Turning a transistor on and off

[Fractal Cat]

Hi,

I know that to turn a transistor on and off you have feed some current in to the base. But how can
I get a transistor to turn on and in doing so turn off another transistor?

FC.

[Pauldf]

You can use a NOT gate to give you the opposite signal, what is the circuit for?

[Fractal Cat]

Hi,

I'm trying to figure OUT how to design a temperature sensor that illuminates an LED depending on the temperature. A green LED lights up
at 5 degrees Celsius; an amber LED lights up at 3 degrees C; and a red LED lights up at zero degrees C. I have done this using transistors and op-amp
comparator circuits. Both circuits work but I'd really like to only illuminate 1 LED at any one time instead of all of them.

Thx

FC.

[Pauldf]

How about using the op amps to supply logic gates?
I'm assuming the circuit uses a positive temp system, i.e. at 0deg the red LED illuminates, at 3deg the red and yellow are illuminated and at 5deg the red, yellow and green are illuminated?
I'm thinking a pair of "and" gates and a pair of "not" gates may suffice such as 7408 and 7404 chips.
The green LED supplied directly from the 5deg op amp.
The yellow LED supplied from an "and" gate, this "and" gate inputs supplied from the 3deg op amp and via a "not" gate from the 5deg op amp.
The red LED supplied from an "and" gate, this "and" gate inputs supplied from the 0deg op amp and via a "not" gate from the 3deg op amp.

[Fractal Cat]

Hi again,

Perhaps I did not explain myself properly. I only want to light one LED at time, not together. Green at 5 degrees Celsius. Amber at 3 degrees C. And red at 0 degrees C.

Thx

FC.

[Pauldf]

Yes,
At the moment do you have:
red at 0ºC,
red and yellow at 3ºC,
red, yellow and green at 5ºC
?

If so then the logic gates above I described should do the trick.

[Fractal Cat]

Hi Paul,

Yes, your logic circuits were spot-on, thank you. I had to tweak a few things but it's now working as required.

Thanks Paul

FC.

[computer blind]

I think you just need to send a pnp to an npn, if u were using bpj transistors.

If you want to go really oldschool and wierd - check this design out, just has resistors, but I warn you, using it past one not is kinda complicated, because it doesnt contain semiconductors, u wind up balancing it with resistors and it gets pretty leaky and confusing.