I Have a device that operates at 3.5 volts but I want to run it of a 9V.Do I need a resistor If so which kind?

Question

I Have a device that operates at 3.5 volts but I want to run it of a 9V.Do I need a resistor If so which kind?

Answer 1

You need to know the current draw of the device before you can compute the value of any resistor or if you need one.  Resistors can be very inefficient ways to drop a voltage.  They can waste more power than the device itself uses.  This shortens the time the battery will run the device and unless it's a rechargeable battery could have you buying a lot more batteries than you really need to.  Unless you have a particular reason for wanting to use a 9 V battery it might not be the best way to go.  

Answer 2

go to radio shack and get an adapter