Wire it up: Transistor Switching

I’ve wired up Experiment 10 from the book Make:  Electronics by Charles Platt.

I show a couple of things in this video.  First off, I’m using an NPN style transistor to switch complete the circuit to an LED.  A NPN transistor has three leads: a collector, a base, and an emitter.  When the base receives current it enters a switched state, which connects the collector and and emitter leads.  The collector is positive and the emitter is negative (this is reversed for a PNP transistor).

In the second part of the video I do the same thing, but ditch the push button in favor of my thumb.  I guess the point here is that the base lead does not need much current to do it’s thing.  When my thumb is dry, you can see the LED barely lighting.  And then when I lick my thumb to improve conductivity the LED is quite bright.  I think this is because unlike a relay, a transistor isn’t simply an on/off switch, but depending on the base lead current the the switched state is more or less amplified.  So this is why the LED was brighter when my thumb was more conductive, even though my thumb wasn’t touching the part of the circuit that powered the LED directly.

Finally the last two parts of this video I hook up a potentiometer to adjust the current, and measure amperage at the base lead and the emitter lead of the transistor.  As I dial up the potentiometer, the amps increase much faster when measuring at the emitter lead.  I think this is because of the amplification effect of the current through the transistor as compared to the current through the base lead.

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