transitor conducts with only wire attached to base

Other Hardware General Electronics
1

Hello,

Could someone explain why the following happens please?

In the following circuit the LED does NOT light up

  • 5V to 330 ohm resistor, to LED, to collector of pnp transistor (2N2222A)
  • emmitter of transitor to GND
  • nothing connected to the base of the resistor

In the following circuit the LED DOESlight up:

  • 5V to 330 ohm resistor, to LED, to collector of pnp transistor (2N2222A)
  • emmitter of transitor to GND
  • a wire connected to the base, but the other end of the wire does not go anywhere
  • (see attached picture)

Why does the transitor allow current to pass from collector to emitter when just a wire is connected to the base :o ?

Thanks,

led.PNG
led.PNG1188×579 1.13 MB

2

1, That's an NPN transistor.
2. If you connect an antenna to something it can pick up all sorts of stray signals particularly if a static generator, like a human being, is close to it or holding it. A length of wire is an antenna.

Steve

3

So I wonder if we touch the wire free standing ungrounded what's happening? I wonder if we were to just look at the base of a transistor as you drew it with a scope what would we see? What do you figure would be applied to the transistor base?

Ron

4

Some touch sensors took advantage of this property. If you chain several transistors to get a large amplification you only need your finger to trigger it.

Either that or...It's FREE ENERGY, MAAAAAN!

5

led.PNG

It's not "static" electricity.

It is picking up by capacitance, the AC field radiated by your house wiring. It is your body that is picking ip the radiation and holing the lead creates a small capacitor connecting it to the base of the transistor.

When the AC voltage picked up is sufficient, the reverse Zener breakdown of the base-emitter junction - about 6 V - charges this capacitance when the voltage goes negative and when it goes positive again, this is fed to the base in the forward direction and the current amplified to light the LED.

The LED is actually flashing at 60 or 50 Hz - the frequency of your power mains. :grinning:

6

Thanks for the answers! Pretty cool stuff, i had no idea about the antenna and AC field in my house.

7

If you have ever plugged a "patch" cord (plug both ends) into the input of your audio amplifier and touched the tip of the other plug, you would be well aware of the AC field!