 transistor (It is NPN). The transistor will be getting the signal from pin 9 on the arduino and will connect to the ground wire of the lock strike. Since I will only need the DC barrel jack for positive (since the ground wire of the strike will be connected to pin 9 via transistor), I need to know whether this barrel jack will draw power from a power source even though it will not be connected to ground.

P.S. The lock strike needs 12VDC.

  • \$\begingroup\$ You're going to need to connect the grounds together for this regardless, otherwise the transistor won't function properly. \$\endgroup\$ Commented Jan 23, 2015 at 3:26
  • \$\begingroup\$ Ah, so one of the pins of the transistor would go to the ground of the DC barrel? (sorry I'm new to this) \$\endgroup\$
    – Kyriazis
    Commented Jan 23, 2015 at 3:29
  • \$\begingroup\$ You have to connect both pins of a barrel jack for it to be useful - "only need it for positive" doesn't mean anything. \$\endgroup\$
    – pjc50
    Commented Feb 26, 2015 at 10:20

2 Answers 2


Below is a basic schematic of how everything will be connected. V1 is the Arduino power supply, either 5V usb, or 7V to 12V dc barrel jack. V2 is the Lock Strike power supply. It will vary based on your lock strike, probably 12V to 24V. Regardless of the combination, the red wire below connects the grounds of both power supplies together (the DC, barrel jack side, not the AC wall side!). Just connect any ground connection on the Arduino to the emitter of the TIP31/ground of the lock strike power supply.


simulate this circuit – Schematic created using CircuitLab


You need a return path, so ground needs to be connected somewhere. On the barrel jack? Not necessarily, but you need to get ground to the circuit. I suppose the Arduino has it's own separate power source with it's own ground, but even in this case, you need to tie the two grounds together or you'll see issues. When you have a 12V source, it is 12V above a reference voltage or ground. If you have unconnected grounds on two separate sources, their references won't be the same, so what's supposed to be 12V might not be.

Now, if you are getting the power to the barrel plug from the same source as the Arduino is using, you don't need the ground, since the ground of the Arduino is from the same source. I assume you're using two different sources though.

Here's an answer on here that goes into using multiple grounds: Connecting multiple grounds


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