I'm trying to use bipolar transistors as the output stage of a circuit controlled by an NXP LPC9221 µC. The illustration shows two of the configurations I've tried, and one about which I have questions.
"On" and "Off" mean current flows or does not flow through the load resistor, in response to a given output from the controller.
The "switch" represents the µC pin, connecting to either +3.3v or ground.
**broken link removed**
Fig 1: shows a low-side NPN based switch. This works fine, but I want to move the transistor to the high-side of the load.
Fig 2: shows my attempt to use a PNP, on the high side, to switch current to a load. In this instance, the transistor never turns off, even when I set the output high. I suspect that's because of the difference in voltage between the load and controller circuits.
Fig 3: shows what I believe will do what I want, switch 12v, from the high side of the load, using a 3.3v signal.
I'd like to know, is there a better solution to this problem, one that uses only one transistor?