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Topic: Arduino output pins controlling Electric Strike (Read 1 time) previous topic - next topic


OK well 450 mA is a fairly big current,  thats why the voltage of your regulator drops.

Thats the same sort of reason why your "12V" power supply has 17 V with no load on it.

It won't harm the solenoid to have 17 V on it.  I'd just have the power supply without having
a second regulator.    Having two regulators isn't doing much for you except wasting power.

Saying the transistor need 1 A base current is strange.    Is that an upper limit or a minimum requirement ?
If you need 1 Amp base current to get 450 mA through the solenoid,   the transistor is not doing much good.


That 1 Amp on the base current is the maximum limit.   Not a requirement to make it work.  That transistor
is also good for 100 V but that doesn't mean you have to use 100 V.

The Base-emitter voltage is in the region of 1.2 volts.    So your base current limiting resistor seems to
be about right 200 ohms, 220 ohms.

It's not shown on your diagram but I would think you would need the ground also connected to your arduino ground.

The gain of the transistor is rather low.   You may not be getting enough current through your solenoid.
It might be a good idea to measure the resistance of the solenoid  to see what the steady state current is
going to be.   One would think,  the resistance would be something like  12V/0.450 = 26 ohms maybe,  but it might not be.

If the gain of the transitor is not enough,  use two transistors or a darlington device.   Or a relay.

Bear in mind also,  there is a voltage drop across the transistor,  maybe 3 volts,  so if you are using a 12 V power
supply,  the solenoid is only going to see about 9 volts.   

If the voltage at the power supply outlet is dropping too much,   that suggests an undersized power supply.

Also,  feeding a 12 V regulator chip with 12V  is not going to work.   Because of the transistors or diodes or
whatever that make the regulator chip work,  it needs an input voltage maybe 2 volts higher than the supposed
regulated output voltage.      Thats the same reason,   you need to supply at least 6 volts and preferably 7 volts to
the input power jack of your arduino.

I am not really a transistor expert.  Someone in a different time zone may have better ideas.


You need at least 2V of "head room" for the 7812 meaning that it wants at least 14V coming in.  If you don't have a heat sink on the 7812, then it's probably getting pretty hot and possibly shutting down.  You should add a .1uF cap on the regulator output in parallel with the cap you already have.

If it were me, I'd be tempted to see if a 1K (or so) resistor across the 12V supply output would load the 17V down to ~12V and skip using the 7812 altogether; either that or get a better regulated 12V supply.
Experience, it's what you get when you were expecting something else.


Don't worry about the 17V - the power supply is not well regulated, but in this application it will have no effect on the performance of the solenoid in the door strike. Hook the positive output of the power supply to your solenoid, the other side of the solenoid to your transistor and the ground side of your power supply to the emitter of your transistor and to Arduino Ground. Put a reverse biased diode across the solenoid to protect your transistor.

Don't add complexity where no complexity is needed. Under load the voltage from your power supply will drop - if it drops too low then you need a larger power supply.


So you're saying to setup up like this?

My original setup works with a bench power supply... 15V in to the regulator for 12V out and another power supply as 5V to simulate an arduino digital out. But what i noticed is when i use the arduino to trigger the transistor, im getting 4v across the collector emitter and 8V at the output of the lock... Idealy, when the transistor is saturated, there should only be approx. 0.2V across the collector emitter, giving me 11.8V at the lock which is enough to trigger the solenoid.

Im assuming the arduino cannot supply enough current to saturate the transistor. Ive used a POT and measured the voltage across the collector and emitter until i got 0.2V but even that limiting resistor value was not enough to turn the transistor on.

If i use the setup that kf2qd suggested, is the diode in the right position? Im pretty new to the electric strike business and im not sure if terminals of the strike have any polarity. When i tested the strike and applied the positive lead of the power supply to either ends it still worked.

Another question, if i decide to use relays, im going to need two relays for this setup right? One thats connected to the arduino output pin which is turned on when the pin is set high and another 12v relay that will turn on when the relay connected to the arduino is on? Im sure the output current of the arduino wont be enough to trigger a 12V relay. If thats the case, what type of relay should i use? ive searched all around and there are so many different types of relays. DPDT, SPDT, SPST i dont even know where to begin :S .... if anyone here can give me some advice as to what i should use or can recommend one that would be greatly appreciated.

Ive also considered using opto-isolators... would that be any better?

Thanks everyone!!!

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