How-To: Pentax intervalometer

The inimitable Randy Sarafan writes:

I decided to make a quality DIY intervalometer for my DSLR Pentax camera. This intervalometer should work with most major brands of DSLR cameras such as Nikons and Canons. It works by triggering the shutter using the camera’s remote trigger port. It can also auto-focus before each shot if so desired (or toggle this on or off at any time). The brains of this intervalometer is an Arduino chip. It may seem very complicated at first glance, but is actually a simple circuit and not that hard to make.