I have the following circuit to drive an unipolar stepper motor (I've used in a parallel port project).
But now I don't know an way to plug the circuit in a safe way to my usb port.
You can control a stepper motor via USB by adding a USBmicro device like a U401, U421, U451 to your interface circuit.http://www.usbmicro.com/U401⁄U421⁄U451 USB I⁄O Interface
The U401⁄U421⁄U451 provides a simple digital i⁄o interface for the PC (Win⁄Linux) or Mac (OSX). Sixteen i⁄o lines from the microcontroller are provided. Commands can be sent to the U401⁄U421⁄U451 that change the i⁄o lines from input to output. I⁄O lines can be individually selected as inputs or outputs. The U401⁄U421⁄U451 supports commands to read the ports, and if the ports are set to output, to write to the ports.
The U401⁄U421⁄U451 is an interface to SPI devices. The firmware on the U401⁄U421⁄U451 provides generic access to read and write SPI devices. The SPI clock rate can be adjusted to 62.5 kHz, 500 kHz, 1 MHz, or 2 MHz. Because additional pins are available as generic i⁄o, the U401 can use these lines as slave select lines and address multiple SPI devices.
The SPI subsystem of the U401⁄U421⁄U451 can be used as a master to communicate with SPI devices such as EEPROMS and A⁄D converters. The U401⁄U421⁄U451 can also be used as a SPI slave to a microcontroller that uses the U401⁄U421⁄U451 as a gateway to the PC (Win⁄Linux) or Mac (OSX) . A PIC or an AVR, for example, can act as a SPI master to communicate data with the U401⁄U421⁄U451, which can then transfer the data to a PC (Win⁄Linux) or Mac (OSX) application.
The U401⁄U421 is a convenient way to interface a standard Hitachi-type of intelligent LCD controller to USB. The commands that support communication to the LCD module are the "standard" LCD commands. Standard commands include writing characters to the display, and controlling the display.
USB interfacing is simple with the U401⁄U421⁄U451 - There are no USB drivers to write and there is no device firmware to develop. There are sample applications that will get you started in minutes. The sample code is available to change for your application.