I am new to electronics as evidenced by the question below.

I understand how you can apply current to a transistor's emitter and base to turn something on. And of course if you remove the current the transistor can turn something off.

However I want to turn something OFF when current is applied to a transistor. For example, if a solar cell starts providing current to a transistor, I want that transistor to turn something off. How is this done? With two transistors somehow?

Thanks!

 

A really simple, cook book way would to use a common-emitter with a bipolar transistor. You can use it as a switch, meaning it will not be operating in its linear range (amplifier), simply on or off.
With a given base current a collector current will flow and drop the supply voltage across the collector resistor. The output at the collector will be essentially 0V.
Open collector really, would a comparator suit you better?

 

Regarding the common emitter with a bipolar transistor, is there a diagram or schematic somewhere that will help me understand how this would work. You're right, I simply need off or on, but I'm not quite following what you're suggesting.

Talk slowly.

 

Regarding the common emitter with a bipolar transistor, is there a diagram or schematic somewhere that will help me understand how this would work. You're right, I simply need off or on, but I'm not quite following what you're suggesting.

Talk slowly.

OK, here's an example. Note that if you have a really high output solar cell, you might need a resistor in series with it to limit the current to the base of Q1.
EDIT: I just saw your post on "the other" forum. Amazingly, I used a simplified version of the circuit you were asking about!

 

LOL Ron. I was kind of worried about being flamed for cross posting so I tried to word the question differently.

...it "steals" the base current from Q2...

That's the kind of spoonfeeding in layman's terms that I needed. Its starting to make sense. Thanks a ton.

 

LOL Ron. I was kind of worried about being flamed for cross posting so I tried to word the question differently.

That's the kind of spoonfeeding in layman's terms that I needed. Its starting to make sense. Thanks a ton.

One thing to remember is that Q2 (or any small-signal bipolar transistor) requires Vbe=0.7V (approx) for it to be ON. If it is much less than that, it will be, for all practical purposes, fully OFF. Also know that Vce will generally be less than 0.2V when a bipolar tranny is saturated. OK, so if Q1 is saturated, it lowers Vbe of Q2 below the voltage that will allow base current to flow into it, "stealing" virtually all of Q2's base current and holding it off.

 

why not just use a transistor with an inverter gate at the base?

 

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