Bipolar Junction Transistors as Common-Emitter Switches - Circuit Simulation 

circuit simulator: www.falstad.com/circuit

Not that I am aware of. There is not a "photoresistor" in the parts list. But you could emulate it by using a mosfet in the linear range and applying a variable voltage to the gate. Question though, since this is a *simulator*, what would you use to shine "light" on your photoresistor? :) The program is open source (java) and the various components are in individual classes. Somebody could certainly model a photoresistor and write a new class for it...

The "output" is the current from Collector to Base ( I had to look ) thus "collector" would derive from something in the arena of collecting electrons; Plausibly from several places; As current flows that flat looking notation in the drawing is a small region which is inversely doped compared to "in and out" When charge is applied to that region the blocking or diode effect is depleted and now "current" can flow to the other region which then becomes a path to the return side of the battery hence current flows; This has a range over which it can occur; This allows varying input to control greater power or may be used as simple switch;

Sorry for the confusion. The controlling (I put) current (turning the transistor on) flows from base to emitter. The controlled (output) current flows from collector to emitter. The collector is the the leg that is switched “on” so that is why we consider it the output, even though current is flowing into it in an NPN transistor. In a PNP transistor the the collector is still the output but current flows in the opposite direction.

It’s just a simple example. The transistor is a current multiplier. So you can use a small current signal (imagine a very tiny switch) to control a larger current load (like a high-powered lamp). Something that your small switch maybe couldn’t handle, but a suitably large power transistor could.

falstad circuit simulator, here is the link www.falstad.com/circuit/