Let's Make Robots!

Choosing a transistor

I've used two types of NPN transistors in the projects I've done so far and I've tried to investigate why, but my conclusion just doesn't seem to make sense (actually, I'm not sure if I've reached one...).

For use with an Arduino, I've used a PN2222 transistor:

 

Whereas, for BEAM solar bots, I've used the 2N3904:

 

I'm just wondering what I should be looking at in a data sheet so that I can make my own decisions about inserting a transistor into a circuit that doesn't come with instructions. Is the 'Emitter-Base Voltage' the most important thing to look at, as that will decide how high the voltage needs to be at the Emitter for the transistor to alllow current to pass through it (at 5 Volts for the NP222, and 6 Volts for the 2N3904)?

And if that's the case, I understand why I would need the NP222 for the Arduino circuit (because 5 Volts is what the Arduino gives out), but why would a BEAM bot (eg; the solarspeeder and photopopper from Solarbotics) need a higher voltage? This is how it is explained - does it mean that the 1381C gives out 6 volts to turn on the transistor?

 

And could I just use this 2N5088 transistor for an Arduino circuit, considering it requires less than 5 volts to activate (how would a lower voltage 'Amplification' transistor be different to a 'General Purpose' one at the exact voltage):

 

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the 1381 comes in verieties triggering at different voltages.
the letter at the end indicates which version it is.

if i remember correctly both types of transistor are used in beam robots,
and the 2222 are used when more current is needed to drive the motors.

beam robots tend to drive motors either directly or trough a transistor H-bridge.

 

For all practical hobby robotics purposes these two are going to be interchangeable and you'll never run out of circuits to use them in. Usually there's a 44 series thrown in there too (4401.) They're ubiquitous and come in mixed 20 packs at RadioShack. I've never seen that 508 series one.