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I wondered if someone could offer advice here.
I have a 3.3v dc motor. Obviously, i can change the direction it turns by flipping the + an - leads round.
Can i acheive the same thing but with 2 npn transistors. Signal to one to turn it one way, and signal 2 for the other.
Can someone advise a simple, safe circuit for this?
Thank you
The circuit you are looking for is called a "H bridge".
http://en.wikipedia.org/wiki/H_bridge
Two transistors will not let you change the direction, you need four as described on wikipedia. And it is also mostly MOSFET or BJT transistors used in "H bridges".
There is also small IC circuits that can be used.
You could get away using 2 transistors if you had a V+, V-, and an extra center ground voltage. Normally what is done with a basic two wire power supply is to use 4 transistors in an H pattern, (with the motor in the middle), this is often called an H-bridge driver. Each transistor pair allows the current to flow (through the center of the H pattern) in only one direction or the other. You can buy motor driver chips with this transistor arrangement already made.
(The unsafe part of the idea happens if you build your own circuit and accidentally turn on the wrong two transistors at the same time.)
While it is possible to create the required circuit with BJT's & construct a H-Bridge
The problem you have is the 3v3 volts you are planning on sourcing the machine from.
Each BJT will, roughly, drop 0.6V, so that is 1.2V taken away from being able to source the electrical machine, which means your top speed & torque will be reduced.
You haven't provided much information with respect to the type of electrical machine & only hinted at how you want to operate it.
A reasonable assumption is that it is a brushedDC machine and you simply want to change direction (not vary its speed)
You could try a DPDT relay
This way you will not have to sacrifice 37% of your available voltage
