The Raspberry Pi computer is eminently suitable for interfacing. It has 17 GPIO lines which can be programmed to be either inputs or outputs as well as various other functions. They are very low power lines however, and on their own can only drive tiny LEDs. With this in mind, I have designed the interface on the left.
As I have built quite a few projects for the Velleman K8055 interface board I designed the pin-out of this interface so that most of my Velleman projects can be connected to it without alterations. The finished product provides eight input lines and eight output lines. The outputs are buffered by a ULN 2803 Darlington driver and can sink – with luck – 500mA each. The board connects to the Raspberry Pi via a parallel cable.
The circuit diagram is on the left.
A resistor network allows eight switches to be connected to the input side. A LED is connected to the 5V line to indicate power on. Because the GPIO 18 line can also be used for pulse width modulation, it is brought out separately. A link will connect it to the Darlington driver instead of GPIO25, should this be required.
For more detail: A Raspberry Pi Interface Board