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Doesn't the voltage and the current coming from the base are added to the emitter?

Yes. But why does that matter to you?


Just a clarification.

So I think that the best solution for me is to use the 5V output pin and try to lower it with a resistance to 3.7V, since the transistor drains 0.7V.

Is this correct?
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Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
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This device http://www.micrel.com/_PDF/mic5219.pdf would be ideal. As it's only available in SMD packages, if you want to breadboad your design then I suggest you get the SOT23-5 version and an adapter board such as https://www.sparkfun.com/products/717.
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So I think that the best solution for me is to use the 5V output pin and try to lower it with a resistance to 3.7V, since the transistor drains 0.7V.

Is this correct?

That will depend on the device. If it consumes a constant current, you have a chance. Otherwise, no.

If you tell us more about this device of yours, we can help you more.
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So I think that the best solution for me is to use the 5V output pin and try to lower it with a resistance to 3.7V, since the transistor drains 0.7V.

Is this correct?

That will depend on the device. If it consumes a constant current, you have a chance. Otherwise, no.

If you tell us more about this device of yours, we can help you more.


It's just a device to control the global heating control of my house. It is powered by two AA batteries, which I took out and tried to connect my arduino to it, with GND to the - pole, and 3V to the +.
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Are you sure it takes as much as 200mA? That sounds rather a lot for a device that is supposed to run for a long time on AA cells.
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It is powered by two AA batteries,

If it is powerred by two AA batteries and then it conumes 200ma, you would have been replacing the batteries every few hours.

Again, before you try to figure something out, spend sometime understanding the characteristics of your device would save you lots of trouble down the road.
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In fact, I am not sure about this, I only have read it somewhere but I think I messed up. I think too that 200mA is way too much, but I can't know how to figure it out. I can only see that if i plug to the + button the 11 pin of the arduino, the display is messed up, and if I place to the + pole the 3.3V of the arduino, everything works just fine.
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Then, connect the + pin to arduino through a 1k resistor and you can measure the voltage drop on that 1k resistor to get some sense as to how much current it is drawing.
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