Basic Stepper Motor Driver Operation
The LM555 (IC 1) astable oscillator produces CLOCK pulses that are fed to PIN 11 of the 74194 (IC 2) shift register.
Each time the output of the LM555 timer goes HIGH (positive) the HIGH state at the 74194's OUTPUT terminals, (PIN's 12, 13, 14, 15), is shifted either UP or DOWN by one place.
The direction of the output shifting is controlled by switch S1. When S1 is in the OFF position (centre) the HIGH output state will remain at its last position and the motor will be stopped.
Switch S1 controls the direction indirectly through transistors Q2 and Q3.
When the base of Q2 is LOW the output shifting of IC 2 will be pins 15 & 14 Reverse states then 13 & 12 Reverse states, etc.
When the base of Q3 is LOW the output shifting of IC 2 will be pins 12 & 13 Reverse states then 14 & 15 Reverse states, etc.
The direction of the output's shifting determines the direction of the motor's rotation.
The four outputs of the 74194 are fed to one of the driver segments of a SN754410NE - H Bridge driver IC (IC 3).
When an input of a SN754410NE segment is HIGH, its darlington transistor will turn ON and that output will source current through one of the motor's two coils and into the output that is LOW.
1.The outputs of the SN754410NE are used in pairs with two of the outputs being HIGH and the two coresponding outputs being LOW at any given time.
As the polarity of the motor's coils are revresed in sequence the motor's armature rotates to follow these changes. Refer to following diagram.