I have a project for my dorm at college. Every student has an ID card with a magstripe on it. I'm trying to build a system that will read the cards and unlock the door for my suitemates and myself.
Right now I'm focusing on the hardware that will physically lock and unlock the door.
So far I've been trying to use some old Futaba servos to pull string that flips the switch on the deadbolt. It works fine while it does work, but I'm having huge problems with the servos not being reliable at all, I've spent 30+ hours troubleshooting just the servo's grounding and phantom spins in the past two week.
Anyway, I've arrived at the point at which I'm so absolutely fed up with trying to get these two servos working that I'm planning to go back to scratch and use either new servos or some other device to open the door.
I've read up on stepper motors, DC motors, and newer servos, and can't really see which will be the best to integrate with an arduino. Flipping the deadbolt switch takes a good bit of torque, enough that my old RC plane servos could barely turn it if they were mounted directly above the pivot and glued to it. So I need something with a little power behind it.
I can already supply 6V 500mA at the door, and could buy a 12V or so adapter if needed.
Does anyone have a suggestion on what may work?
It's about 11:20 over here, and I really need to get to bed, class at 8 tomorrow. I'm thinking of opening up a new thread somewhere (hopefully) appropriate and just laying out as much detail as I can about my project. I'm a computer engineering freshman, and I really want to understand this stuff, not just expect other people to use google for me on a forum..
also, I found some servos that are reasonably priced (money's an issue x.x but so is this grade) and they claim to have 80 oz per inch of torque.
Through this page:
http://wiki.answers.com/Q/How_many_pounds_is_80_oz
I found that that torque specification is about 5 pounds from a one inch arm, and I'd only count on 3 or 4 because I don't want to max out hardware. I think that's enough to turn my deadbolt, maybe. I'll try to find some way to measure the torque it takes to turn the knob...
Why not extend the knob/lever of the lock; that will reduce your torque requirements (though increase the length of travel needed, of course). If you are worried about modifying the lock, find out what make/model it is, and contact the manufacturer for a replacement knob kit - modify -that- part, and install it in place of the lock on the door.
Another option would be to use something other than a servo (perhaps a larger gearmotor, and figure out your own feedback/positioning system)...
cr0sh,
Right now I'm toying with the idea of trying to mount the servo "upside down" directly above the knob's pivot and just hot-glue it in place. If I extend the knob, then I would need the servo to pull a piece of string attached to the end of the knob or something, and then I'd need two servos on each side to pull it in two directions. That's kind of the setup I have right now.
Your point on modifying the lock was very helpful, I took a look at the shroud between the knob and the actual door, and I can be unscrewed, which releases the knob! It turns out that the knob attached to a 1/16" by 4" by 1/4" ish shaft (looks like the end of a medium sized flathead screw driver) that goes into the door and actually turns the gear for the deadbolt.
I think it would be kinda neat to re-create the shaft and put my own lever on the top of it for the servo/motor to interface with. It wouldn't be too much modification of the lock, and I could just replace the stock knob if asked. Also, it might work well with the upside down servo idea.
cjdelphi,
I don't think I can use a solenoid door lock, I need to make my project integrate with the existing door hardware. But then again, I don't really know what a solenoid lock is :~ I'm thinking of something that I'd have to totally replace my deadbolt with.
Cool, I'll try to get pictures and upload them later.
Rather than trying to turn the lock, you open the striker instead. This means that you can still use a key, and it preserves any emergency or fire escape systems that are already built into the lock.
Wizdum,
That's really neat! I think something like the lock strike you linked to would be the best option, especially considering the price, but I don't think that the Resident Director will let me remove the deadbolt all together or add another one. The people in charge of my dorm want to be able to physically override the servo/motor with their own key on the other side of the door if they need to.
Also, I've attached a picture of the knob on my door.
Then I realized the electric strike only works if there's a space in the doorframe for the strike to move through. I can't saw a hole in the doorframe because this is a dorm and I'll get charged to repair that if I can't undo it when I move out.
Crap. One stupid lack of opening in the doorframe and now I can't do this.
