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Hi
    I need to power up an external device which uses 3V batteries with arduino, based on a GPRS call. I already have the GPRS shield, and this is working fine, I can switch on a pin whenever I want. The problem is about the circuit needed.

I tried to use the 5V ouput into a transistor controlled by the base, a 27 Ohm resistor and then connect it to my device, but this doesn't work. So I tried to use my out pin (which gives me around 4.5V) and use the same resistance, and then this goes to my device's + pin, but this didn't work either.

I am pretty newbie on electronic, can anyone give me an advice to do that?

Thank you very much
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Did you try the 3.3v pin on the arduino?
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A 27R on the base is far too small make it a 2K7 at least.
Are you trying to control the power to this device or send a signal to it?
If a signal then :-
Then on the collector you need a pull up resistor, say 1K to the 3V line.
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Did you try the 3.3v pin on the arduino?

If i connect it to the 3.3 pin, i got 2.6 v since 0.7 is drained by the transistor.
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A 27R on the base is far too small make it a 2K7 at least.
Are you trying to control the power to this device or send a signal to it?
If a signal then :-
Then on the collector you need a pull up resistor, say 1K to the 3V line.

I'm trying to give power to it.

I used a 27 resistance since i used the ohm law: 1,3/0.05 = 27?

Did i get something wrong?
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Did i get something wrong?
Maybe. Where did the 1.3 and 0.05 come from?
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Did i get something wrong?
Maybe. Where did the 1.3 and 0.05 come from?

1.3 is the voltage i want to drain (from 4.3 to 3) and 0.05 is 50 mA, which is the current that the Vout pin has.
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I used a 27 resistance since i used the ohm law: 1,3/0.05 = 27?

Did i get something wrong?

Yes this is not the way to do anything. You can not use resistors to reduce the voltage.
You need to get a 3V voltage that is capable of powering your device. How much current does your device take?
Then you need to switch that with a transistor.
Arduino output -- 2K7 --- transistor base
Transistor emitter to ground
device ground to transistor collector
device power to your 3V supply +ve
3V supply ground to arduino ground.
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Perhaps th best way would be a low dropout 3.3v regulator, 
or second maybe cheaper just 2 plain diodes to drop 1.4 volts to 3.6, maybe a germanium diode to drop aanother.3v, that will work the only problem is the voltage dropped changes slightly with current being drawn
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