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So I'm trying to do some insane home DIY with my room blinds. I'm trying to make them adjust (go up and down) automatically.

Now for this, obviously, I will need a motor.

The blinds I'm working with don't require much strength to pull down, but a fair bit to pull them up (using the little string/cord), So I'll need something fairly powerful, but slow.

I'm taking a look at a few gear motors on this site, but I have no idea which one would be right to use for this kind of task.

Could anyone help me out?

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    Try using a cheap fishing scale to determine the force required to raise the blinds. You can then calculate the necessary motor torque as a function of your pulley diameter. – HikeOnPast Jul 25 '12 at 01:17
  • Is your question how to pull this off in general or just how to select the motor? – Jason Jul 25 '12 at 01:19
  • @Jason I figured out how to pull it off already, and I did try a crappy little motor that you get in kids toys, which worked fine for pulling the blinds down, but not up. So I just need a more powerful motor. –  Jul 25 '12 at 01:45
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    If pulling one direction takes a lot more force than the other, use a counterweight to ballance the two directions. That minimizes the force the motor needs to put out and the mechanical system has to be able to take. – Olin Lathrop Jul 25 '12 at 12:22
  • Hear, hear. A perfectly designed counter-weight system (similar to those used in lifts/elevators) would reduce the force requirement to a little over what's required to overcome static friction. You may not need exotic high torque motors at all. – shimofuri Jul 25 '12 at 16:27
  • If those blinds are like mine (AKA venetian blinds, http://en.wikipedia.org/wiki/Window_blind) the counterweight solution unfortunately doesn't works well, because the weight being lifted is variable (lighter when the blinds is fully down, it gets heavier during the lifting as more "blades" rest on the blind lowest bar) – Axeman Jul 26 '12 at 13:34

3 Answers3

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Using electric motor to adjust blinds
The blinds I'm working with don't require much strength to pull down, but a fair bit to pull them up (using the little string/cord), So I'll need something fairly powerful, but slow.

Cord pull in kg ~= (Torque in N.cm)/10 on a 1 cm RADIUS = 0.8" dia drum
Cord pull in pounds = kg x 2.2
Bigger drum = smaller pull. Smaller drum =- bigger pull.

Torque on your chart is given in N.cm (chart image at end)

kg.cm ~ N.cm/10
1cm radius = 2cm diameter ~= 0.8 inch.
So, if you have a 0.8 inch diameter or 2 cm diameter drum for the cord then the cord pull in kg = N.cm/10.
So a 20 N.cm motor will provide 2 kg cord pull.
Pounds force ~= kgf x 2.2
Kg to Pounds: Double pounds, THEN add 10%.
eg 2 kgf -> 2 x2 = 4, +10% = 4.4 lbf
5 kgf = 11 lbf etc.

Looking at their 1st line of motors you have torques including
25, 50, 100, 300, 900 N.cm
= 2.5, 5, 10, 30, 90 kg.cm
= same cord pull on a 1cm radius = 2cm dia = 0.8 inch dia drum.
= 5.5, 11, 22, 66, 198 lbf cord pull.

Use a fishing scale (as DeanB suggests) or tie on bottles of water or exercise weights or ... to get some idea of needed oull.

I'd expect 2.5 kg to be low, 10 kg to be getting OK, 90 kg to be tearing the cord off. YMMV.

Double drum dia = half cord pull.
Halve drum dia = double cord pull.

DIY

You can make your own rotary to linear gearboxes using threaded rod.
Rotate a captive nut on a rod and rod moves.
Rotate a captive rod in a nut and the nut moves.
Simple pulley speed reductions from motor to rod increase overall effective reduction.

enter image description here

Russell McMahon
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I used for this exact purpose a 4 kgcm standard RC servo modified for continous rotation. Cheap, replaceable, easy to be driven from a microcontroller.

It was exactly this servo. There are servos with more torque, if you need it. 4 kgcm is enough for my blinds.

m.Alin
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Axeman
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Stepper motors may be a good choice. Counting the number of steps will tell you how many revolution did the motor make and therefore how high the blind is.

Szymon Bęczkowski
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    AFAIK a stepper motor without a strong gear reduction can't generate enough torque to move the blind drum. – Axeman Jul 25 '12 at 08:44
  • Depends on the blinds. I have some old blinds that even I can't move and some that can be operated with one finger down ad up. Stepper motors will also have some cogging torque to help keep the blinds at desired position. – Szymon Bęczkowski Jul 25 '12 at 18:10