Not 100% Sure what I'm looking for

Hi all,

I’m currently in the middle of a project and could use some advice/clarification. What I’d like to be able to do is use the arduino to spin a hand of a clock to an arbitrary position. I know most servos have a limited span through which they’ll rotate (ie, 0-180 degrees), which clearly isn’t ideal for this, so what exactly should I be looking for?

I just need something small that I can take a 0-359 degree angle and spin to it. Thoughts?

A servo and a 1:2 gear set?

Thanks, just wanted to make sure that a standard servo is the right thing to be looking for, not a continuous rotation servo or something like that.

A normal continuous rotation servo doesn’t have the ability to turn to a specified angle.
It’s really just a motor with a convenient three wire interface.
There may, however, be specialised servos that turn > 180 degrees, but you’d have to find an R/C supplier.

Use a stepper motor. You can turn it to exactly where you want at whatever speed you want.

Stepper. Repeatable, and less work than servos (IMO).

Stepper. Repeatable

Possibly, though it may still need a detent switch of some sort, like the old track zero interrupter on a stepper-driven disk drive head.

Possibly, though it may still need a detent switch of some sort, like the old track zero interrupter on a stepper-driven disk drive head.

Never heard of it…what does it do and how does it work?

As long as the current is high enough and there isn’t a large load, it will probably work repeatably without detents.

The idea is that at the start of the day, a stepper doesn’t know where it is.
Whilst it was powered off, someone could have manually moved the shaft, so stepping can only ever be relative to an initial position.

Disk drives with stepper actuators used to have a little crank arm attached to the shaft that on power up, the stepper would rotate until the arm interrupted an interrupter LED/phototransistor pair.

So, now the stepper controller knows where track zero, or twelve o’clock or whatever, is.

An encoder disk and IR Beam will sort that one out.

So now we’re at little more than a stepper motor…
:wink:

Ok, I’ve got a couple TowerPro SG90 servos left over from something else so it would be great if I could use those.

As far as finding a 1:2 gear set goes, I’ve poked around online and I’m not having much luck finding anything that looks suitable. Any suggestions on where to look (cheaper would be better)?

If you want 360 degree rotation though you’d have to mod them for continuous rotation and ocne you do that you lose positional control.

Stepper motors and encoders sound like the best solution for what you want to do.

I don’t need particularly fine control, I just need to be able to spin a hand to about 12 positions or so… problem is those positions are all around a circle.

If there’s an easy way to use a couple gears to augment the ~180 degree travel range of on of the servos I already have that would be preferable for this project (although I’ll definitely look into a stepper motor for future stuff).

There seem to be a number of gear combination sets available. For example, check out

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=4950
http://scientificsonline.com/product.asp_Q_pn_E_3082458

You don’t have to have exactly a 2:1 ratio on the gear teeth. If you match 32 teeth to 8 teeth, you would move your servo through 90 degrees to get a 360 rotation on the clock hand. Do the math if you get gears with other ratios.

[edit]Whether or not you can move the servo 90 degrees is another question, as Mike Mc has pointed out.[/edit]

Stepping motor:-

Possibly, though it may still need a detent switch of some sort,

Well not if you always move the hands to a set position before you start the Arduino. In this application it sounds like this is possible.