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Wednesday, 11 March 2009

UNIPOLAR Stepper Motor Driver (74194)


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.

The stepper motor driver on this page replaces the Unipolar Stepper Motor Driver (74194) 2007 That was previously available through this web site.

A printed circuit board is available for this circuit.

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

  • IC 1 - LM555 - Timer, normally configured as an astable oscillator but can be used a monostable timer for 1 step at a time operation or can be used as a buffer between external inputs and IC2. (See later Diagrams.)

  • IC 2 - 74194 - 4-Bit Bidirectional Universal Shift Register. The shift register provides the logic that controls the direction of the drivers output steps.

    This circuit can use either the 74LS194 or the 74HC194 shift register IC. Their logic functions are identical but the 74HC194 IC is a CMOS type that can be damaged by static electricity discharges. Antistatic precautions should be used when handling the 74HC194 to avoid damage.

    If you are purchasing your own parts use the 74LS194 IC if it is available.

  • IC 3 - ULN2803 - 8 Segment, Darlington, High Current, High Voltage Peripheral Driver. Each segment can handle currents of up to 500 milliamps and voltages up to 50 volts. In this circuit 2 output segments are connected in parallel, this allows a maximum output current of 1 amp per phase.

  • IC 4 - LM7805 - Positive 5 Volt Regulator. Provides low voltage power to the driving circuitry and can also power external control circuits.

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

  • Due to the lack of error detection and limited step power, this circuit should not be used for applications that require accurate positioning. (The driver is designed for hobby and learning uses.)

  • There are links to other stepper motor related web pages further down the page. These may be helpful in understanding stepper motor operation and control.

  • For the parts values shown on the schematic, if the external potentiometer (RT) is set to "ZERO" ohms, the calculated CLOCK frequency will be approximately 145 Hz and a motor will make 145 steps per second. This step rate should be slow enough for most motors to operate properly.

    The maximum RPM at which stepper motors will operate properly is low when compared to other motor types and the torque the motor produces drops rapidly as its speed increases. Testing may be needed to determine the minimum values for RT and C1 to produce the maximum CLOCK frequency for any given motor. Data sheets, if available, will also help determine this frequency.

  • If RT is set to 1 Megohm, the calculated step rate will be 0.73 Hz and the motor would make 1 step every 1.39 seconds.

    There is no minimum step speed at which stepper motors cannot operate. Therefore, in theory, the values for RT and C1 can be as large as desired but there are practical limitations to these values. The main limitation is the 'leakage' current of electrolytic capacitors.

  • External CLOCK pulses can also be used to control the driver by passing them through IC 1 via the "T2" terminal of the circuitboard. Using IC 1 as an input buffer should eliminate "noise" that could cause the 74194's output to go into a state where more than one output is HIGH.

  • If stepping rates greater than 145 per second are needed, capacitor C1 can be replaced with one of lower value.

    A 0.47uF capacitor would give a calculated range of 1.5 to 310 steps per second.

    A 0.33uF capacitor would give a calculated range of 2.2 to 441 steps per second.

    Alternately, capacitor C1 can be removed from the circuitboard and an external clock source connected at terminal 'T2'. With C1 removed, the practical limit on the step rate is the motor itself.

  • In the above items the "calculated" minimum and maximum CLOCK frequencies are valid for the nominal part values shown. Given the tolerances of actual components and the leakage currents of electrolytic capacitors the actual CLOCK rates may be lower or higher.

  • The direction of the motor can be controlled by another circuit or the parallel output port of a PC. This will work as long as the voltage at the bases of Q2 and Q3 can be made lower than 0.7 volts. Additional NPN transistors may be required to achieve this result, depending on the method used.

  • If the bases of both Q2 and Q3 are made LOW at the same time the SN74194 will go into a RESET mode. This will cause the step sequence to stop and on the next clock pulse pins 15 and 14 will go to a HIGH state.

    Making the bases of both Q2 and Q3 LOW at the same time can be used to reset the SN74194 to its starting position without having to remove the circuit power.

  • Each stepper motor will have its own power requirements and as there is a great variety of motors available. This page cannot give information in this area. Users of this circuit will have to determine motor phasing and power requirements for themselves.

    Power for the motors can be regulated or filtered and may range from 12 to 24 volts with currents up to 1,000 milliamps depending on the particular motor.

    Motors that operate at voltages lower than 12 volts can also be used with this driver but a separate supply of of 9 to 12 volts will be needed for the control portion of the circuit in addition to the low voltage supply for the motor.

  • A LED connected to the output of the LM555 timer (IC 1) flashes at the CLOCK frequency. If a direction has been selected, The motor will move one step every time the led turns ON.

  • There is no CLOCK output terminal on the circuitboard but there is a pad to the right of the LED that can be used if a clock output signal is required. This pad is connected to pin 3 of the LM555 IC.

  • The LM7805, positive 5 volt regulator used on the circuitboard can also be used to provide power for external control circuits. With its tab trimmed off, the regulator can easily dissipate up to 1 watt.

    For a 12 volt supply, external circuits can draw up to 100 milliamps.

    For a 24 volt supply, external circuits can draw up to 25 milliamps.

  • The photo of the circuitboard shows the tab of the 7805 regulator cut off, this is an option that is available on request.

74194 Stepper Driver Initialization Notes

  • When power is applied to the 74194 Stepper Driver circuit there is a very short delay before stepping of the outputs can begin. The delay is controlled by Capacitor C2, resistor R4 and transistor Q1.

  • The function of the delay is to allow the outputs of IC 2 to be set with pin 12 in a HIGH state and pins 13, 14 and 15 in a LOW state before direction control becomes active. The delay also prevents IC 1 from oscillating until IC 2 has been set.

  • If the power to the circuit is turned off, there should be a pause of at least 10 seconds before it is reapplied. The pause is to allow capacitor C2 to discharge through R4 and D2.

  • If the initialization delay were not used, IC 3 could have: none, any or all of its outputs in a high state when stepping is started. This would cause the motor to move incorrectly or not at all during normal operation.

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 photo of the circuitboard shows the tab of the 7805 regulator cut off, this is an option that is available on request.

The price for 1 circuitboard is 9.50 dollars US plus postage.

The price for 1 kit of parts and a circuit board is 19.00 dollars US plus postage.

The price for 1 Assembled circuitboard is 22.00 dollars US plus postage.

If you are interested in a circuitboard and parts for this circuit please send an email to the following address:

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.

  • When the voltage at the centre tap of R IN is between the HIGH and LOW voltages set by resistors R1, R2, and R3 the motor will be stopped.

  • When the voltage at the centre tap of R IN is above the HIGH voltage between R1 and R2 the motor will be step in the FWD direction.

  • When the voltage at the centre tap of R IN is below the LOW voltage between R2 and R3 the motor will be step in the REV direction.

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.

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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