Wednesday, February 8, 2012

Understanding a transistor (hint: I don't)

Right, so I have set up a circuit. I'm afraid I'm not much with photoshop, so here's a worded description.

5V+ is wired into what I think is the collector
An Arduino output pin is wired through a 330 Ohm resistor into what I think is the base
The emitter is wired to a connection point
5V - is wired to another connection point
My Arduino is programmed to emit a HIGH pulse for 2 seconds every 2 seconds to the base.

Using the volmeter to complete the circuit and measure the volts saw a change every 2 seconds as expected. However, it was measuring 1.5V, then an increase to 3.4V and back again.

Can someone please explain why the voltage doesn't drop to zero?

Then, for bonus points, why does it not reach 5V? I can accept that the HIGH level might not be quite 5V for a variety of reasons such as the Arduino itself using some power from its source, some current going towards the base which is then not available to the 5V output from the Arduino etc etc.

But why on earth is there 1.5V present when the transistor's base recieves no current???

Now, for the next question. If I detach the wire from the Arduino output pin to the transistor altogether, the circuit from 5V to collector to emitter measures 3.3V! Surely in this case it should be either 1.5V (as per above) or 0.

I just don't get it!