maximite stepper motor interface

This simple circuit and program listing allows the Maximite microcomputer (SILICON CHIP, March-May 2011) to control a stepper motor. It could be expanded to allow for the control of multiple motors, with four of the Maximite`s external I/O pins used to control each motor with identical driver circuits. A ULN2003 Darlington transistor array (IC1) s
maximite stepper motor interface - schematic

witches current through the stepper motor`s two windings in either direction. When one of the four Maximite output pins (8, 12, 16 & 20, corresponding to I/Os 19, 17, 15 & 13) goes high, the corresponding output pin on IC1 goes low, sinking current through a motor winding. Conversely, when these pins are high, the corresponding Darlington transistor is off and so no current flows through that portion of the winding. The centre tap of each motor winding is connected to a current source comprising PNP Darlington transistor Q1 and some resistors. The maximum current is determined by the resistive divider driving its high-impedance base, setting the base voltage to around 9. 1V when it is fully on. By adding Q1`s base-emitter voltage (1. 4V at 0. 5A, as per the data sheet) we can determine that there will be around 1. 5V across the 3. 3O resistor (12V - 10. 5V), resulting in a current of 1. 5V G· 3. 3O = ~450mA. Transistor Q1 must be fitted with a medium-sized flag heatsink (Jaycar HH8504, Altronics H0637) or larger to handle its maximum dissipation of (10. 5V - 4. 9V) x 450mA = 2. 5W. When one of the Darlington transistors switches off and current flow through the corresponding motor winding ceases, the inductive winding generates a back-EMF current which causes the voltage across that winding to spike. IC1 has internal free-wheeling  diodes from each output to the COM pin, which is connected to the +12V supply. The back-EMF...

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