The circuit shown above can be used to control a unipolar stepper motor which has FOUR coils (I've swiped it off an old fax machine). The above circuit can be for a motor current of up to about 500mA per winding with suitable heat sinks for the SL100. For higher currents power transistors like 2N3055 can be used as darlington pair along with SL100. The diodes are used to protect the transistor from transients.

Activating sequence:

+-------------+---------------+ |   Inputs    |Coils Energised| |-------------+               | |  D0     D1  |               | |-------------+---------------| |  0      0   |     A,B       | |-----------------------------| |  0      1   |     B,C       | |-------------+---------------| |  1      0   |     C,D       | |-------------+---------------| |  1      1   |     D,A       | |-----------------------------|

To reverse the motor just reverse the above sequence viz. 11,10,01,00.

Alternately a 2bit UP/DOWN counter can also be used to control the direction , and a 555 multi-vibrator can be used to control the speed.