Arduino + Solenoid Lock I was trying to use a solenoid lock alongside the arduino, using a 12V 2A DC adapter, TIP120, and a diode.

The circuit looks something like this. However, the lock isn't going in automatically (it requires a push).

I've tested the circuit using a multimeter, and the output given by the transistor, is indeed 12V, however, when connected to the lock, the potential difference is around 8V. I don't know if that's normal, but the lock isn't going in on it's own.

Can anyone help me with this issue?
It sounds like your 12 volt adaptor is not powerful enough to operate the lock solenoid. If you power up the solenoid only and directly using the adaptor it should work. If it doesn't, you'll need a more powerful adaptor. If it does work, then the problem is somewhere in your driver circuit or Arduino sketch.