DC motor with 4 pins !?

Hello everybody,
Recently I broke an old Phillips casette player and brought out two powerful DC motors (they are quite big in size). Now the problem is each of them have got 4 pins !! One of them is marked +, another is marked -, the third one is marked A and the last one is not marked any thing. The motors are rotating at full speed when the + and - are connected to 5v supply and nothing happens with the other two. I have not yet tried anything with an arduino. And I am not planning to connect the motor directly to my Arduino but are those two extra pins anything for signals to controll speed digitally?? Don't know! Not getting exact description of it anywhere on the internet. Please help me if anyone knows about them.
Thank you.

"brought out two powerful DC motors (they are quite big in size). "

Not a lot to go on.


They could be speed encoders - do you get anything on those wires when the motor is moving? Even just connecting a multimeter on resistance mode will show you if something changes, then you can drill down with the Arduino or oscilloscope to work out pulses per revolution.

They really could be anything. Have fun exploring and let us know what you find.

The other two contacts are likely a tachometer - try connecting to an oscilloscope
and rotating the motor by hand - if you see a DC or AC voltage its a tacho, and this
would have been used to sense the rotation speed in the casette players speed
feedback control loop.

To me a "two powerful DC motors (they are quite big in size). " means things that
need a crane to lift. There's this really neat system called "measurement" these days
that can be used for communicating size :wink:

Thanks for the suggestions, yeah I think its nothing more than just speed encoder or a tachometer.
I used the multimeter but there was no change in the reading in resistance mode, it showed discontinuity. But there were readings for potential difference.
When I spun it with my fingers it showed: 0.58V
When I used a 3.7V Li-ion battery: 3.49V
When I used a 5V adapter: 5.67V
When I used a 12V adapter: 11.76V

And sorry for the "they are quite big in size". They have a diameter of 33mm and a height of 25mm.

Big is relative. Your motor is big compared to a vibrator motor:)


In the original circuit there would be a feedback circuit generating a PWM duty
cycle derived from the difference between tacho output and a reference voltage.

Removing the bottom cover you can see how it is done, and where wires are connected