Stepper Motor Driver (74194)

  This page features simple and inexpensive, stand alone UNIPOLAR stepper motor driver using parts that are available from many sources.

  The driver is designed for medium and low speed applications with motors that draw up to 1.0 amperes per phase.

  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. (Slower and faster step rates are also possible. - See notes.)

  The only step angle for this driver is the design step angle of the motor itself. 'Half-stepping' is not possible.

  A 74194 - Bidirectional Universal Shift Register from the 74LS or 74HC - TTL families of logic devices to produce the stepping pattern.

A printed circuit board is available for this circuit.


  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: 74194 Unipolar Stepper Driver - ULN2803

Stepper Motor Driver PCB Circuit

  The following schematic is for the printed circuitboard version of the 2008 stepper motor driver.

Basic Controls For The Stepper Driver

  The direction is selected by an ON-OFF-ON toggle switch.

  The stepping rate is shown being set by a 1 Megohm potentiometer (RT). 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.

Basic Stepper Motor Driver Operation

  1.   The LM555 (IC 1) astable oscillator produces CLOCK pulses that are fed to PIN 11 of the 74194 (IC 2) shift register.

  2.   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 - 13 - 12 - 15; .etc.

      When the base of Q3 is LOW the output shifting of IC 2 will be pins 12 - 13 - 14 - 15 - 12; .etc.

      The direction of the output's shifting determines the direction of the motor's rotation.

  3.   The outputs of the 74194 are fed to four sets of paralleled segments of a ULN2803 Darlington driver (IC 3).

    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.

  4.   As the coils of the motor are turned ON in sequence the motor's armature rotates to follow these changes. Refer to following diagram.

Inputs Vs. Outputs Waveforms

  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). The output is shown stepping in one direction only.

Integrated Circuit Chips Used

  It is not the purpose of this page to provide full explanations of how these devices work. Detailed explanations can be found through datatsheets that are available from many source on the internet.

74194 Stepper Motor Driver Notes

74194 Stepper Driver Initialization Notes

  The 2008 version of the stepper motor driver is ready to start operation as soon as the the initialization delay is complete.

Stepper Circuit Board Parts List

Qty.   Part #   DigiKey Part #   DigiKey Description
1 - IC 2* - 296-9183-5-ND - IC BI-DIR SHIFT REGISTER 16-DIP
1 - IC 3 - 497-2356-5-ND - IC ARRAY EIGHT DARLINGTON 18 DIP
1 - IC 4 - LM7805ACT-ND - IC REG POS 1A 5V +/-2% TOL TO-220
  -   -   -  
3 - Q1, 2, 3 - 2N3904FS-ND - IC TRANS NPN SS GP 200MA TO-92
1 - D1 - 160-1712-ND - LED 3MM GREEN DIFFUSED
1 - D2 - 1N4148FS-ND - DIODE SGL JUNC 100V 4.0NS DO-35
1 - D3 - 1N4001FSCT-ND - DIODE GEN PURPOSE 50V 1A DO41
  -   -   -  
4 - R1, 2, 8, 9 - 3.3KQBK-ND - RES 3.3K OHM 1/4W 5% CARBON FILM
3 - R4, 6, 7 - 10KQBK-ND - RES 10K OHM 1/4W 5% CARBON FILM
1 - R3, 5 - 470QBK-ND - RES 470 OHM 1/4W 5% CARBON FILM
  -   -   -  
1 - C1 - P5174-ND - CAP 1.0UF 50V ALUM LYTIC RADIAL
2 - C2, 3 - P5177-ND - CAP 4.7UF 50V ALUM LYTIC RADIAL
1 - C4 - P5168-ND - CAP 470UF 35V ALUM LYTIC RADIAL
  -   -   -  

  * - The DigiKey part number for IC 2 is for the 74HC194 - CMOS IC. This IC is a CMOS type that can be damaged by static electricity discharge.

  DigiKey does not carry the 74LS194 in small quantities. It is available for other sources such as Mouser Electronics - stock number 47053 and Jameco Electronics - stock number 59574LS194AN as well as many other sources. Be sure that the IC's have the DIP package.

  * - The Part Number for Q1, Q2 and Q3 is for 2N3904s. Almost any NPN, Switching or Small signal type will work, the 2N4400 is one example.

Circuitboards And Parts

  The following picture is of an assembled circuitboard for the Unipolar Stepper Motor Driver. The board measures 2 inches by 3 inches and has been commercially made. The board is not tinned or silkscreened.

  The relative positions of the terminal blocks at the sides and ends of the circuitboard correspond with those in the schematic diagram and the control circuit examples.


 The price for 1 circuitboard is 10.00 dollars US plus postage.


 The price for 1 kit of parts and a circuit board is 20.00 dollars US plus postage.


 The price for 1 Assembled circuitboard is 23.00 dollars US plus postage.


  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: 74194 Unipolar Stepper Driver - ULN2803

Please Read Before Ordering

  Due to delays in acquiring 74LS194 type ICs, the assembled circuitboards and kits will use the 74HC194 - CMOS type IC. The 74HC194 will be mounted in a socket to eliminate soldering this device during assembly.

  Although the 74HC194 is sensitive to damage from static discharge, once it is installed in its socket the IC is very safe as all of its pins are connected to the 5 volt supply or to common through low impedance paths.

  When handling the board, avoid nonconductive surfaces such as plastics or glass. If the circuit board is to be placed in a plastic case, do the assembly work on a wood or metal surface that is connected to earth. Also avoid carpeted areas during assembly.

  A good practice is to touch the work surface before touching the circuitboard.

PCB Parts Placement Diagram

Other Information And Diagrams

Wiring for longer distances.

  If the motor is some distance from the circuit board or power supply, it might be best to separate the motor's power supply lead from the circuit board's supply as illustrated in the next diagram. The motor could be connected using larger gauge wire.

  This will keep most effects of the motors current pulses away from the supply to the circuit board. A filter capacitor could be placed in the motor's supply circuit as well.

Connecting A 6 Lead Motor to the Stepper Driver

  It may be necessary to move the coil leads around to get the motor to turn properly. Leave one wire connected permanently and change the other three coil leads as needed.

Single-Step Input

  The connections in the following diagram will allow the motor to make single steps. A toggle switch could be used to select between single and continuous steps if the 1 Megohm potentiometer was included in the circuit.

External Controls Using Transistors

External Controls Using Optoisolators

  The use of optoisolators provides complete isolation between the driver and the external control circuit.

Automated Motor Control Circuit - (Voltage Comparators)

  The circuit above replaces the direction control switch with a "window" type voltage comparator circuit. Potentiometer "R IN" could be a temperature or light sensing circuit.

  In a practical application the direction of the motors load, a heating duct damper for example, would bring the temperature represented by the voltage at R IN back to the range between the HIGH and LOW voltage setpoints.

  The limit switches at the outputs of the comparators are used to prevent the damper from going beyond its minimum and maximum positions by to stopping the motor.

  Also see Voltage Comparator Information And Circuits - Voltage Window Detector Circuit.

Slower Step Rates

  Additional capacitance can be added to the IC 1 circuit to provide slower motor step rates. There is a limit to this approach as control of the step rate becomes less accurate as the capacitance increases and at some point the timer will stop working due to the leakage currents of the capacitors.

Fast External Clock

  An external clock with a step rate greater than 145 steps per second can be connected to the driver circuit by removing capacitor C1. There is no limit on how slow the clock input can be.

Using Low Voltage Motors

  Stepper motors that require less than 12 volts can be controlled by the driver by removing diode D3 from the circuitboard and connecting a external power supply to the control section of the driver.

  Also, IC 4 could be removed from the circuitboard and a regulated 5 volt supply connected at the '+5V' terminal.

Single Input Direction Control

  The following circuits allow the direction of the motor to be controlled by as single, ON-OFF input. The maximum input voltage is 5 Volts.

Using Higher Current Motors

  The next circuit uses TIP125 Darlington type transistors to increase the current capacity of the 74194 driver circuit to 5 amps per winding.

  Depending on the current required for the motor, small heatsinks may be needed for the transistors.

Other Information

  Animated operation of stepper motors.


  For the motor driver circuit on this web page, only 1 coil is ON at a time so the rotor of the motor would be aligned with one of the stator's poles and not half way between poles as shown in the animation.

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



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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.


  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: 74194 Unipolar Stepper Driver - ULN2803

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04 July, 2016