# How to transform pozitive signal into negative?

This may be not clear but let me explain.
If you got postive signal(a power, coming from battery, but its positive = + ) and you want to make it negative ( like coming from negative of battery, = - ) how you can do it. Some kind of tranzistor?
I wanted to use it in LPT port. i getting signal from pin 1(its +) and i want to change it to negative.

Please use picture as example(it's paint drawed, sorry).
picture(mirror 1) (mirror 2)

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6 years ago
If you just need to drive an LED then a transistor will invert the input signal. So if you drive an NPN transistor on with the positive input the collector will go negative.
dava_2 (author)  rickharris6 years ago
It's actually led matrix. No, your way dosen't work.
6 years ago
Rick is correct. You can drive the rows with one (pnp or npn) and the columns with the other. It would work.
dava_2 (author)  frollard6 years ago
Here's way I tried which didn't worked: http://i25.tinypic.com/148jgbs.jpg
6 years ago
There are no current limiting resistors on either the transistor base or the LEDs.

.So basically you want to turn ON your LEDs when the logic is 0V and OFF when its +5 ?
dava_2 (author)  steveastrouk6 years ago
No, opposite. I want it on when is 5+, and off when nothing of power.
6 years ago
Then wire them as collector loads on the transistor.
LED Anode to resistor, resistor to supply +ve.
LED cathod to transistor collector
Transistor emitter to 0V
Transistor base to 4K7 Ohm resistor, resistor to +Ve supply.
Connect logic pin to transistor base
dava_2 (author)  steveastrouk6 years ago
On this you said, of coruse, led lits up. What do you think at "logic pin"?
6 years ago
Connection to the computer that controls the matrix
dava_2 (author)  steveastrouk6 years ago
Hmm. Yes, on some logics it should light up. Thanks for support ! I'm marking this as a best answer. I this dosen't working, I will start a question again. Thanks everyone !
steveastrouk6 years ago
Then wire them as collector loads on the transistor. LED Anode to resistor, resistor to supply +ve. LED cathod to transistor collector Transistor emitter to 0V Transistor base to 4K7 Ohm resistor, resistor to +Ve supply. Connect logic pin to transistor base
Re-design6 years ago
He's not changing it to negative. He's using the transistors as switches to turn on an odd the leds.
dava_2 (author)  Re-design6 years ago
Example? Some sketch or something..
steveastrouk6 years ago
To literally turn say +5 into -5 relative to the same ground, you need to use something like an ICL 7660
dava_2 (author)  steveastrouk6 years ago
Something simplier? I would need 8 of those then.. For example: some guy here(http://www.jbprojects.net/projects/led/) done it with some tranzistors...