Transistor Help

Using Arduino General Electronics
1

I need to get a 9V power source switched down to 5V. I would love if this could be done using a NPN transistor. Supply 9V to the emitter, have an arduino I/O pin hooked into the base, and have 5V be passed through the collector when the transistor is closed. I was wondering if I could also trigger the transistor to close when a certain amount of electric is supplied (.01V - 5V) and have it remain open when receiving no power?

Currently I am using a PNP transistor. Supplying 5V to the emitter, base connected to I/O pin on an arduino, and collector going to devices. I need to supply 5V on the base line to open the circuit. I would much rather supply some type of voltage when the devices need power, not when I do not want them to be powered.

2

Supply 9V to the emitter, have an arduino I/O pin hooked into the base, and have 5V be passed through the collector when the transistor is closed.

No that is not going to work.
You need a regulator to make the 9V into 5v.
Alternatively put 5.7V onto the base and the COLLECTOR to 9V and the emitter will have 5V on it.

I would much rather supply some type of voltage when the devices need power, not when I do not want them to be powered.

Sorry but that is just being silly. This is electronics, it works by switching things.

3

I think what you are really after is a voltage regulator with a shutdown control pin.

An example:

4

I didn't know they made such devices!

I found this just now! Looks like it can use it as it has a shutoff pin!

Will i need a resistor between this device and my arduino i/o pin?

5

You know what, cross posting is a great way to piss people off on this forum.

6

The link to the part you posted didn't work, but using the first link from CrossRoads the current entering the enable pin is supposed to be 2 microamps or less. This means that the enable pin has an input impedance greater than 2.5Mohms, so you won't need a resistor to limit the current. That said, adding a 10K resistor in line with the pin wouldn't hurt anything and would also make sure that if you mess something up your Arduino stays safe.