Going from 18.5V to 12 V is easy. All you need is a simple 7812 voltage regulator and a few capacitors. Going from 18.5V to 24V is a little harder, you need a DC to DC converter. You could build your own, or buy it neatly pre-packaged in a variety of different power and input voltages.Here's one with a 2W rating, and accepts an input voltage between 18 and 36V. But if you want to make your own, there are plenty of designs out there. Linear Technologies has lots of ICs designed specifically for this purpose. Visit their site and take a look through their extensive selection.