I'm learning about NPN transistors and have wired up this:
I understand some of its behavior, but one is a complete mystery for me.
First, if I tie "B" (green dangling wire) to "G" (blue dangling wire) then the LED turns off, as expected.
If I let "B" float, the LED glows faintly (leakage current?). If I touch "B" the LED turns on brightly, it intensifies even if I just wave my hand near it. Again, expected, that's why I built it in the first place, to try this.
What is a complete mystery for me, is that if I touch "G" (just touch it, not hook "B" to "G"), the LED turns off. Same thing if I hook up "G" t o the ground from a power outlet. I don't understand this, I didn't expect it to affect anything. Again, not "B", I (sorta) understand that this setup is very sensitive to any signal on the base of Q1, but "G", which is the emitter of Q2 / negative of the power source.
Note, the "battery" on the schematics is a standard USB charger. I want to try this with a 9V battery, but didn't get to yet.
So, my main question is why/how touching "G" affects anything? Why does the LED turns off? More generally, I'd like to understand how to properly think about these things…
Update: I experimented more with it. Changed it to a proper Darlington configuration (which didn't change anything). And tried it with a 9V battery instead of a USB charger as a power source. This completely eliminated the mysterious behavior.
My conclusion is that the USB charger doesn't provide a pure DC power, and the 50 Hz noise gets greatly amplified by this circuit. And my body with its capacitance (and just being a big bulky antenna) can affect it in various ways...