Switching a transistor with "body voltage"

[Braeden Hamson]

So I was messing around with a S9018
http://vakits.com/sites/default/files/S9018 Transistor.pdf
In which 3 V from my PS was connected to the collector and an LED was connected to the emitter. The LED was then connected to the - of the power supply. I connected the base of the transistor to the 3v rail of my breadboard and nothing happened. Figures, the collector-base voltage is 4V. However I then touched the jumper wire that was connected to the base and the LED turned on. Upon further investigation I found the same thing happens with the 2222A transistor, however this particular transistor turns on when I connected it to my 3.3v but the strangest thing is that it turns on when I connected it to the ground of my arduino which is plugged into my laptop.

What in the sweet hell is going on here?

I swear, ever, single, time I pull out transistors black magic happens.

 

[Braeden Hamson]

Update, connecting the grounds of my PS and arduinos together makes the transistor turn off when the base is connected to a "0V" analog output. Which makes sense, my laptop's ground must be higher than the power supply. So everything that happened with the 2222A makes sense. However the transistor not turning on at 3.3V but turning on when I touch it doesn't make sense.

 

[WHONOES]

Your body makes an excellent broadband R.F. (radio Frequency) antenna. You are probably injecting R.F into your circuit via capacitive coupling from you body.

 

[shrtrnd]

The old CMOS chips were pretty static sensitive (newer ones less-so), but you'll find a lot of low-power
'touch-switch' type applications with those. As mentioned by WHONOES, capacitive-coupling from your body.