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I'm looking to raise my blinds every morning using an rpi/Arduino and a motor. The blinds are lifted by a cord that loops near the bottom. I've checked that the cord requires about 20N force to pull down, and the gear turning the cord loop at bottom will have a diameter of 6 cm.

Which motor should I buy?

Min torque needed ~ 20*3 = 60 Ncm. Min speed ~ 20 rpm

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  • \$\begingroup\$ You have force so now you can convert that to a torque. You are missing your speed so you need to add that in and convert it to RPM because you are probably going to require a gear motor. Edit the calculations into your question. \$\endgroup\$
    – Transistor
    Commented Aug 31, 2018 at 19:28
  • \$\begingroup\$ Well, obviously you need a motor with torque above 1.2 N-m (12 kg-cm, 170 oz-in). If you calculate RPM as @Transistor suggested above and choose acceptable voltage range, you'd have enough parameters to start browsing. This last part you'd have to do yourself, of course. \$\endgroup\$
    – Maple
    Commented Aug 31, 2018 at 19:32

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You can choose any miniature DC gear motor with a suitable torque with a down limit stop switch and DPDT relay to reverse with another up limit stop switch. (mechanical or opto). The coil can be a latching or non-latching type relay with snubber reverse diodes across coil and contacts to driving voltage.

Alternatively you can ramp the voltage slowly to limit start acceleration current from 8x down to 1x then detect some current rise sensed to stop immediately and latch off logic that drives a dual FET bridge that acts as a DPDT switch. This protects the cable from excess force in the up direction but not the down direction where a limit switch is still needed unless you detect a very slight rise in current at the end of travel due to lack of gravity pull.

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