hi there,
i would like to create something that i can put inside my place door on the linchpin of the lock and rotate it enough to lock/unlock the door... but i am looking to servos and gearmotors and i can't find anything... can someone help me pointing in the right direction?
what i need
1 something strong enough to turn the lock (how much is strong enough? 400N/cm? how do i calculate it?)
2 something that is not too big
3 something that can rotate multiple times clockwise and counterclockwise (even 3 times)
4 i would like to be able to use the key from outside, so something that is not self blocking
5 something that has "limits", so it can turn the right angle (>360°) to lock and unlock
6 something that can be aware if it was turned with the key, so that it can understand if the door is locked or unlocked, and when started it can invert the status of the lock

i hope someone can give me some advice. thanks in advance

 

Welcome to AAC!
How is simply rotating a lock part going to work? Locks have levers or tumblers which have to be aligned with a key before any rotation can occur.

 

Hi there, thanks for the quick reply!
From the outside you need a key, from the inside there is a knob, like this one

I would like to remove the knob and attach the device on the linchpin that connects the knob to the lock.

 

hi there,
i would like to create something that i can put inside my place door on the linchpin of the lock and rotate it enough to lock/unlock the door... but i am looking to servos and gearmotors and i can't find anything... can someone help me pointing in the right direction?
what i need
1 something strong enough to turn the lock (how much is strong enough? 400N/cm? how do i calculate it?)
2 something that is not too big
3 something that can rotate multiple times clockwise and counterclockwise (even 3 times)
4 i would like to be able to use the key from outside, so something that is not self blocking
5 something that has "limits", so it can turn the right angle (>360°) to lock and unlock
6 something that can be aware if it was turned with the key, so that it can understand if the door is locked or unlocked, and when started it can invert the status of the lock

i hope someone can give me some advice. thanks in advance

look at
https://www.safewise.com/resources/electronic-door-locks-buyers-guide/

Picbuster

 

1 something strong enough to turn the lock (how much is strong enough? 400N/cm? how do i calculate it?)

(1) Attach one end of a radially-directed lever to the knob, attach a spring-balance (luggage scales) to the other end and measure the force needed to rotate the knob. Force times lever length = torque. Double that (at least) should give a fair approximation of the motor torque needed.
(2 and 3) Geared DC motors are available online or from model/craft shops.
(4) That could be a problem. Any geared motor with enough grunt to turn the lock would show considerable resistance to being back-driven by mechanical key force alone. Methinks you would need an electronic key/code reader to signal the motor to reverse.
(5) Motor stall current could be sensed to establish limits.
(6) See (4).

Given the effort and expense in providing all the features you have set out, it would probably be cheaper (and a lot less hassle) to buy an off-the-shelf electronic lock such as in Picbuster's link above.

 

Thanx for your replies!
I am well aware of August and Sesame, which are the closer in the market (with others) to what i need.
But I would like to build it myself, to learn something and have some fun.

@Alec_t: why a geared motor with enough grunt to turn my lock should necessarily oppose force? Any way I can add some "clutch"?
What are motor stalls? You mean when the motor can't turn anymore in one direction? I was oping to find something like roller shutters motors that "know" how many times they have turned and how much they can turn in way or the other.

 

Stepper motors can deliver high torque when powered, be easily turned when not powered, and you can count steps to know accurately how far it has turned. I think that would be your best bet.

Bob

 

@Alec_t: why a geared motor with enough grunt to turn my lock should necessarily oppose force?

Friction in the gearing. Magnetic forces.

What are motor stalls? You mean when the motor can't turn anymore in one direction?

Yes. Motor current rises dramatically when the motor can't rotate.

 

While I understand the desire to build your own, I would still recommend the August lock.

I put an August lock in about 4 months ago. It is absolutely amazing and I love it. It senses when the door is open or not completely closed to prevent motor jamming. The app on the phone works great.

Just a note - the batteries failed after 2 weeks when installed. I initially thought this was going to be a problem (replacing batteries every 2-3 weeks and the lock doesn't get heavy use!). Anyway, they were cheap batteries that came with unit. I put in decent batteries and haven't had a problem since.

 

While I understand the desire to build your own, I would still recommend the August lock.

I put an August lock in about 4 months ago. It is absolutely amazing and I love it. It senses when the door is open or not completely closed to prevent motor jamming. The app on the phone works great.

Just a note - the batteries failed after 2 weeks when installed. I initially thought this was going to be a problem (replacing batteries every 2-3 weeks and the lock doesn't get heavy use!). Anyway, they were cheap batteries that came with unit. I put in decent batteries and haven't had a problem since.

Hi!
As I said, i think too that the august lock can be a good device, but I would like to build mine so that it can meet my expectations and let me have some fun. It's like if I were asking for a cake recipe 'cause I like to cook (even if I don't have a clue) and you are proposing to buy one
Apart from that, August differs from what I am looking for for these reasons (that may be corrected aftermarket):
1 I want to power the device with the line, and maybe have capacitors/batteries only for when there is no power. I don't want to change batteries not even once a year (mainly because I am not home often and my wife would complain a lot if something doesn't work), I want to install the thing and forget it
2 I don't want to open it using an app, I would prefer rfid/nfc (so if I want to use my smartphone, I still have nfc)
3 On amazon it is priced 386 €... I was hoping to build something recycling stuff and spending less... like a lot less.

 

Hi!
As I said, i think too that the august lock can be a good device, but I would like to build mine so that it can meet my expectations and let me have some fun. It's like if I were asking for a cake recipe 'cause I like to cook (even if I don't have a clue) and you are proposing to buy one
Apart from that, August differs from what I am looking for for these reasons (that may be corrected aftermarket):
1 I want to power the device with the line, and maybe have capacitors/batteries only for when there is no power. I don't want to change batteries not even once a year (mainly because I am not home often and my wife would complain a lot if something doesn't work), I want to install the thing and forget it
2 I don't want to open it using an app, I would prefer rfid/nfc (so if I want to use my smartphone, I still have nfc)
3 On amazon it is priced 386 €... I was hoping to build something recycling stuff and spending less... like a lot less.

I completely understand building your own. However, the mechanics are where things can get really complicated. Also, the August app does have NFC - when I approach my house, it unlocks automatically. When I leave, it will lock about 1 minute after departure. Additionally I can check the status of my door from anywhere - open/closed/locked/unlocked.

I'm not trying to discourage you in any manner - it's definitely more fun to build/create things! But sometimes, being lazy is fun also!

So, for your project, watch out for several things - mainly detecting whether the door is open/closed, and if closed, that is closed ALL THE WAY. If not, the lock could activate and jam the bolt or damage the motor (unless you put motor protection circuits). I like the suggestion of a stepper motor - they have excellent torque for their size and you can control how many steps to move. Keep us posted!

 

I like the suggestion of a stepper motor - they have excellent torque for their size and you can control how many steps to move. Keep us posted!

I like it too, but as far as I know stepper motors need a driver circuit, I hope it can be something not too big.
I will look for some documents and try to understand what to do.
Thanks!

 

One advantage on a stepper motor is the the motor current is always at the plate rated , and is not exceeded, even at zero rpm or stall, or should never be.
IOW if the rated plate voltage is used, then this will apply.
In this application, where you may not need high rpm, then this makes the controller much simpler, as stepper controllers used in high rpm applications use a higher supply voltage in order to maintain current the rated current as inductive reactance increases with RPM.
Max..

 

A mechanical thought. Use a small reversible DC gear motor, small reduction, feeding a worm drive but gear engagement is by a solenoid. add limit switches.

 

@airone12
Where are you located?
Princess Auto in Canada has a selection of geared surplus automotive motors.
You mention also you want to turn the pin with a Key operation also, this will preclude the use of worm and pinion drive motor/GB combo as you cannot back-feed a W&P gearbox.
Max.

 

I like it too, but as far as I know stepper motors need a driver circuit, I hope it can be something not too big.
I will look for some documents and try to understand what to do.
Thanks!

You are going to need a driver no matter what. A microcontroller plus a few MOSFETS can do it, while also handling the communication. This is a simple project with an Arduino and a couple of shields.

Bob

 

@MaxHeadRoom : I am in Italy

@BobTPH: I will look into Arduino, thanks

 

Sound like a good learning project.
Regarding the stepper motor, you could connect it to the lock via a toothed belt and appropriate pulleys.
Good luck.
Keep us posted please.

 

I agree with @btebo
By the time you get all the stuff needed, rig it to fit on your door, program it.......yada,yada,yada
you could buy one cheaper and better
https://august.com/products/august-...MIvKSPgO2l5AIVrP_jBx1xMAVuEAQYASABEgINzvD_BwE
Brzrkr

 

Hi there,
still working out the details, I bought an Arduino and looking for a suitable stepper motor with low detent torque.
One question: how do you suggest to connect the shaft of the stepper motor with the linchpin of the lock?
Thanks again