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everyone. I am asking you to help me out for making original musical instrument. The goal is to vibrate a small motor by square wave signal witch is generated in a synthesizers in my Mac.

Here is what I tried;

Square wave sound (in my laptop) -> stereo output -> stereo cable -> headphone amplifier -> stereo cable -> motor

I cut the end of the stereo cable to connect the left(or right) wire and the ground wire. (please check the photo below) enter image description here

However, it doesn't work. I could hear the sound from the amplifier by headphone, so there might be problems between the amp and the motor.

Let me tell you the specs of the amplifier; make/model: Pyle Audio, PHA40 input impedance: 100K-ohm, Unbalanced Max Input Level: +15dBu output impedance: 80-Ohm, Unbalanced Max output Level: 40mW in 100ohm Max Gain: 20dB S/N Ratio: More than 90dB THD: Less than 0.03%

And the motor is DC 1.5-6V 1750-7000(<- I have no idea what does this, mA?)

Hope I can get any ideas/answers from you guys.

Thanks,

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    \$\begingroup\$ First of all, 40 mW isn't likely to budge your motor (even a tiny cellphone vibrator requires about 80mA @ 3V = 240 mW), and secondly, your amplifier is providing an AC output and it's a DC motor. \$\endgroup\$
    – Dave Tweed
    Commented Jan 16, 2015 at 22:57
  • \$\begingroup\$ Thank you for your answer! So seems I have made two big mistakes. It was great help for me to fix this problem. \$\endgroup\$
    – Takuto
    Commented Jan 17, 2015 at 8:06
  • \$\begingroup\$ 1750-7000 is speed in RPM. That means 1750rpm at 1.5V and 7000rpm at 6V. From whch you can determine roughly the DC voltage you need for any given pitch. \$\endgroup\$
    – user16324
    Commented Jan 17, 2015 at 9:57
  • \$\begingroup\$ Hello, Brian. Thank you for your participating in this. From your answer I notice that I might misunderstand something crucial. I tried to control the rotation rate of the motor by changing Hz of the synthesizer (I mean, square wave oscillator), but do you mean that changing Hz cannot change the rpm but only volt can, which I have to install some tool somewhere at this signal flow? \$\endgroup\$
    – Takuto
    Commented Jan 21, 2015 at 21:40

2 Answers 2

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The voltage from your amplifier is AC so on average it's 0v, and even if it's swinging above 1.5v it's not doing so for long enough, or with enough current capability, to even get the motor started.

First try creating a source of DC for the motor, perhaps with a transistor, then you can think about making the speed controllable / modulatable.

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  • \$\begingroup\$ Mark, I really appreciate that you guys are spending your time to help me. I even didn't know that amp uses AC until I posted here. So I understand there must be like transistor for transfer AC to DC, but do I still need other tools to get enough power(volume?) of mA or volt? \$\endgroup\$
    – Takuto
    Commented Jan 17, 2015 at 8:20
  • \$\begingroup\$ Audio amplifiers are designed to reproduce an audio waveform, and an audio waveform is always AC. A transistor is a device that amplifies power or a signal, it doesn't convert AC to DC. An example of a DC source would be a battery. You could mix together DC + AC signals at the base of a transistor, using appropriate resistors to balance the proportions of each. I'm not sure that the AC waveform will audible, though, as the motor's inertia will act as severe a low pass filter, averaging the waveform to almost zero. The main factor in what you hear will be the level of DC voltage. \$\endgroup\$
    – Mark Hadman
    Commented Jan 18, 2015 at 11:00
  • \$\begingroup\$ Thank you for still sharing your time and knowledge for me. So it seems I would need a converter from AC to DC, and a transistor to bring up the signal volume. But I just would like to make sure. Just converting from AC to DC doesn't help changing the rpm by oscillator's pitch, right? \$\endgroup\$
    – Takuto
    Commented Jan 21, 2015 at 21:54
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I'm not quite sure what you are trying to achieve. Are you trying to make the motor speed represent the frequency from your oscillator?

RPM is Rotations Per Minute. 1750 - 7000 RPM would correspond to about 29 Hz through about 117 Hz.

If you want motor speed to be proportional to the incoming frequency, you can use a circuit block called a "Frequency To Voltage" converter.

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