How can I drop a 15 volt DC supply to 12 volt dc?

Answer 1

Best way is use a DC-DC converter (LM317T Positive *cross* NTE 956 *cross* Radio Shack 276-1778) It's a very simple circuit that requires 2 resisters (or one resister and a pot if you want it adjustable) and 2 capacitors.
It will convert the 12 VDC in to an adjustable output of 0.5 - 11 VDC.
I use these all the time to install new (and inexpensive) 1 1/2 VDC quartz clock mechanisms in older, mechanical 12 VDC clocks in vintage and classic cars.

Answer 2

A three terminal linear regulator would work well for this if you need less than 1A of current. Have a look at the LM7812 data sheet for the application circuit. These are fairly inexpensive and easy to find.

http://www.fairchildsemi.com/ds/LM/LM7812.pdf

A more simple, but less precise way would be to add a zener diode, or three forward biased silicon power diodes (1N4001 or similar) in series. This would give you approximately 12V (+/-1.5V), depending on the load current, characteristics of the diodes, and the temperature.

Answer 3

the downright easiest and laziest way is:

put 3 1 volt diodes on the positive terminal.