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Help explaining why this circuit does what it does

Just reading into capacitors, and there is a section in my text book with an exercise that says to make this circuit and explain why it functions the way it does. I was very suprised when I built and tested the circuit, as it didn't do what I expected.
5zL89.png
This is the circuit. While the button is not pressed, the motor and LED are both off. When the button is held, the motor starts, but LED remains off. When you release the button, the LED flashes once and the motor stops.

I am very confused as to why the LED lights at all. Any help would be appreciated!

Thanks!
(edited 10 years ago)
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Avatar for ellisvlad
ellisvlad
OP
Ok, I gave my grandpa a call. He is an electrical engineer, and he explained the circuit for me. Basically, when the button is pressed, a current is allowed through the transistor, and the motor begins to turn. As expected. When you release the button, the motor is still spinning and acts, temporarily as a generator. The energy that the motor still has as it is spinning, causes a current to be induced in the reverse direction as to before, and so the LED is briefly powered. This can be seen a little easier in the circuit below:
http://puu.sh/5zMXB.png
When the motor is spinning and the button is released, current flows through the LED, lighting it briefly. :smile:

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