Hi I'm just about to start a project of electric blinds for my shed, I've searched a lot and trying to figure out if you could have the motor running the exact amount of time it takes for the blind to go fully down. And same on returning up again. This will be from a voice command through Alexa for example or a remote fob button.
Thanks Steve
You could definitely go by time, simply using the millis() function to see how much time has elapsed.
I don't know what these blinds look like, but would limit switches to detect the position work? That would be a better option depending on the specific application IMO.
Or if these blinds roll up on a spindle, perhaps use an absolute encoder so you measure by revolutions rather than time?
EDIT: I didn't catch the "stepper motor" part for some reason. Like everyone else said, a DC motor would be better. Along with my other suggestions.
I did a project for work last year that uses a similar concept except with an actuator that moves a large sheet of metal back and forth. On startup, I made it calibrate a known "zero" position by moving the metal sheet until a fixed limit switch is tripped. Then I knew where it was.
The actuator has a built-in hall sensor that lets me keep track of movement in fixed intervals and what direction the movement is, so after I have the zero position determined, I can control the actuator and move the sheet to any desired position between fully open and fully closed.
I suspect it would be much simpler just to put a limit switch at the top and the bottom and run the motor until the switch is triggered. With that arrangement a simple geared DC motor would do the job and would be a lot more energy efficient than a stepper motor. With a worm drive the DC motor will hold position without any power.
With a stepper motor you will need at least one limit switch so that the Arduino can move the blinds to trigger the switch when the Arduino starts so it knows where the ZERO count for the steps is. And a direct drive stepper motor needs full power all the time to hold position.
IMHO time is not a very practical way to judge the movement. With a DC motor the speed can vary slightly depending on friction and other things. With a stepper motor there would be no need to use time - just count steps.
Using stepper motors You preferably use a certain number of steps rather then time. If the motors are strong enough it could work. Some way the system needs to be started from a known situation. Any unexpected trouble will tilt the system "for good".
Using DC motors and end switches is another possibikity.
I've never used limit switches before but they are venetian blinds so they have no bottom rail so I'm not sure how I could wire a limit switch at the bottom with wires trailing down. To make that a little bit harder these are on 2 opening doors so the idea is the blinds go up then you open the doors so the blind isn't banging around etc. Thanks for the replys lots of things I hadn't even thought about!
Maybe a sensor at both top and bottom mounted to the wall. Stop the motor when the lower one is blocked on the way down. And stop on the way up when the upper one is no longer blocked.
Just another idea to throw in the pot.
(An infrared proximity sensor or an ultrasonic range finder come to mind)
Ok if I run a DC motor with a gear system to the pulley etc. What is the best way to get the motor to stop at the top and the bottom? Obviously don't want it to be trying to pull the blind past the very top position and snap the strings.
Limit switches not ideal as don't really want wiring at the bottom. Could the revolutions be used in this situation?
Can You enlarge Your thoughts regarding wiring? I don't understand wiring at the bottom
Yes, its a double door. Both doors open 1 blind on each. the wiring to the motor etc thats attached to the blind will come from above the door only long enough to allow the door to open without pulling on the wires. I don't want a wire running down the door if I can help it as I would have to make a channel for it to fit in and hide it, and it can't come from the bottom as that will be where you walk to get through the doors.
Ok. Can You produce pictures of the blinds, especially the machinery?
An end switch at the top ought to be possible, cables being close to the cables for the motors. Then, some way, have good controll of the lowering and it might work.
Thats the style of blind, the pull string that pulls the blind up and down will be removed, then will now be using a DC motor and getting a piece 3d printed to link the motor to a mechanism which winds the string up to raise the blind and reverses to lower the blind. I could possibly fit a limit switch at the top within the top rail of the blind and then would have to be a set amount of rotations to lower it maybe?
Ok. That kind of blinds will have the longest life if the blades are set horisontal before they are lifted up.
How/where do You plan to attach the motor?
Yes was hoping to maybe use a 180 degrees servo to open the blind before it raises and its also an added feature.
The motor will be housed in the top rail where the mechanisms are, if the motor with gear system and mounts are bigger and they come out of the top I will make a trim to go over it all so you can't see any of it but its all concealed within the top rail of the blind.
I doubt You will manage to hide any motor inside the console at the top. There is a long bar across the entire length inside that console.
The number of turns of that bar can be a Little bit uncertain depending on how the lifting strings wind up on the drum inside. But, keep on thinking! I have been thinking of something like this at home.
I know its not a bit area but it can come out the top and be screwed to it, then make a nice finishing trim to hide all the wiring mechanism motor servo etc,
Only other option I'm thinking of is make a full frame so the blind slides down a rail each side and then make a bottom rail and put the lower limit switch within that, I'm guessing this would be the most accurate for the blind operation and reliability. Thinking of wrapping the frame in 4 way stretch that I've used in campers previously
wilco1989:
I've never used limit switches before but they are venetian blinds so they have no bottom rail so I'm not sure how I could wire a limit switch at the bottom with wires trailing down.
One option is to choose a DC motor that will not be damaged by excessive current if it stalls AND whose stall torque will not damage the blind mechanism. With such a motor you could just operate for (say) 50% more time than is needed for an UP or a DOWN and you would know that the blind must be fully open or closed.
Another option would be to add a screw- mechanism to the blind mechanics arranged so that as the blind drops the nut moves until it triggers a limit switch. When the blind rises the nut will move in the other direction and eventually trigger the other limit switch,
Another option is to fit an extra piece of cord running from the bottom of the blind over an extra pulley at the top and running down behind tensioned with a weight. Two small obstructions fitted near each end of the cord could trigger limit switches attached at the top of the blind mechanism.
I'm sure there are many more ideas
...R
I try to send 2 pictures.
My blinds have like rails keeping the blades staying in their place even if the window is tilted, upper part of the window opened, leaning into the room. Being opened like doors, no problem. The blinds will not fly around.
@Robin2
Lowering that kind of blinds too much might make a string mess up up there, or make the blinds even start moving upwards again.
Stalling motors are not an option as I think. That will surely shorten the life of the strings liftinglowering the blades.
Seen them blinds, look ideal tbh. Think my way forward is making a full frame top bottom and sides with limit switches top and bottom.
Just need to figure the Alexa control part along with a remote if possible and get parts 3d printed to control the up and down motion for the blinds. Be well worth it though if it works!
Railroader:
Lowering that kind of blinds too much might make a string mess up up there, or make the blinds even start moving upwards again.
A problem for every solution 
I was hoping to get the OP to do a little thinking outside his box.
...R
