Wayne,
Whoops, I see now that instead of insulting some anonymous person who posted their pet project on the Web for others to duplicate, I see that I insulted you, who posted a circuit here asking that it be critiqued.
To produce 5V to be used as a logic supply from the 15V primary power source, use a three-legged regulator, such as an LM317 or LM7805. The output from a voltage divider is very dependent on the current being drawn from the tap. With digital logic, the supply current to all of the logic chips is all over the place as the chips switch, so the 5V source needs to be "regulated". Don't forget to bypass the logic chips (Vdd to Vss) using 0.01uF or 0.1uF capacitors (at least three).
Another possibilty: couch the circuit in terms of 4000 or HC series CMOS, which can be operated directly off 15V.