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HDDJ: Turning an old hard disk drive into a rotary input device

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Step 3: Probing the motor output

Picture of Probing the motor output
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This is not really a necessary step, but more an illustration of what exactly we are trying to do.

If you have a access to an oscilloscope with multiple inputs, connect three of them to three of the wires soldered to the spindle motor contacts in the previous step (it doesn't matter which three). Connect the probes' ground clips to the fourth wire, then set the platter spinning.

The scope images below show the three waveforms that are generated when the HDD platter is spun by hand (the scale is set to 500mV per division in the vertical axis, and 20ms per division in the horizontal axis). Three perfect phase-shifted sinusoidal waveforms!

The three different pictures show what happens to the waveforms as the platter gradually slows down: they all decrease in both frequency and amplitude by the same amount.

These waveforms carry a lot of information, not only how fast the platter is spinning, but also in which direction it is spinning (clockwise, or anti-clockwise). More on this later.

The raw signals, as generated by spinning the motor by hand, are simply too subtle to be sampled directly by a microcontroller, so the next step is to amplify them into useful levels.

 
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