I do not believe that the HC is specified as 5V tolerant inputs. Not sure.
should be, it just cant take the current to drive older ls devices
The inputs aren't 5V tolerant when powered at 3V. The Phillips spec
is Vcc + 0.5V. So the maximum specified voltage would be 3.5V.
If you want much higher (and don't care too much about speed)
you could use zener clamps and a series resistor. This would clamp the voltage
and limit the current. A lot more parts but you would easily get to
the 15V you wanted.
(* jcl *)