Here is a simple variable voltage regulator using ic LM317 circuit. The voltage regulator is an LM317T, and should accept up to about 14 volts without problems. It can handle up to 1 amp, but you WILL need a heatsink on the voltage. If you only plan to use items that run on one voltage, this is a very useful feature and will save plugging in and damaging your valuable (or not so valuable) equipment. You can even add a relay to switch off the power if the over voltage LED turns on, but bear in mind it will have to work from the voltage of the zener diode right up to the input voltage. The over voltage LED uses a zener diode to switch on the LED at a certain preset voltage, this can be varied depending on the voltage of the zener diode, I used a 6.2v zener diode.
Here is a schematic variable voltage regulator:
Simple Variable Voltage Regulator Circuit
part of list regulator circuit:
R2: 2K Cermet or carbon preset potentiometer
C2: 1uF tantalum
LM317T Voltage regulator