I can't seem to turn on a 2n2222a with the output of a LM339 comparator, as soon as the base is connected to the output, directly or with a resistor, even if the output is 5 volts, 9 volts, I always get 0.6 volts to the base of the transistor?? How odd...there is a pull-up resistor on the output of LM339. I also tried a super simple comparator setup with LM324 op-amp, and same thing, I only get 0.6volts when the output is connected to base...
What am I missing? Why always 0.6 volts?
Your circuit needs to look something like this:
simulate this circuit – Schematic created using CircuitLab
Comparator outputs are typically open-collectors, so they'll sink current in order to output a "0". You need a pull-up in order to have the output actually go high, feeding current into the base of your switching transistor. R1 needs to be high enough so you don't cram too much current into the comparator when it's trying to pull low (20 mA might be absolute max, so go with a bit less than that), but low enough to fully saturate Q2 in order to drive whatever load you're planning on using. R2 can (probably should) be zero.
As Ignacio pointed out, the base of Q2 will stay around VBE(sat), typically 0.6-1.0 V for most BJTs. If you manage to exceed that by very much, you'll destroy the device. The voltage isn't really relevant anyways; BJTs are current devices.
