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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?

  • Welcome to Arduino:SE. You might find this helpful: How to ask a good question for Arduino Stack Exchange – sempaiscuba Apr 22 '18 at 20:47
  • lock isn't going in ... what does this mean? .... lock or unlock? – jsotola Apr 22 '18 at 20:47
  • @jsotola by this I mean that it is not unlocking. When I provide it direct 12V dc, it is going unlocking perfectly. – MaJoR21 Apr 22 '18 at 21:17
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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.

  • Well, the lock works (goes in) when I power it directly. I don't know what am I doing wrong. :/ – MaJoR21 Apr 22 '18 at 21:17
  • The next step is to connect the lock to the TIP120 driver circuit. Apply 12V to the base of the transistor through a 1k ohm resistor. The solenoid should work. – VE7JRO Apr 22 '18 at 21:32
  • Well...I still have to push the lock in that configuration.. :/ Don't tell me the lock is faulty... – MaJoR21 Apr 22 '18 at 21:49
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    I think the power adaptor and lock are OK. The TIP120 driver circuit is not allowing enough current to flow through the solenoid. The next step would be to verify the driver circuit has been built correctly. Are any of the components getting hot? With everything connected, try using a jumper wire between the TIP120 emitter and collector. The solenoid should activate. – VE7JRO Apr 22 '18 at 22:07
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    For some reason, the TIP120 circuit is not able to operate the lock solenoid, so your question is more of a "general electronics" question, not an Arduino question. For the price of $1 CDN, you can get a relay that will definitely do the job: aliexpress.com/item/…. There is a chance that your power adaptor is "just powerful enough" to engage the relay, yet when combined with the transistor's voltage drop it is not. You could check this with a power adaptor that can deliver more current. – VE7JRO Apr 22 '18 at 23:15

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