Need some help using transistors for switching

[smoguzbenjamin]

Hey guys,

I've taken a two week break from electronics and I've forgotten how to use a friggin transistor :mrgreen:

What kind of transistor would I use if I want to use it as a switch in the following fashion: I want the Collector to be high at all times, and for example an LED at the Emitter. I want the LED to be on (so the emitter is high) when the base is Low, and I want the transistor to turn off (making emitter low) when the base is high. I'd like to do this with as little parts as possible, preferably with one BJT. Thanks in advance!

Ben

[StephenGiles]

BJT?
Stephen

[Gilles C]

bipolar junction transistor.

Well you got everything right except for a limiting missing resistor in series with the LED...

So you would like a wiring diagram, right?

[R.G.]

I want the Collector to be high at all times, and for example an LED at the Emitter. I want the LED to be on (so the emitter is high) when the base is Low, and I want the transistor to turn off (making emitter low) when the base is high.

You have a problem. You have just described a follower (collector always high, LED at the emitter) on the output side, and an inverter (base high when emitter is low) on the input.

There are no BJTs that invert from base to emitter. Emitters always follow bases.

If what you really wanted was an LED to be on when the control signal is low and off when the control signal is high, the you want a PNP transistor with emitter to + voltage, collector to LED/resistor to ground, and a resistor into the PNP base.

[smoguzbenjamin]

Ah, ok. Lesson learnt, thanks RG. Now I can get to playing with logic. Boolean logic is fun! :mrgreen:

[Gilles C]

Arg... You're right R.G.  :oops: my mind switched off while I was reading...  :cry: