I've had a few problems with GPIO devices up till now, but not like this dratted solenoid. I got mine from Adafruit ($6.95 for plastic). I also ordered the suggested transistor, diode and 9V power supply.. To be extra careful and protect the Rpi from the wrong voltage, I also bought a second breadboard and a multimeter. I wired up the breadboard following (as best I could) the seemingly-identical diagrams below.
Anyway, when I try it, the solenoid activates as soon as I connect the 9V+ and –. Changing the poles of the TIP102 doesn't work.
I need help. Any suggestions?