I have extracted a small stepper motor from a 3.5 inch floppy drive, that I intend to use with an Arduino or Raspberry Pi. The problem is: initially a thin ribbon cable was attached to it, with no labels.

I couldn't use them, so I soldered wires to it. how can I determine which cable is what? I don't have a multimeter at hand, are there any other ways?

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  • \$\begingroup\$ You are not going to get very far without a multimeter, it's one of the basic bits of kit for any electronics engineer. Cheap ones are likely less than five of whatever your local currency might be (pounds, euros, dollars). \$\endgroup\$ – John U Oct 7 '14 at 14:41
  • \$\begingroup\$ @JohnU "I don't have a multimeter at hand" I will be near a multimeter in a couple of days, but right now I am not. I would need to sort this out though, do you think it's possible? \$\endgroup\$ – marchello Oct 7 '14 at 14:46
  • \$\begingroup\$ Anything's possible. Can you use some other device/circuit to measure relative resistances - for example, a small light bulb or LED where you could connect to/through different connections on the motor and observe dim/bright/off to work out the connections. Think about how a stepper is wired, then think about what you might be able to try with whatever things you have to hand. \$\endgroup\$ – John U Oct 7 '14 at 14:49
  • \$\begingroup\$ wow, thanks! I hooked up an LED to a breadboard and tried the wires in pairs, while rotating the shaft. I got some flashes with the 2nd and 4th wire (from above the second image, from left to right) \$\endgroup\$ – marchello Oct 7 '14 at 15:03

4 wires indicates bipolar windings. There are only 3 different ways the coils could be wired (1-2 + 3-4, 1-4 + 2-3, 1-3 + 2-4) so just try each combination until it steps. If it doesn't step continuously in the same direction then reverse the polarity of one winding. If it goes backwards then reverse both windings.

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  • \$\begingroup\$ I wired it following this tutorial: learn.adafruit.com/… but it did not work. maybe the Pi can't supply it with enough current? \$\endgroup\$ – marchello Oct 7 '14 at 19:07
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    \$\begingroup\$ You should not be powering it from the pi - neither trying to drive it directly from port pins, or even trying to power a driver chip through the pi. \$\endgroup\$ – Chris Stratton Oct 7 '14 at 19:21
  • \$\begingroup\$ @ChrisStratton but isn't this exactly what they did in the tutorial? \$\endgroup\$ – marchello Oct 7 '14 at 19:25
  • \$\begingroup\$ Their claims about it not being a problem are rather dubious. But their tutorial is for a unipolar motor, while you said your motor is bipolar. \$\endgroup\$ – Chris Stratton Oct 7 '14 at 19:28
  • \$\begingroup\$ It should work if you use an L293 (the ULN2803 is for unipolar motors only). The Pi won't have any trouble driving the L293. However the motor could draw over 500mA, so make sure that your power supply can handle the extra load. It might be safer to use a separate power source for the L293 (four AA Alkaline cells will do if you don't have a suitable a mains power pack). \$\endgroup\$ – Bruce Abbott Oct 8 '14 at 4:40

looks like a bipolar stepper - you'll need to find out which pins have continuity between them. Easiest way to do it would be an ohmmeter where you'll se a couple ohms resistance between some pair of pins. Since you dont have a DMM on hand, the suggestion of using an led, some series resistance for current limiting and a 9v battery could work. Just wire one end of the led+resistor to a pin and check which complementary pin makes your circuit light up - you've found the fist coil, and pigeonholed the next.

After you've found the pairs that make a coil, hook them up to your full-bridge or stepper driver circuit. There can be problems with direction of current here, so you might have to adjust.


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