I'm looking for a very slow motor. Like half a round in a second, or even less. The movement must be as fluid as possible. I don't care about velocity variation on start or on stop, but I care about the constant velocity.
I've tried with a small DC motor, powering it with both 5V and 3V and a 5 and 10 kohm potentiometer but slowing down after a while it does not move anymore.
Then I've tried with a small stepper motor and an Arduino motor shield and with micro step I'm getting the desired velocity but the movement is no fluid at all and I can see the steps.
I think that I need to use a full velocity DC-motor with some reduction gear but I don't know how to get the right ones. Can someone advice me some pre-made gear set for very low-velocity motor or at least how to calculate them and try to make by myself (3D printing?)
EDIT: no particular torque is needed, I'd like to rotate really light object (lddd then 100g)
Along with the comments about gearhead motors and gearbox motors, for links see JYelton's comments to your question, you can also use a "360 degree Servo" or a "Continuous Servo" with the fun of it having a PWM interface. It's really just a cheap servo with the limitation removed. The smallest ones can be had for about $3 and come quite close to 10 to 30 RPM (= half a round per second or less) with pretty decent continuous motion and a good amount of strength as well.
Of course this does depend on whether you'll be wanting to drive it with PWM or not, but even if you don't, you can remove the back housing, snip out the PCB that controls it and put 1 to 5V on the motor yourself directly to control rotational speed, torque and direction with your own electronics.