Stepper Motor Driver (74194)

  This page features an inexpensive stepper motor driver that could be used to power slow speed projects on the layout or other hobby applications.

  Based on the SN74LS194 - Bidirectional Universal Shift Register the circuit is designed to drive UNIPOLAR type stepper motors and provides only basic control functions - Forward, Reverse, Stop and Speed adjustment.

  The only step angle for this driver is the design step angle for the motor.

  The circuit is not complex and is cheaper than many dedicated driver/controller devices and the parts are easy to find.

  For the purposes of this page the direction control function is selected by an ON-OFF-ON type toggle switch. This could be easily replaced by another method such as transistors controlled by a PC's parallel output port.

  Speed control is by means of a potentiometer but the circuit could accept pulses or controls from other sources such as a push button or a simple computer interface. The direction could also be controlled by a computer interface.

  This web page uses integrated circuits from the SN74LSxx family of TTL devices. It is not the purpose of this page to provide detailed explanations of how these devices work and an understanding of simple logic circuits would be helpful.

  Do not be discouraged by this however as the circuit's operation is quite simple.

N.B. - Due to the lack of error detection or correction this circuit should not be used for application that require accurate step control or positioning accuracy. The circuit is intended for hobby use only.

Stepper Motor Driver Circuit

  The following diagram is for the main circuit of the motor driver.

  A testing version is shown near the end of this page. It is laid out differently and shows the SN7474 in logic block form and LED's are used to indicate the motor coils being switched.

Stepper Motor Driver circuit

  The blue line on the drawing is the path that the CLOCK pulses that drive the circuit follow.

  The stepper motor would not be connected as shown on the schematic as the motors usually have a common and four coil leads.

  Also, the filter capacitor at the power supply to the circuit would not be connected as shown.

Stepper Motor Driver Operation

  1.   The 555 astable oscillator produces a series of CLOCK pulses that are fed to PIN 11 of the SN74194 integrated circuit.

  2.   Each time the CLOCK pulse goes positive the HIGH state at the SN74194's OUTPUT terminals, (PIN's 12, 13, 14, 15), is shifted either UP or DOWN. Refer to the "Stepper Motor Driver Waveforms" diagram.

      The direction of this shifting is controlled by switch S2. When S2 is in the center OFF position the HIGH output state will remain in its last position and the motor will be stopped.

      When the base of Q1 is LOW the shifting will be PIN 12 - 15 - 14 - 13 - 12  .etc.

      When the base of Q2 is LOW the shifting will be PIN 12 - 13 - 14 - 15 - 12  .etc.

      The direction of the pulse shifting determines the direction of motor rotation.

  3.   The pulses from the OUTPUT's of the SN74194 are fed to four segments of the ULN2003 Driver. When a segments input is HIGH the transistor will turn ON and its OUTPUT will conduct current through one of the motors coils.

  4.   As the coils of the motor are turned ON in sequence the motor rotates to follow these steps. Refer to following diagrams.

  The following diagram shows the Stepper Motor Driver circuit with its circuit boards external connection terminals. The motor coils have been omitted from the diagram.

Stepper Motor Driver - Circuit Connections

Integrated Circuit Chips Used

  The next diagram shows the basic waveforms for the stepper motor driver circuit.

Stepper Motor Driver Waveforms

  The next diagram shows a very simplified diagram of the step funtion of the 74194 chip.

Stepper Motor Driver Equivalent

  The following diagram shows the stepping order of the inputs to ULN2003 Peripheral Driver for forward and reverse motor directions. Pin numbers are not indicated as this depends on the PCB layout.

  Each positive pulse at the SN74194's - OUTPUT terminals turns ON one of the stepper motors coils.

ULN2003 Motor Driver Stepping Order

74194 Stepper Motor Driver Notes

The motors used to test this circuit were:

  1. JAPAN SERVO CO.  (From an old floppy drive)
    TYPE KP4M4-001
    75 OHM / PHASE
    0.15 AMP / PHASE

  2. AIRPAX : LA82720-M1  (From a chart drive)
    24 VOLT
    60 OHMS / COIL
    7.5 DEGREES / STEP

Some Stepper Motor Related Links

  The following links are for stepper motor related pages and have a lot of good information on other types of driver circuits and motors.

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Stepper Motor Driver Test Circuit

Stepper Motor Driver circuit initial testing version.

  This schematic shows the SN7474 in logic block form with its two "D" type FLIP-FLOP's. This circuit was used to test the stepper motor driver circuits operation.

  Section FF1 acts as a binary divider while FF2 acts as a RS FLIP FLOP. After one division step the FLIP FLOP is SET to Q-high.

  This allows the SN74194 to "SET" its output states to PIN 15 - HIGH and PINs 12, 13 and 14 - LOW before the DIRECTION control switching transistors, Q1 and Q2, become active.

  The POWER (14), COMMON (7) and CLEAR (CLR) (1,13) connections of the SN7474 are not shown on the schematic diagram to make the drawing less cluttered. The CLEAR terminals are connected to the +5 volt supply.

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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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