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Author Topic: Arduino and 2-wire actuator (power door lock)?  (Read 2016 times)
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Hi,

I want to use an arduino to activate a two-wire actuator (automobile power door lock mechanism).

The actuator runs off 12 volts . When power is applied, the actuator flips in one direction. If you reverse the polarity on the actuator and apply power, it flips in the other direction.

I only need it to flip in one direction, so I am not worried about the polarity reversal (I will manually be moving the actuator to "lock" it).

However, I believe  I need to use a relay to power the actuator since I am running off a 12 volt battery, and this thing will be plugged in 24/7.

I dont want 12+ volts powering the arduino since I believe there may be heat issues, but I also want to send the full 12 volts to the actuator.

Am I right that I need to use a relay in this situation. Can you give me a recommendation for the type of relay to use?

Thanks for any help you can provide!
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I did a few auto security and remote start projects when I first started using the Arduino. The best way I found to interface to actuators, etc was to order a small relay with a 5VDC, low current coil and use it as a pilot relay to trip an automotive relay with a 12VDC coil. For the automotive relays I used the common DEI ones you would get a car stereo shop.

In the end your Arduoino will only be providing 5 volts to the coil of the pilot relay. The contacts of the pilot relay will switch the cars 12VDC power through the coil of the automotive relay. The automotive relay contacts that will do the actual work are rated at around 10 amps.
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Will the actuator still be wired to the car?  If so,  you're probably stuck with the relay.  Plus,  even though you intend to "lock" it manually,  you may need to worry about whether some other part of the system is trying to "lock" it while the Arduino is trying to "unlock" it:  that would cause a 12V-to-ground short circuit.

If not,  you can wire the actuator with the + side always hot,  and use a transistor or FET to connect the ground side.  There are lots of examples in the forums and on the Playground:  just be aware that you'll probably need a much stronger transistor than the 2N2222(-like) ones usually used for small relays,  because that actuator almost certainly draws a lot more current.

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Sorry, didn't mean to imply it was being used in a car. It's not. I'm using it to trip the latch on a chicken coop door. (I don't like getting up at the crack of dawn to let them out.)

I already have 12v power there running the electric fence. Nothing else will be connected to the actuator, just the arduino.

I like the transistor idea, I'll look into it some more.

I don't think the actuators draw much current, but I will see if I can dig up a spec. There was no data sheet in the box, but it was $2.99 with free shipping so I am not complaining smiley
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Hey, that's funny. I automated my chicken coop a while back and then took everything back off. My wife was always bugging me with "what if.....?' scenarios. She had no faith in the microcontroller. Here's the secret,  highly technical solution I use now.

I make my kids do it.....

On a side note here's link to a company that sells one. They don't show any useful pics of the assembly but they use a simple hobby DC motor to roll the string up. You could do the same thing with a $10 servo modded for continuous rotation. Then you wouldn't need the relays.
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Haha, I know what you mean. But my kids are still too young. I don't like them disconnecting the electric fence to open the coop door.

I've seen the prefab door openers. I have a door on a hinge now, and I have all the mechanical pieces put together to open the door when the latch is pulled (it is currently set up with a modified sprinkler timer to dump some water in a bucket... total rube goldberg device. It worked good, except when it is < 32 degrees F outside, which was last night).

I'm also wiring in a real time clock and will get it set up so that it opens a little after sunrise, depending on the date.

Thanks!
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I don't think the actuators draw much current
If it's a solenoid moving a door lock know,  it's very likely it draws multiple Amps.  I've seen some (just in catalogs) that are geared motors,  though:  those would probably draw quite a bit less.

You can get a rough idea of the current draw by measuring the resistance and using Ohms Law.

Don't leave out the "backwards" protection diode you'll see in the examples of using transistors to drive relays,  motors,  etc.:  you really do need it to keep the residual energy in the coil from zapping your transistor when you switch the motor/solenoid off.
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I automated my chicken coop a while back and then took everything back off. My wife was always bugging me with "what if.....?' scenarios.

And that's when you tell her "Free chicken dinner!"...

 smiley-grin
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I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

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