I believe that I am using the NPN transistors correctly, but I'm not sure about the PNPs. I made diagrams of the ways I can light an LED as the load.

From everything I've read about PNP transistors, I should be biasing the base to ground (I've tried, but I can't make it work). I'm not sure why it works with the base of the PNP biased to the source voltage (+5v). I want to be able to turn on/off a load alternately by connect the bases of the BJTs to the output of a 555 (pin 3). What I expect is that an LED (NPN load) will turn on when the timer output is high, and an LED (PNP load) will turn on when the timer output is low (source and sink, conventional current flow model). How do I turn on a PNP transistor?

 

R3 and R6 need to be connected to 0V to turn on the PNP transistors. If those resistors were connected to a '555 then it would work as you describe.

 

The parts in the diagrams are the correct values. The BC547B is the PNP that I am using. I can't get it to turn on with the base connected through the resistor to ground. Why does it turn on with the base connected through the resistor to source (+5v)? I have more than one part, and I have tried them all without success. Am I using the NPNs correctly?

 

The BC547B is an NPN transistor.
The NPN circuits are OK (and the BC548 is NPN).

 

OOOPS! The BC547B is an NPN. Oh man, do I feel stupid. Now I know why it is working like an NPN, because it is! thanks for the replies.

 

So now the question is, Why does the LED light up with the emitter connected high?

 

What pin connections did you follow connecting the '547?
Looking at the flat side of the package with the legs downwards, from left to right collector, base, emitter.
If they pins are connected as the diagram shows then the 10k base resistor will feed current through the forward biased base collector junction to the LED. That current would be about 0.25mA which may be enough to light the LED though I wouldn't expect it to be very bright.

 

I thought they were all EBC. I didn't know the BC547B pins are arrange CBE.

 

TO-92 package, it helps to read the data reference. Collector, Base, Emitter. OK, thanks. I'm smarter now.

 

The same connections apply to the '548 as well.