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I have the following circuit schema.  The engine is not spinning, but if I replace it with a led, the led shines. Could anybody give me an explanation and how to fix it? Thank you.

The engine spins if I directly use the 3v source.

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  • \$\begingroup\$ What engine? and what is that image supposed to be? \$\endgroup\$
    – PlasmaHH
    Feb 22 '16 at 20:45
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    \$\begingroup\$ You are supplying your motor via a resistor, there will not be enough current to drive a motor but there is enough for an LED \$\endgroup\$ Feb 22 '16 at 20:46
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    \$\begingroup\$ To me, "engine" means a car engine that runs on gasoline or diesel. You probably mean "electric motor". An electric motor (even a very small one) will draw much more current than an LED so will not work in your circuit - almost all the voltage will be dropped across the collector resistor, leaving very little voltage for the motor. Also, you need a resistor between your "5V" and the transistor base to avoid destroying the transistor - try 1K or so. \$\endgroup\$ Feb 22 '16 at 20:56
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    \$\begingroup\$ When we say not enough current, we mean nowhere near enough. You would not supply a 3V motor through a resistor at all. No resistor. Just the transistor. And make sure the transistor is big enough and you give it enough base current. The motor will probably take around 100 times as much current as you need to light an LED \$\endgroup\$ Feb 22 '16 at 21:03
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    \$\begingroup\$ Thank you for the feedback. Constructive or destructive, but it solved my problem. I learned something: some people can be very mean but others have the integrity to teach me the right way. \$\endgroup\$
    – nucandrei
    Feb 22 '16 at 21:12
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In your original circuit, you are using the transistor to create a short circuit around the LED or motor - this would be very wasteful if the collector resistor could pass enough current to allow the motor to operate.

(By the way, in English, "engine" usually means something like a car engine, running on petrol/gasoline or diesel. You are talking about an electric motor.)

The preferred circuit to operate a motor is something like:

schematic

simulate this circuit – Schematic created using CircuitLab

When V1 is 3 volts, the transistor will conduct, turning the motor on. If V1 is zero volts, the transistor will be turned off, and the motor will stop.

Edits suggested by comments:

D1 is a "flyback diode" to suppress voltage spikes when the motor is turned off. The transistor must be rated to carry the motor current.

LED D2, and a current-limiting resistor should be connected as shown (with or without the motor).

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    \$\begingroup\$ I would suggest bigger transistor and smaller base resistor \$\endgroup\$ Feb 22 '16 at 21:32
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    \$\begingroup\$ I assume Q1 will be used as a switch, so I would suggest a flyback diode across the motor. \$\endgroup\$
    – Tut
    Feb 22 '16 at 21:38

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