transistors

How do I use transistor to take a negative input and output a positive.  I was thinking maybe I could use a 741 op-amp with the inverting amplifier cofiguration vin/vout =-r2/r1 with r2=r1 12v(-) in should output 12 v(+) correct? Or can I use an npn or pnp transistor to do this?  If so how do it do it and what specific transistor should I use.  I am trying to make an led that I will put in my shift knob turn on with the dash lights, which is simple, a 1k resistor and 12v led then ground it and hook up to the dash light option in the fuse box, but I also want the same led to turn on when the door is open, which operates using a ground switch.  I will also need to use diodes so the doors dont turn on the dash lights and vis versa.  Does anyone have a solution?Andy Huot

This can easily be done with a relay and a diode, if nobody else helps you with this today, I can draw you a diagram tonight. If the LED is a 12 volt device the 1K resistor is already built into it. If it is a standard LED you will need the resistor.

Yep, PNP would be the ticket, might try a higher value resistor for the base. Since it's only switching an LED; a 2.2k should be fine, since the current gain is probably at least 20. The base current would only be about 5mA, which would still be able to switch 100mA.

Yeah, that wouldn't be a bad idea.  The 220 ohm value is the value I use for 5vdc systems.  It would still work, on 12vdc but a higher value wouldn't hurt.  I usually try to limit the base current to around 25mA; a 560 ohm resistor will acomplish that.  For your small LED load, dualsport is correct, you could go much, much higher.  The beta of that transistor, I believe, is around 50.Kevin Pierson