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Topic: Fingerprint reader to open electric strike (Read 6121 times) previous topic - next topic

Let me start by saying this is my first Arduino project, and I'm a little confused with some of the hardware requirements. I should be good on the coding, as I've been programing for a long time now.

I want for my first project to build a fingerprint scanner that will unlock an electric strike if the correct finger print is scanned.

As far as the fingerprint scanner goes, I was going to use this one, which has its Arduino source code on Github:

https://www.adafruit.com/products/751

So far, I'm okay with the hardware. As I'm looking more into the electric strike hardware configuration, I'm getting confused.

From what I can tell, the way they work is that I supply a constant supply of electricity to the strike, and then from the Arduino I can send a signal to interrupt this power supply, so that the strike releases.

The strike I was planning on using was this one:

http://www.smarthome.com/5190S12/Electric-Door-Strike-for-Schlage-Locks-8-12VDC/p.aspx

With this power supply:

http://www.smarthome.com/8194B/Linear-Corporation-AAE00381-12-Volt-DC-2000mA-Power-Supply-/p.aspx

From what I can tell, the strike has two wires. One of those wires I would plug into the breadboard, and the other would go to the power supply. Is this correct? Would it come with instructions specifying which one is for which? Also, if my thoughts on how the strike was locked and released above are true, wouldn't I need to interrupt the power supply to unlock the strike, or is that current separate?

I was originally going to follow this guide on how to make the strike setup, but it doesn't provide clear instructions on how to wire it:

http://www.instructables.com/id/Easy-Bluetooth-Enabled-Door-Lock-With-Arduino-An/

Further searching led me to this image, which seemed to show me the correct way to wire this:

http://itp.nyu.edu/physcomp/uploads/arduino_bb_pot_transistor_motor2.png

Except I don't need everything in that diagram. For instance (the parent url is http://itp.nyu.edu/physcomp/Tutorials/HighCurrentLoads), I wouldn't need the potentiometer, and I'd replace the motor with the strike. Then my question goes back to what I asked above, about the two different wires on the strike, as to which one would go to the breadboard, and which one would receive the power.

I understand these are very basic questions, especially for the experienced users, but if anyone can offer some insight on my questions above, that would be awesome. I'll be sure to post pictures once I get this completed (and oh, I'm getting this completed).

johnwasser

That strike says " your visitors will be able to push or pull the door open when power is applied."  That seems to mean it unlocks when you apply power.

You will need a transistor to switch the 200 mA power.  Most any NPN transistor will do.  Connect +12V from you power supply to one side of the strike.  Connect the other side of the strike to the Collector of the transistor.  Connect the Emitter of the transistor to both the - side of the power supply and your Arduino ground.  Connect your digital output pin through a 1K resistor to the Base of the transistor.  Turning on the digital output pin will power the strike and allow the door to open.
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Thanks for the reply, this is a lot of help.

I made a schematic in Fritzing, which I'll post below, and I'm wondering if I followed your instructions correctly:


Full size: http://i.imgur.com/1rl68.jpg

Couple notes on that
• The 9V battery was all I could find for a power supply. Obviously I'd use the wall power source I posted above, which I'm assuming has two wires coming off of it instead of the one I can see (is this correct?).
• I used a motor for the strike since there was no strike in the application

Is this correct? Also, what would I do with the fingerprint scanner, as it also needs to connect to the ground of the Arduino, just connect it to the ground on top of this one?

johnwasser

You have your Arduino Ground and digital output swaped.  You want the Ground going to battery - and the Emitter.  You want the digital output going through the resistor to Base.
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So like this?



Also, would a 2N3904 NPN transistor and a 1k? resistor work?

johnwasser

The circuit looks right now.  The transistor pins are EBC when looking at the flat side.

The 2N3904 switches 200 mA which is just enough to handle the strike.  You might want to use a 2N2222 which is designed for higher current.
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Awesome! We're getting somewhere :)

Now, the power supply has both a negative and positive wire. Which goes where?

johnwasser

Negative wire to Ground.  Positive wire to the strike.  Just like in your diagram (Red is positive and Black is negative).
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Awesome. Now if I think I might buy a different electric strike, as this one is pretty expensive. The one I'm looking at now though is a 12V DC, with a current of 450 mA. Will I need to purchase a 12V DC wall supply that is rated for 450 mA, or can I exceed the limit at all? I'm finding it tough trying to find a company to buy the strike and power supply from for an affordable price.

johnwasser

A Google Shopping search turned up these:

$20.39  YS-130NC
http://www.dinodirect.com/electronic-mortise-lock-standardtype-electric-strike.html?cur=USD&AFFID=15

$22.69 YS-130.NO
http://www.dinodirect.com/electronic-strike-mortise-lock-ys-130no.html?cur=USD&AFFID=15

I'm guessing the NO model is unlocked when power is off and NC is locked when power is off.
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Yeah I saw those. I'm a little weary of DinoDirect... those are rated for a 200mA current, does that mean that the DC wall power needs to be 200mA? I realize that's a terribly basic question, but I'm wondering if there's any room for a higher or lower current.

PaulS

Quote
but I'm wondering if there's any room for a higher or lower current.

Current is pulled by a device. The power supply for that device has to be able to supply, at a minimum, the current required by that device. Otherwise, the device may not work properly.

Think of a car starter and a weak battery.

Having a 500A power supply for a 200mA device is not a problem. Well, affording one might be, but using one won't be.

That makes sense, and will make my life a lot easier.

Would it be superfluous to use an SPDT Relay in this?

PaulS

Quote
Would it be superfluous to use an SPDT Relay in this?

A single pole double throw relay combines a normally closed path and a normally open path in one relay. Activating the relay opens one circuit and closes another. Do you need both?

Personally, I thought the setup above was fine. But I read about them and I felt like they would work well because the strike itself includes one, I'm assuming, but then I thought that since it included one it would be pointless for another.

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