I'm trying to control a 12V electronic door strike lock from my 3V3 BeagleBone.

Here's a diagram of my circuit:

Simple Circuit Diagram

My circuit probably needs a NPN transisor of some sort, but I'm not sure which one would work. Also, because the strike lock is a solenoid, do I need any diodes/other protection in my circuit to prevent over-voltage/other bad things?

Thanks. Your help is much appreciated!

  • 3
    \$\begingroup\$ I don't know what your power source for the Beaglebone is, but I'd probably use a separate supply for the solenoid. The BB has/needs information level power, not do-mechanical-things power. Why force that through it? Plus, you may be able to avoid needing the step up circuit. Nor does the solenoid need highly regulated power at a precise tolerance. Yes, a reverse connected protection diode across the solenoid to deal with the jolt when power is removed and its magnetic field collapses is always a Good Idea. (Better qualified persons will answer your which transistor question.) \$\endgroup\$ Jun 27, 2012 at 3:18
  • \$\begingroup\$ Do you have 12V available? GPI01 via 1k resistor to transistor base. Emitter to ground. collector to strike terminal 1 (either). 12V+ to strike terminal 2. 1N400x diode across strike coil - Cathode = line side to =!2V. Transistor BC337-40. Many others work but if you have none that's as good as most. Available Digikey and w=elsewhere. \$\endgroup\$
    – Russell McMahon
    Jun 27, 2012 at 4:56

1 Answer 1


You'll have to connect the lock on the collector side of the transistor; common collector like you've drawn doesn't work.

enter image description here

The BC337-40 suggested by Russell is a good choice; it has a high \$H_{FE}\$, and can draw up to 500 mA.

Your power supply needs some attention. You need 12 V at 250 mA, that's 3 W. If you have a boost converter with 85 % efficiency this will need more than 1 A from the BB, and that's probably more than it can supply. Definitely too much if you're powering the board from USB. I would power the lock from a 12 V/1 A wall wart.

  • \$\begingroup\$ I strongly recommend wiring the diode directly across the lock's terminals rather than at the transistor circuit board (which I assume will be at or very near the BeagleBone board). \$\endgroup\$
    – MikeJ-UK
    Jun 27, 2012 at 12:38

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