Ultimately I would like to have a 12v motion detection flood light activating system for my RV. The ones you can buy are pretty expensive but I was wondering if it would be feasible to convert one of the 120ac (cheaper) units to 12v. Or are the components just too much of a mis-match?

 

All depends on the design of the normal 120V ones and good luck getting any sort of schematic on one. I would think designing a new one would be in order, might even be able to re-use the detector part of the 120V one to drive a circuit optimized to switch 12V.

Then again the PIR detectors they use in home alarm systems are usually 12V by nature, simple matter of hooking the output up to a relay.

 

I did this when i build my home alarm system. I take one of those night lite types with the IR motion detector.

Inside the unit are 2 PCB's The first one got the relay soldered to it and the next board was the IR detector. Remove all the components from the relay board except for the lm7808 . Solder a 12v relay to the spot where the 24v was soldered.You'll see a small lm7808 voltage regulator which you have to leave on the board. It give power to the detector.Just remember to add a diode to the relay for the back EMF.

On the detector board i just cover the LDR which turn the unit off in daytime, but maybe you wont need to do this.All you have to do now is to replace 120v supply with a 12v supply and that's it