Using a transistor instead of a relay to control a DC motor

I've made a simple relay circuit to switch power to a small DC motor

Here is a diagram of what I got right now:

Is it possible to replicate that with a transistor. Which one would be a good choice?

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We'll need to know about the motor. Is it a brush DC motor? What is its peak current? – HikeOnPast Sep 27 '12 at 22:58
Its a DC motor, not sure if its brush or not? Its for a auto winding mechanism in my Airsoft rifles box mag. So when the trigger is pulled, it will start winding BB's up in the mag, and the 2nd switch is for pre-winding without shooting. – Christian Bekker Sep 28 '12 at 12:14
If you run it by simply applying a voltage to it it will be a brush motor. BLDC (BrushLess DC) motors need a motor controller. – stevenvh Oct 4 '12 at 12:42

You can use a logic gate MOSFET, like the FDC855, and then your circuit will look like this:

A FET will give you a high current if needed (probably not necessary, since you're powering the motor from AA cells).

Or a BJT like the BC547, and then you would make it like this:

With the 1 kΩ base resistor you'll have minimum 100 mA current for the motor. If you need more I'd recommend the FET solution.
The diode can be a 1N4148.

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 Just to be sure. You write 3.7V lipo, it is a 7.4V Lipo? Yeah i know its a bit strange, actaully it started out with a practical problem with the 2 power sources being far apart. But know i remake it, i could just use the lipo i guess? so i would just use a big resistor between the motor and lipo? – Christian Bekker Sep 28 '12 at 12:18 @Christian - Sorry, I missed that 7.4 V. A LiPo is typically 3.7 V, so yours will be two in series. A series resistor is not a good idea to regulate a voltage down. Suppose the motor draws 1 A, then you would need a (7.4 V-3 V) / 1 A = 4.4 $\Omega;$ series resistor to get 3 V, but if the current will drop to 0.5 A, for instance because of no load, then the resistor will only drop 2.2 V, and the motor will get 5.2 V. Granted, this will increase the current again, but it still is a bad way to control a voltage. maybe you can power the motor from one LiPo cell? – stevenvh Sep 28 '12 at 12:26 @Christian - Fixed LiPo voltage in schematic – stevenvh Sep 28 '12 at 12:34 Well my lipo is one big battery pack. and it powers some other stuff too. so i guess im just sticking with the 2 AA's to power the motor... Ill just take a look at the schematic and test it out with your suggestions – Christian Bekker Sep 28 '12 at 13:48 @Christian - do you know about switchers? They're very efficient voltage regulators, which can downconvert the 7.4 V to the 3 V the motor needs. – stevenvh Sep 28 '12 at 13:51