Stepper Motor Driver Card (Single Motor)

Stepper Motor Driver Card (Single Motor) provides Opto-isolation Interface with PC/μP/μC. The circuit of Stepper Motor Driver Card described here can control single motor up to 6Kg Torque. It converts 4 TTL-level signals into an equal number of outputs with a drive capability of 2A each. It is suitable for any other project where single unipolar stepper motor is to be driven. It is generally assigned to work with the PC interface; the power driver card can be connected to any other type of computer that provides 4-bit wide I/O port.   
About the Circuit :
The operation of the circuit is shown in figure is identical for all 4 channels. The input is connected to the PC interface card or any other port. Each signal drives a LED D1 to D4 and also drives an opt coupler IC1 to IC4. LED D1 to D4 indicates the status of input signal from PC interface. Each opt coupler is followed by an inverter consists in IC5, which passes the signal to a driver contained in IC6. This IC is capable of supplying the necessary base current for the power transistors T1 to T4. The logic level applied to pin1 of IC6 defines the data direction. The bit pattern supplied by the inverters is fed direct to the power transistors. The active low input of the bus drivers in IC6 are taken logic high by pull-up resistors, the relevant IC6 is switched to its high-impedance output mode(three-state), so that all eight power transistors driven by it are switched off. The base currents of the eight driver transistors T1 to T4 are limited by 1K resistors R10 to R13. Transistor is switched on with the associated LED D5 to D8 lights. The LED arrays are intended mainly to assist you while running an initial test on the stepper motor control. The LED bar shows the status of all eight motors at a glance, and will be found much handier. Each collector-emitter junction of the power transistors is shunted by diodes D9 to D12, which suppress back emf pulses generated when inductive loads are switched.  The circuit requires DC power supply of 5V, which is designed using regulator IC7.