Hall Effect Sensor Simulation

I have a robotics project that I am working on at my University. I need to gain control of the accelerator and brake pedals of a gold cart. They both use hall effect sensors that vary a voltage between .5 and 4.5 volts. .5 being the minimum braking and acceleration and 4.5 the maximum. The output voltages from the sensors get fed into a motor controller. All of the equipment that I am putting on the cart will be running off a power source separate from the 48V source the cart has. My plan is to use a basic stamp micro-controller that will be sent serial commands from the computer to adjust the voltages to certain levels at a certain rate. Great, that will work out fine. The part I do not understand is that how do I control an external power source which is in fact different from the source that is powering the stamp. The stamp can simulate a 0 to 5v analog output, but that would mean running it off of the carts power source. Any suggestions? Thanks.

This is what switching transistors are for.

Thanks, I am not sure why I didn't see that myself, but appreciate the reply. I knew there was a simple solution.

Probably thinking about it too hard =)