The output is shown stepping in one direction only.
The following diagram shows the stepping order for the outputs of the ULN2803 (IC 3) as compared to the input and output of the 74194 (IC 2).
Īs the coils of the motor are turned ON in sequence the motor's armature rotates to follow these changes. When an input of a ULN2803 segment is HIGH, its darlington transistor will turn ON and that OUTPUT will conduct current through one of the motors coils. The outputs of the 74194 are fed to four sets of paralleled segments of a ULN2803 Darlington driver (IC 3). The direction of the output's shifting determines the direction of the motor's rotation. When the base of Q3 is LOW the output shifting of IC 2 will be pins 12 - 13 - 14 - 15 - 12. When the base of Q2 is LOW the output shifting of IC 2 will be pins 15 - 14 - 13 - 12 - 15. Switch S1 controls the direction indirectly through transistors Q2 and Q3. When S1 is in the OFF position (centre) the HIGH output state will remain at its last position and the motor will be stopped. The direction of the output shifting is controlled by switch S1. Įach 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 LM555 (IC 1) astable oscillator produces CLOCK pulses that are fed to PIN 11 of the 74194 (IC 2) shift register. Using the component values shown for R1, RT, R2 and C1, the calculated step rate range is between 0.72 steps per second (1.39 seconds) to 145 steps per second. The stepping rate is shown being set by a 1 Megohm potentiometer (RT). The direction is selected by an ON-OFF-ON toggle switch. The following schematic is for the printed circuitboard version of the 2008 stepper motor driver. If you are interested in printed circuit boards please send an email to the following address: Subject: 74194 Unipolar Stepper Driver - ULN2803 
A printed circuit board is available for this circuit.
'Half-stepping' is not possible.Ī 74194 - Bidirectional Universal Shift Register from the 74LS or 74HC - TTL families of logic devices to produce the stepping pattern. The only step angle for this driver is the design step angle of the motor itself. (Slower and faster step rates are also possible. This driver provides only basic control functions such as: Forward, Reverse, Stop and has a calculated Step rate adjustment range of 0.72 (1.39 sec) to 145 steps per second. The driver is designed for medium and low speed applications with motors that draw up to 1.0 amperes per phase. This page features simple and inexpensive, stand alone UNIPOLAR stepper motor driver using parts that are available from many sources. 74194 Unipolar Stepper Motor Driver UNIPOLAR Stepper Motor Driver (74194)
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